Re: a quest for a better scheduler
I don't want to open any old wounds, but I just got a summary from a colleague of mine, Dan Tsafrir, who measured the context switch overhead on Linux with multiple processes. You can find the document at: http://www.cs.huji.ac.il/~dants/linux-2.2.18-context-switch.ps The measurements were taken on a quad Pentium III 550MHz IBM NetFinity server with 1GB RAM. Even though this was done on the older 2.2.18 kernel, this is quite intereseting with regard to the current scheduler discussion. Yoav Etsion - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Fri, Apr 06, 2001 at 11:06:03AM -0700, Timothy D. Witham wrote: > Timothy D. Witham wrote : > > I propose that we work on setting up a straight forward test harness > > that allows developers to quickly test a kernel patch against > > various performance yardsticks. The Linux Test Project would like to help out here. At the least, we would like to add the scripts, wrappers, and configuration files used to create the test systems to LTP. Making test systems available is definitely a great step forward. Showing people how to build similar test systems is another step forward. Let us know what parts you need and we'll see what we can come up with. > Further comments? I will start contacting folks who have expressed > interest. If anyone has loose programs that they use to test the scheduler, please submit them to the Linux Test Project. Chances are other people will also find them useful and add functionality. It doesn't have to be a formal test program or use the test libraries that we use. We can take care of that when we add it to the CVS tree. While we probably aren't going to package up full applications for testing purposes, we could definitely keep track of useful configuration files and scripts that people find useful for testing. -- Nate Straz [EMAIL PROTECTED] sgi, inc http://www.sgi.com/ Linux Test Projecthttp://oss.sgi.com/projects/ltp/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Missing an important one, our VPN's routinely run on 16 MG Ram, no HD or swap.. Loaded from an initrd on a floppy.. Don't we need to test on minimalistic machines as well :) > So the server hardware configurations have evolved to look like > the following. > > 1 way, 512 MB, 2 IDE > 2 way, 1 GB, 10 SCSI (1 SCSI channel) > 4 way, 4 GB, 20 SCSI (2 channels) > 8 way, 8 GB, 40 SCSI (4 channels) maybe Fibre Channel (FC) >16 way, 16 GB, 80 FC (8 channels) > -- "Catch the Magic of Linux..." Michael Peddemors - Senior Consultant LinuxAdministration - Internet Services NetworkServices - Programming - Security WizardInternet Services http://www.wizard.ca Linux Support Specialist - http://www.linuxmagic.com (604)589-0037 Beautiful British Columbia, Canada - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Timothy D. Witham wrote : [...] > I propose that we work on setting up a straight forward test harness > that allows developers to quickly test a kernel patch against > various performance yardsticks. [... (proposed large server testbeds) ...] OK, so I have received some feedback on my proposal to provide a reference set of machines so that any kernel modifications could be checked across a range of machines and a range of tests. It was pointed out that there are lots of smaller servers out there and they should be part of any test plan. There was also some concern that a 4 way server didn't add any value in a test lineup. But I have to think that with the number of 4 ways out there they should be included. One additional piece of feedback was that any comprehensive characterization plan should include desktops, tablet devices and older machines and the performance tests that address those configurations usage models and I agree that it is something that needs to be done. But as for providing hardware for that effort the OSDL is not the group to do that. Hopefully somebody with an interest in these configurations will step forward to do that portion of the job. So the server hardware configurations have evolved to look like the following. 1 way, 512 MB, 2 IDE 2 way, 1 GB, 10 SCSI (1 SCSI channel) 4 way, 4 GB, 20 SCSI (2 channels) 8 way, 8 GB, 40 SCSI (4 channels) maybe Fibre Channel (FC) 16 way, 16 GB, 80 FC (8 channels) The types of server applications that I have heard people express concern are: Web infrastructure: Apache(SPECWEB99???) TUX (SPECWEB99???) Jabber(have their own) Corporate infrastructure: NFS (SPECSFS2.0???) raw TCP/IP performance Samba (Have their own) email (SPECMAIL2001???) Database performance: Raw storage I/O performance (various) OLTP workload(something like TPC-C???) OLAP workload General usage: compile speed (usually measured by kernel compile) Further comments? I will start contacting folks who have expressed interest. -- Timothy D. Witham - Lab Director - [EMAIL PROTECTED] Open Source Development Lab Inc - A non-profit corporation 15275 SW Koll Parkway - Suite H - Beaverton OR, 97006 (503)-626-2455 x11 (office)(503)-702-2871 (cell) (503)-626-2455 (fax) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
RE: a quest for a better scheduler
Torrey, nothing to worry about (or at least not yet). The new scheduler only replaces the current SMP scheduler, not the single cpu scheduler. So the devices that you refer to are not affected at all by these changes. The desire for interactivity etc, lead us to stick to the current proposed MQ scheduler semantics, namely not to completely isolate the runqueues from each other (e.g. the HP-MQ does so), but do some cross checking to ensure that high priority tasks are run when they need to. First you get the same (similar good or bad scheduler semantics as the current one) but at a significantly lower cost for medium to high end loads (defined as #runnable tasks > #cpus) Here is a simple approximate arithmetic. Assume the following: - Let X be the fixed overhead to get through the scheduler (cur or MQ) once. - Let Y the overhead of touching a task to inspect its goodness - Let N be the number of tasks - Let C be the number of cpus. The cost for the current scheduler is: X(cur) + N*Y. The cost for the MQ scheduler is: X(MQ) + (N/C)*Y + C*Y I assumed here equal runqueue length. You can see that this ballpark math shows that if X(cur) ~ X(MQ) MQ is expected to be better when (N/C) + C < N or if ( N > C*C/(C-1) ) C=2 N>4 C=3 N>4 C=4 N>5 C=5 N>6 C=6 N>7 C=7 N>8 C=8 N>9 Turns out that for C>2 this amounts to N > C+1 MQ will do better. Now this is in the face of lack of runqueue lock contention. When runqueue lock contention surfaces, then this will shift in favor of MQ. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Torrey Hoffman <[EMAIL PROTECTED]>@vger.kernel.org on 04/05/2001 07:53:27 PM Sent by: [EMAIL PROTECTED] To: "'Timothy D. Witham'" <[EMAIL PROTECTED]>, Linux Kernel List <[EMAIL PROTECTED]> cc: Subject: RE: a quest for a better scheduler Timothy D. Witham wrote : [...] > I propose that we work on setting up a straight forward test harness > that allows developers to quickly test a kernel patch against > various performance yardsticks. [... (proposed large server testbeds) ...] I like this idea, but could the testbeds also include some resource-constrained "embedded" or appliance-style systems, and include performance yardsticks for latency and responsiveness? It would be unfortunate if work on a revised scheduler resulted in Linux being a monster web server on 4-way systems, but having lousy interactive performance on web pads, hand helds, and set top boxes. How about a 120Mhz Pentium with 32MB of RAM and a flash disk, a 200 Mhz PowerPC with 64 MB? Maybe a Transmeta web pad? For the process load, something that tests responsiveness and latency - how about a set of processes doing multicast network transfers, CPU-intensive MPEG video and audio encode / decode, and a test app that measures "user response", i.e. if X Windows was running, would the mouse pointer move smoothly or jerkily? The "better" scheduler for these applications would make sure that multicast packets were never dropped, the MPEG decode never dropped frames, and the "user interface" stayed responsive, etc. Also, I would not mind a bit if the kernel had tuning options, either in configuration or through /proc to adjust for these very different situations. Torrey Hoffman [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
--On Thursday, April 05, 2001 15:38:41 -0700 "Timothy D. Witham" <[EMAIL PROTECTED]> wrote: > Database performance: > Raw storage I/O performance >OLTP workload You probably want to add an OLAP scenario as well. --Chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
RE: a quest for a better scheduler
Timothy D. Witham wrote : [...] > I propose that we work on setting up a straight forward test harness > that allows developers to quickly test a kernel patch against > various performance yardsticks. [... (proposed large server testbeds) ...] I like this idea, but could the testbeds also include some resource-constrained "embedded" or appliance-style systems, and include performance yardsticks for latency and responsiveness? It would be unfortunate if work on a revised scheduler resulted in Linux being a monster web server on 4-way systems, but having lousy interactive performance on web pads, hand helds, and set top boxes. How about a 120Mhz Pentium with 32MB of RAM and a flash disk, a 200 Mhz PowerPC with 64 MB? Maybe a Transmeta web pad? For the process load, something that tests responsiveness and latency - how about a set of processes doing multicast network transfers, CPU-intensive MPEG video and audio encode / decode, and a test app that measures "user response", i.e. if X Windows was running, would the mouse pointer move smoothly or jerkily? The "better" scheduler for these applications would make sure that multicast packets were never dropped, the MPEG decode never dropped frames, and the "user interface" stayed responsive, etc. Also, I would not mind a bit if the kernel had tuning options, either in configuration or through /proc to adjust for these very different situations. Torrey Hoffman [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Exellent idea Assuming that you have set up these environments already, what would be a real treat is to get the average runqueue length at a given time, for instance every second or so, while running some of these more sophisticated server oriented applications that you mention. >From that we can see which of the various assumption regarding runqueue length is holding up, when the runqueue is not empty. This would help the current discussion trememdously as we don't seem to be able to even agree on this. Simple bincount could do. If you want a kernel patch that counts every scheduling cycle I'll write it. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 "Timothy D. Witham" <[EMAIL PROTECTED]>@vger.kernel.org on 04/05/2001 06:38:41 PM Sent by: [EMAIL PROTECTED] To: Linux Kernel List <[EMAIL PROTECTED]> cc: [EMAIL PROTECTED] Subject: Re: a quest for a better scheduler I have been following this thread and thinking that everybody has some truth in what they are saying but with the absence of a repeatable test environment there really isn't a way of arriving at a data driven decision. Given the following conditions. 1)The diversity of the problem sets that developers are working on results in a real concern that another developers solution to their performance issue might result in a worsening of my performance situation. 2)Most of the developers have a given set of tests that they use when they make changes to determine if they change did what they want. 3)The Open Source Development Lab has the faculties for providing a large scale testing environment on several different configurations that remain the same over time. I propose that we work on setting up a straight forward test harness that allows developers to quickly test a kernel patch against various performance yardsticks. If we use the same set of hardware for the generation of the performance data from patch to patch there will be a correlation between the runs allowing for a real comparison of the different changes. The following should be taken only as a starting point. As for the base hardware configurations I propose: 2 way with 1 GB main memory and 2 IDE drives 4 way with 4 GB main memory and 16 disk drives 8 way with 8 GB main memory and 24 disk drives The types of applications that I have heard people express concern are: Web infrastructure: Apache TUX Jabber Corporate infrastructure: NFS raw TCP/IP performance Samba Database performance: Raw storage I/O performance OLTP workload General usage: compile speed (usually measured by kernel compile) The OSDL also has the ability to make some of the "for fee" benchmarks available for use for testing that is done onsite (network access to OSDL machines counts as onsite) so that people don't have to purchase individual copies. Things like SECMAIL2001, SPECSFS2.0 and SEPCWEB99 come to mind. Comments, suggestions, volunteers? -- Timothy D. Witham - Lab Director - [EMAIL PROTECTED] Open Source Development Lab Inc - A non-profit corporation 15275 SW Koll Parkway - Suite H - Beaverton OR, 97006 (503)-626-2455 x11 (office)(503)-702-2871 (cell) (503)-626-2455 (fax) On Wed, Apr 04, 2001 at 03:54:54PM -0700, Christopher Smith wrote: > --On Wednesday, April 04, 2001 15:16:32 -0700 Tim Wright <[EMAIL PROTECTED]> > wrote: > > On Wed, Apr 04, 2001 at 03:23:34PM +0200, Ingo Molnar wrote: > >> nope. The goal is to satisfy runnable processes in the range of NR_CPUS. > >> You are playing word games by suggesting that the current behavior > >> prefers 'low end'. 'thousands of runnable processes' is not 'high end' > >> at all, it's 'broken end'. Thousands of runnable processes are the sign > >> of a broken application design, and 'fixing' the scheduler to perform > >> better in that case is just fixing the symptom. [changing the scheduler > >> to perform better in such situations is possible too, but all solutions > >> proposed so far had strings attached.] > > > > Ingo, you continue to assert this without giving much evidence to back it > > up. All the world is not a web server. If I'm running a large OLTP > > database with thousands of clients, it's not at all unreasonable to > > expect periods where several hundred (forget the thousands) want to be > > serviced by the database engine. That sounds like hundreds of schedulable > > entities be they processes or threads or whatever. This sort of load is > > regularly run on machi
Re: a quest for a better scheduler
I have been following this thread and thinking that everybody has some truth in what they are saying but with the absence of a repeatable test environment there really isn't a way of arriving at a data driven decision. Given the following conditions. 1)The diversity of the problem sets that developers are working on results in a real concern that another developers solution to their performance issue might result in a worsening of my performance situation. 2)Most of the developers have a given set of tests that they use when they make changes to determine if they change did what they want. 3)The Open Source Development Lab has the faculties for providing a large scale testing environment on several different configurations that remain the same over time. I propose that we work on setting up a straight forward test harness that allows developers to quickly test a kernel patch against various performance yardsticks. If we use the same set of hardware for the generation of the performance data from patch to patch there will be a correlation between the runs allowing for a real comparison of the different changes. The following should be taken only as a starting point. As for the base hardware configurations I propose: 2 way with 1 GB main memory and 2 IDE drives 4 way with 4 GB main memory and 16 disk drives 8 way with 8 GB main memory and 24 disk drives The types of applications that I have heard people express concern are: Web infrastructure: Apache TUX Jabber Corporate infrastructure: NFS raw TCP/IP performance Samba Database performance: Raw storage I/O performance OLTP workload General usage: compile speed (usually measured by kernel compile) The OSDL also has the ability to make some of the "for fee" benchmarks available for use for testing that is done onsite (network access to OSDL machines counts as onsite) so that people don't have to purchase individual copies. Things like SECMAIL2001, SPECSFS2.0 and SEPCWEB99 come to mind. Comments, suggestions, volunteers? -- Timothy D. Witham - Lab Director - [EMAIL PROTECTED] Open Source Development Lab Inc - A non-profit corporation 15275 SW Koll Parkway - Suite H - Beaverton OR, 97006 (503)-626-2455 x11 (office)(503)-702-2871 (cell) (503)-626-2455 (fax) On Wed, Apr 04, 2001 at 03:54:54PM -0700, Christopher Smith wrote: > --On Wednesday, April 04, 2001 15:16:32 -0700 Tim Wright <[EMAIL PROTECTED]> > wrote: > > On Wed, Apr 04, 2001 at 03:23:34PM +0200, Ingo Molnar wrote: > >> nope. The goal is to satisfy runnable processes in the range of NR_CPUS. > >> You are playing word games by suggesting that the current behavior > >> prefers 'low end'. 'thousands of runnable processes' is not 'high end' > >> at all, it's 'broken end'. Thousands of runnable processes are the sign > >> of a broken application design, and 'fixing' the scheduler to perform > >> better in that case is just fixing the symptom. [changing the scheduler > >> to perform better in such situations is possible too, but all solutions > >> proposed so far had strings attached.] > > > > Ingo, you continue to assert this without giving much evidence to back it > > up. All the world is not a web server. If I'm running a large OLTP > > database with thousands of clients, it's not at all unreasonable to > > expect periods where several hundred (forget the thousands) want to be > > serviced by the database engine. That sounds like hundreds of schedulable > > entities be they processes or threads or whatever. This sort of load is > > regularly run on machine with 16-64 CPUs. > > Actually, it's not just OLTP, anytime you are doing time sharing between > hundreds of users (something POSIX systems are supposed to be good at) this > will happen. > > > Now I will admit that it is conceivable that you can design an > > application that finds out how many CPUs are available, creates threads > > to match that number and tries to divvy up the work between them using > > some combination of polling and asynchronous I/O etc. There are, however > > a number of problems with this approach: > > Actually, one way to semi-support this approach is to implement > many-to-many threads as per the Solaris approach. This also requires > significant hacking of both the kernel and the runtime, and certainly is > significantly more error prone than trying to write a flexible scheduler. > > One problem you didn't highlight that even the above case does not happily > identify is that for security reasons you may very well need each user's > requests to take place in a different process. If you don't, then you have > to implement a very well tested and secure user-level security mechanism to > ensure things like privacy (above and beyond the time-sharing).
Re: [Lse-tech] Re: a quest for a better scheduler
This concept I think is used in Solaris .. as they have dynamic loadable schedulers.. Zdenek Kabelac <[EMAIL PROTECTED]> on 04/05/2001 05:43:15 PM To: Andrea Arcangeli <[EMAIL PROTECTED]> cc:(bcc: Amol Lad/HSS) Subject: Re: [Lse-tech] Re: a quest for a better scheduler Hello Just dump idea - why not make scheduler switchable with modules - so users could select any scheduler they want ? This should not be that hard and would make it easy to replace scheduler at runtime so everyone could easily try what's the best for him/her. [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
Hello Just dump idea - why not make scheduler switchable with modules - so users could select any scheduler they want ? This should not be that hard and would make it easy to replace scheduler at runtime so everyone could easily try what's the best for him/her. [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
--On Wednesday, April 04, 2001 15:16:32 -0700 Tim Wright <[EMAIL PROTECTED]> wrote: > On Wed, Apr 04, 2001 at 03:23:34PM +0200, Ingo Molnar wrote: >> nope. The goal is to satisfy runnable processes in the range of NR_CPUS. >> You are playing word games by suggesting that the current behavior >> prefers 'low end'. 'thousands of runnable processes' is not 'high end' >> at all, it's 'broken end'. Thousands of runnable processes are the sign >> of a broken application design, and 'fixing' the scheduler to perform >> better in that case is just fixing the symptom. [changing the scheduler >> to perform better in such situations is possible too, but all solutions >> proposed so far had strings attached.] > > Ingo, you continue to assert this without giving much evidence to back it > up. All the world is not a web server. If I'm running a large OLTP > database with thousands of clients, it's not at all unreasonable to > expect periods where several hundred (forget the thousands) want to be > serviced by the database engine. That sounds like hundreds of schedulable > entities be they processes or threads or whatever. This sort of load is > regularly run on machine with 16-64 CPUs. Actually, it's not just OLTP, anytime you are doing time sharing between hundreds of users (something POSIX systems are supposed to be good at) this will happen. > Now I will admit that it is conceivable that you can design an > application that finds out how many CPUs are available, creates threads > to match that number and tries to divvy up the work between them using > some combination of polling and asynchronous I/O etc. There are, however > a number of problems with this approach: Actually, one way to semi-support this approach is to implement many-to-many threads as per the Solaris approach. This also requires significant hacking of both the kernel and the runtime, and certainly is significantly more error prone than trying to write a flexible scheduler. One problem you didn't highlight that even the above case does not happily identify is that for security reasons you may very well need each user's requests to take place in a different process. If you don't, then you have to implement a very well tested and secure user-level security mechanism to ensure things like privacy (above and beyond the time-sharing). The world is filled with a wide variety of types of applications, and unless you know two programming approaches are functionaly equivalent (and event driven/polling I/O vs. tons of running processes are NOT), you shouldn't say one approach is "broken". You could say it's a "broken" approach to building web servers. Unfortunately, things like kernels and standard libraries should work well in the general case. --Chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 03:23:34PM +0200, Ingo Molnar wrote: > > On Wed, 4 Apr 2001, Hubertus Franke wrote: > > > I understand the dilemma that the Linux scheduler is in, namely > > satisfy the low end at all cost. [...] > > nope. The goal is to satisfy runnable processes in the range of NR_CPUS. > You are playing word games by suggesting that the current behavior prefers > 'low end'. 'thousands of runnable processes' is not 'high end' at all, > it's 'broken end'. Thousands of runnable processes are the sign of a > broken application design, and 'fixing' the scheduler to perform better in > that case is just fixing the symptom. [changing the scheduler to perform > better in such situations is possible too, but all solutions proposed so > far had strings attached.] > Ingo, you continue to assert this without giving much evidence to back it up. All the world is not a web server. If I'm running a large OLTP database with thousands of clients, it's not at all unreasonable to expect periods where several hundred (forget the thousands) want to be serviced by the database engine. That sounds like hundreds of schedulable entities be they processes or threads or whatever. This sort of load is regularly run on machine with 16-64 CPUs. Now I will admit that it is conceivable that you can design an application that finds out how many CPUs are available, creates threads to match that number and tries to divvy up the work between them using some combination of polling and asynchronous I/O etc. There are, however a number of problems with this approach: 1) It assumes that this is the only workload on the machine. If not it quickly becomes sub-optimal due to interactions between the workloads. This is a problem that the kernel scheduler does not suffer from. 2) It requires *every* application designer to design an effective scheduler into their application. I would submit that an effective scheduler is better situated in the operating system. 3) It is not a familiar programming paradigm to many Unix/Linux/POSIX programmers, so you have a sort of impedance mismatch going on. Since the proposed scheduler changes being talked about here have been shown to not hurt the "low end" and to dramatically improve the "high end", I fail to understand the hostility to the changes. I can understand that you do not feel that this is the correct way to architect an application, but if the changes don't hurt you, why sabotage changes that also allow a different method to work. There isn't one true way to do anything in computing. Tim -- Tim Wright - [EMAIL PROTECTED] or [EMAIL PROTECTED] or [EMAIL PROTECTED] IBM Linux Technology Center, Beaverton, Oregon Interested in Linux scalability ? Look at http://lse.sourceforge.net/ "Nobody ever said I was charming, they said "Rimmer, you're a git!"" RD VI - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
I give you a concrete example: Running DB2 on an SMP system. In DB2 there is a processes/thread pool that is sized based on memory and numcpus. People tell me that the size of this pool is in the order of 100s for an 8-way system with reasonable sized database. These determine the number of agents that can simultaneously execute an SQL statement. Requests are flying in for transactions (e.g. driven by TPC-W like applications). The agents are grepped from the pool and concurrently fire the SQL transactions. Assuming that there is enough concurrency in the database, there is no reason to believe that the majority of those active agents is not effectively running. TPC-W loads have observed 100 of active transactions at a time. Ofcourse limiting the number of agents would reduce concurrently running tasks, but would limit the responsiveness of the system. Implementing a database in the kernel ala TUX doesn't seem to be the right approach either (complexity, fault containment, ...) Hope that is one example people accept. I can dig up some information on WebSphere Applications. I'd love to hear from some other applications that fall into a similar category as the above and substantiate a bit the need for 100s of running processes, without claiming that the application is broke. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Mark Hahn <[EMAIL PROTECTED]> on 04/04/2001 02:28:42 PM To: Hubertus Franke/Watson/IBM@IBMUS cc: Subject: Re: a quest for a better scheduler > ok if the runqueue length is limited to a very small multiple of the #cpus. > But that is not what high end server systems encounter. do you have some example of this in mind? so far, noone has actually produced an example of a "high end" server that has long runqueues. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 10:49:04AM -0700, Kanoj Sarcar wrote: > Imagine that most of the program's memory is on node 1, it was scheduled > on node 2 cpu 8 momentarily (maybe because kswapd ran on node 1, other > higher priority processes took over other cpus on node 1, etc). > > Then, your patch will try to keep the process on node 2, which is not > neccessarily the best solution. Of course, as I mentioned before, if > you have a node local cache on node 2, that cache might have been warmed > enough to make scheduling on node 2 a good option. Exactly it made it a good option, and more important this heuristic can only improve performance if compared to the mainline scheduler. Infact I tried to reschedule the task back to its original node and it dropped performance, anyways I cannot say to have done an extensive research on that, I believe if we take care of some more history of the node migration we may understand it's right time to push it back to its original node but that was not obvious and I wanted a simple solution to boost the performance under CPU bound load to start with. Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
> > It helps by keeping the task in the same node if it cannot keep it in > the same cpu anymore. > > Assume task A is sleeping and it last run on cpu 8 node 2. It gets a wakeup > and it gets running and for some reason cpu 8 is busy and there are other > cpus idle in the system. Now with the current scheduler it can be moved in any > cpu in the system, with the numa sched applied we will try to first reschedule > it in the idles cpus of node 2 for example. The per-node runqueue are mainly > necessary to implement the heuristic. > Yes. But this is not the best solution, if I can add on to the example and make some assumptions. Imagine that most of the program's memory is on node 1, it was scheduled on node 2 cpu 8 momentarily (maybe because kswapd ran on node 1, other higher priority processes took over other cpus on node 1, etc). Then, your patch will try to keep the process on node 2, which is not neccessarily the best solution. Of course, as I mentioned before, if you have a node local cache on node 2, that cache might have been warmed enough to make scheduling on node 2 a good option. I am not saying there is a wrong or right answer, there are so many possibilities, everything probably works and breaks under different circumstances. Btw, while we are swapping patches, the patch at http://oss.sgi.com/projects/numa/download/sched242.patch tries to implement per-arch scheduling. The current scheduler behavior is smp_arch_goodness() and smp_pick_cpu(), but the patch allows the possibility for a specific platform to change that to something else. Linus has seen this patch, and agrees to it in principle. He does not consider this 2.4 material though. Of course, I am completely open to Ingo (or someone else) coming up with a different way of providing the same freedom to arch specific code. Kanoj - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
> Just a quick comment. Andrea, unless your machine has some hardware > that imply pernode runqueues will help (nodelevel caches etc), I fail > to understand how this is helping you ... here's a simple theory though. > If your system is lightly loaded, your pernode queues are actually > implementing some sort of affinity, making sure processes stick to > cpus on nodes where they have allocated most of their memory on. I am > not sure what the situation will be under huge loads though. Exactly. If a given task has run on a particular nodes for a while, its memory will tend to be allocated on that node. So preferentially running it on another CPU on that same node should get you better memory latency than would running it on some other node's CPUs. In addition, continuing to run the task on a particular node means that more of that task's memory is from that node, which again means good memory latency. In contrast, if you move a task back and forth between nodes, it can end up with its memory spread over many nodes, which means that it does not get good memory latency no matter where you run it. Thanx, Paul > As I have mentioned to some people before, percpu/pernode/percpuset/global > runqueues probably all have their advantages and disadvantages, and their > own sweet spots. Wouldn't it be really neat if a system administrator > or performance expert could pick and choose what scheduler behavior he > wants, based on how the system is going to be used? > > Kanoj - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
Correct, that's true. Our patch does various things. (a) limit search for a task to a admin specified set of cpu's during schedule().. (b) limits search for a preemptable task to another set of cpu's during reschedule_idle() (c) loadbalancing, i.e. moving from queue to queue. Currently we balance within a set and across sets. Obviously in NUMA one could specify (a) such that multiple sets fall into the same node no node crossings. (b) specify this set to at least span a node (c) do some intelligent moving based on memory maps etc. I guess (c) would be first instance on where to plug architecture dependent information, e.g. how much memory footprint does a task have on a particular node and how much would the moving cost. The loadbalance we provide is a simple sceleton to tickle you mind, not a solution. Nevertheless, one can see it can have some impact. See for results for various combinations of poolsizes and balancings: http://lse.sourceforge.net/scheduling/results012501/status.html#Load%20Balancing Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Kanoj Sarcar <[EMAIL PROTECTED]> on 04/04/2001 01:14:28 PM To: Hubertus Franke/Watson/IBM@IBMUS cc: [EMAIL PROTECTED] (Linux Kernel List), [EMAIL PROTECTED] Subject: Re: [Lse-tech] Re: a quest for a better scheduler > > > > Kanoj, our cpu-pooling + loadbalancing allows you to do that. > The system adminstrator can specify at runtime through a > /proc filesystem interface the cpu-pool-size, whether loadbalacing > should take place. Yes, I think this approach can support the various requirements put on the scheduler. I think there are two degrees of freedom that are needed in the scheduler. One, as you say, for the sysadmin to be able to specify what overall scheduler behavior he wants. Secondly, from the kernel standpoint, there needs to be perarch hooks, to be able to utilize nodelevel/multilevel caches, NUMA aspects etc. Kanoj - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, Apr 03, 2001 at 09:21:57PM -0700, Fabio Riccardi wrote: > I was actually suspecting that the extra lines in your patch were there for a > reason :) > > A few questions: > > What is the real impact of a (slight) change in scheduling semantics? > > Under which situation one should notice a difference? I assume your questions are directed at the difference between local and global scheduling decisions. As with most things the impact of these differences depends on workload. Most multi-queue scheduler implementations make local scheduling decisions and depend on load balancing for system wide fairness. Schedulers which make global decisions handle load balancing via their global policy. The HP multi-queue implementation you are using performs no load balancing. Therefore, in a worst case situation you could have low priority tasks sharing one CPU while high priority tasks are sharing another. To be fair, I have talked to the HP people and this scheduler was never intended to be a general purpose solution. Therefore, it makes little sense to comment its merits as such. > As you state in your papers the global decision comes with a cost, > is it worth it? My primary motivation for attempting to perform the same global decisions as the current scheduler was so that meaningful comparisons could be made. Also, by using the same global decision policy I was able to avoid the issue of load balancing. In most multi-queue implementations, load balancing algorithms take considerable effort to get working in a reasonable well performing manner. > > Could you make a port of your thing on recent kernels? There is a 2.4.2 patch on the web page. I'll put out a 2.4.3 patch as soon as I get some time. -- Mike Kravetz [EMAIL PROTECTED] IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Well put, this how we can eliminate searching all bins or lists and that's how we do it under. http://lse.sourceforge.net/scheduling/2.4.1-pre8-prioSched. If you have a list per priority level, then you can even pick the first one you find if its on the same level. That's what I tried in a more recent implementation. Also combined that with using a bitmask to represent non-empty tasks. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Davide Libenzi <[EMAIL PROTECTED]>@ewok.dev.mycio.com on 04/04/2001 12:12:54 PM Sent by: [EMAIL PROTECTED] To: Ingo Molnar <[EMAIL PROTECTED]> cc: Linus Torvalds <[EMAIL PROTECTED]>, Alan Cox <[EMAIL PROTECTED]>, Linux Kernel List <[EMAIL PROTECTED]>, Hubertus Franke/Watson/IBM@IBMUS, Mike Kravetz <[EMAIL PROTECTED]>, Fabio Riccardi <[EMAIL PROTECTED]> Subject: Re: a quest for a better scheduler On 04-Apr-2001 Ingo Molnar wrote: > > On Tue, 3 Apr 2001, Fabio Riccardi wrote: > >> I've spent my afternoon running some benchmarks to see if MQ patches >> would degrade performance in the "normal case". > > no doubt priority-queue can run almost as fast as the current scheduler. > What i'm worried about is the restriction of the 'priority' of processes, > it cannot depend on previous processes (and other 'current state') > anymore. > > to so we have two separate issues: > >#1: priority-queue: has the fundamental goodness() design limitation. > >#2: per-CPU-runqueues: changes semantics, makes scheduler less > effective due to nonglobal decisions. > > about #1: while right now the prev->mm rule appears to be a tiny issue (it > might not affect performance significantly), but forbidding it by > hardcoding the assumption into data structures is a limitation of *future* > goodness() functions. Eg. with the possible emergence of CPU-level > threading and other, new multiprocessing technologies, this could be a > *big* mistake. This is not correct Ingo. I haven't seen the HP code but if You store processes in slots S : S = FS( goodness(p, p->processor, p->mm) ) and You start scanning from the higher slots, as soon as you find a task with a goodness G' that is equal to the max goodness in slot You can choose that process to run. Again, if You haven't found such a goodness during the slot scan but You've found a task with a goodness G' : G' >= SG - DD where : SG = max slot goodness DD = SG(i) - SG(i - 1) You can select that task as the next to spin. This was the behaviour that was implemented in my scheduler patch about 2 years ago. Beside this, I this that with such loads We've more serious problem to face with inside the kernel. - Davide - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 09:50:58AM -0700, Kanoj Sarcar wrote: > > > > I didn't seen anything from Kanoj but I did something myself for the wildfire: > > > > >ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1 > > > > this is mostly an userspace issue, not really intended as a kernel optimization > > (however it's also partly a kernel optimization). Basically it splits the load > > of the numa machine into per-node load, there can be unbalanced load across the > > nodes but fairness is guaranteed inside each node. It's not extremely well > > tested but benchmarks were ok and it is at least certainly stable. > > > > Just a quick comment. Andrea, unless your machine has some hardware > that imply pernode runqueues will help (nodelevel caches etc), I fail > to understand how this is helping you ... here's a simple theory though. It helps by keeping the task in the same node if it cannot keep it in the same cpu anymore. Assume task A is sleeping and it last run on cpu 8 node 2. It gets a wakeup and it gets running and for some reason cpu 8 is busy and there are other cpus idle in the system. Now with the current scheduler it can be moved in any cpu in the system, with the numa sched applied we will try to first reschedule it in the idles cpus of node 2 for example. The per-node runqueue are mainly necessary to implement the heuristic. > cpus on nodes where they have allocated most of their memory on. I am > not sure what the situation will be under huge loads though. after all cpus are busy we try to reschedule only on the cpus of the local node, that's why it can generate some unbalance yes, but it will tend to rebalance over the time because some node will end with all tasks with zero counter first if it's less loaded, and so then it will start getting tasks with has_cpu 0 in the runqueues out of other nodes. You may want to give it a try on your machines and see what difference it makes, I'd be curious to know of course. Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
> > > > Kanoj, our cpu-pooling + loadbalancing allows you to do that. > The system adminstrator can specify at runtime through a > /proc filesystem interface the cpu-pool-size, whether loadbalacing > should take place. Yes, I think this approach can support the various requirements put on the scheduler. I think there are two degrees of freedom that are needed in the scheduler. One, as you say, for the sysadmin to be able to specify what overall scheduler behavior he wants. Secondly, from the kernel standpoint, there needs to be perarch hooks, to be able to utilize nodelevel/multilevel caches, NUMA aspects etc. Kanoj - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
Kanoj, our cpu-pooling + loadbalancing allows you to do that. The system adminstrator can specify at runtime through a /proc filesystem interface the cpu-pool-size, whether loadbalacing should take place. We can put limiting to the local cpu-set during reschedule_idle back into the code, to make it complete and compatible with the approach that Andrea has taken. This way, one can fully isolate or combine cpu-sets. here is the code for the pooling. http://lse.sourceforge.net/scheduling/LB/2.4.1-MQpool loadbalancing and /proc system combined in this module. http://lse.sourceforge.net/scheduling/LB/loadbalance.c a writeup explaining this concept is available under http://lse.sourceforge.net/scheduling/LB/poolMQ.html Prerequisite is the MQ scheduler... http://lse.sourceforge.net/scheduling/2.4.1.mq1-sched We need to update these for 2.4.3 (coming) Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Kanoj Sarcar <[EMAIL PROTECTED]>@lists.sourceforge.net on 04/04/2001 12:50:58 PM Sent by: [EMAIL PROTECTED] To: [EMAIL PROTECTED] (Andrea Arcangeli) cc: [EMAIL PROTECTED] (Ingo Molnar), Hubertus Franke/Watson/IBM@IBMUS, [EMAIL PROTECTED] (Mike Kravetz), [EMAIL PROTECTED] (Fabio Riccardi), [EMAIL PROTECTED] (Linux Kernel List), [EMAIL PROTECTED] Subject: Re: [Lse-tech] Re: a quest for a better scheduler > > I didn't seen anything from Kanoj but I did something myself for the wildfire: > > ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1 > > this is mostly an userspace issue, not really intended as a kernel optimization > (however it's also partly a kernel optimization). Basically it splits the load > of the numa machine into per-node load, there can be unbalanced load across the > nodes but fairness is guaranteed inside each node. It's not extremely well > tested but benchmarks were ok and it is at least certainly stable. > Just a quick comment. Andrea, unless your machine has some hardware that imply pernode runqueues will help (nodelevel caches etc), I fail to understand how this is helping you ... here's a simple theory though. If your system is lightly loaded, your pernode queues are actually implementing some sort of affinity, making sure processes stick to cpus on nodes where they have allocated most of their memory on. I am not sure what the situation will be under huge loads though. As I have mentioned to some people before, percpu/pernode/percpuset/global runqueues probably all have their advantages and disadvantages, and their own sweet spots. Wouldn't it be really neat if a system administrator or performance expert could pick and choose what scheduler behavior he wants, based on how the system is going to be used? Kanoj ___ Lse-tech mailing list [EMAIL PROTECTED] http://lists.sourceforge.net/lists/listinfo/lse-tech - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 09:39:23AM -0700, Kanoj Sarcar wrote: > example, for NUMA, we need to try hard to schedule a thread on the > node that has most of its memory (for no reason other than to decrease > memory latency). Independently, some NUMA machines build in multilevel > caches and local snoops that also means that specific processors on > the same node as the last_processor are also good candidates to run > the process next. yes. That will probably need to be optional and choosen by the architecture at compile time too. The probably most important factor to consider is the penality of accessing remote memory, I think I can say on all recent and future machines with a small difference between local and remote memory (and possibly as you say with a decent cache protocol able to snoop cacheline data from the other cpus even if they're not dirty) it's much better to always try to keep the task in its last node. My patch is actually assuming recent machines and it keeps the task in its last node if not in the last cpu and it keeps doing memory allocation from there and it forgets about its original node where it started allocating the memory from. This provided the best performance during userspace CPU bound load as far I can tell and it also better distribute the load. Kanoj could you also have a look at the NUMA related common code MM fixes I did in this patch? I'd like to get them integrated (just skip the arch/alpha/* include/asm-alpha/* stuff while reading the patch, they're totally orthogonal). ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/00_alpha-numa-1 If you prefer I can extract them in a more finegrinded patch just dropping the alpha stuff by hand. Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
> > I didn't seen anything from Kanoj but I did something myself for the wildfire: > > >ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1 > > this is mostly an userspace issue, not really intended as a kernel optimization > (however it's also partly a kernel optimization). Basically it splits the load > of the numa machine into per-node load, there can be unbalanced load across the > nodes but fairness is guaranteed inside each node. It's not extremely well > tested but benchmarks were ok and it is at least certainly stable. > Just a quick comment. Andrea, unless your machine has some hardware that imply pernode runqueues will help (nodelevel caches etc), I fail to understand how this is helping you ... here's a simple theory though. If your system is lightly loaded, your pernode queues are actually implementing some sort of affinity, making sure processes stick to cpus on nodes where they have allocated most of their memory on. I am not sure what the situation will be under huge loads though. As I have mentioned to some people before, percpu/pernode/percpuset/global runqueues probably all have their advantages and disadvantages, and their own sweet spots. Wouldn't it be really neat if a system administrator or performance expert could pick and choose what scheduler behavior he wants, based on how the system is going to be used? Kanoj - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
> > > On Wed, 4 Apr 2001, Hubertus Franke wrote: > > > Another point to raise is that the current scheduler does a exhaustive > > search for the "best" task to run. It touches every process in the > > runqueue. this is ok if the runqueue length is limited to a very small > > multiple of the #cpus. [...] > > indeed. The current scheduler handles UP and SMP systems, up to 32 > (perhaps 64) CPUs efficiently. Agressively NUMA systems need a different > approach anyway in many other subsystems too, Kanoj is doing some > scheduler work in that area. Actually, not _much_ work has been done in this area. Alongwith a bunch of other people, I have some ideas about what needs to be done. For example, for NUMA, we need to try hard to schedule a thread on the node that has most of its memory (for no reason other than to decrease memory latency). Independently, some NUMA machines build in multilevel caches and local snoops that also means that specific processors on the same node as the last_processor are also good candidates to run the process next. To handle a single layer of shared caches, I have tried certain simple things, mostly as hacks, but am not pleased with the results yet. More testing needed. Kanoj > > but the original claim was that the scheduling of thousands of runnable > processes (which is not equal to having thousands of sleeping processes) > must perform well - which is a completely different issue. > > Ingo > > > ___ > Lse-tech mailing list > [EMAIL PROTECTED] > http://lists.sourceforge.net/lists/listinfo/lse-tech > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On 04-Apr-2001 Ingo Molnar wrote: > > On Tue, 3 Apr 2001, Fabio Riccardi wrote: > >> I've spent my afternoon running some benchmarks to see if MQ patches >> would degrade performance in the "normal case". > > no doubt priority-queue can run almost as fast as the current scheduler. > What i'm worried about is the restriction of the 'priority' of processes, > it cannot depend on previous processes (and other 'current state') > anymore. > > to so we have two separate issues: > >#1: priority-queue: has the fundamental goodness() design limitation. > >#2: per-CPU-runqueues: changes semantics, makes scheduler less > effective due to nonglobal decisions. > > about #1: while right now the prev->mm rule appears to be a tiny issue (it > might not affect performance significantly), but forbidding it by > hardcoding the assumption into data structures is a limitation of *future* > goodness() functions. Eg. with the possible emergence of CPU-level > threading and other, new multiprocessing technologies, this could be a > *big* mistake. This is not correct Ingo. I haven't seen the HP code but if You store processes in slots S : S = FS( goodness(p, p->processor, p->mm) ) and You start scanning from the higher slots, as soon as you find a task with a goodness G' that is equal to the max goodness in slot You can choose that process to run. Again, if You haven't found such a goodness during the slot scan but You've found a task with a goodness G' : G' >= SG - DD where : SG = max slot goodness DD = SG(i) - SG(i - 1) You can select that task as the next to spin. This was the behaviour that was implemented in my scheduler patch about 2 years ago. Beside this, I this that with such loads We've more serious problem to face with inside the kernel. - Davide - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [Lse-tech] Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 09:44:22AM -0600, Khalid Aziz wrote: > Let me stress that HP scheduler is not meant to be a replacement for the > current scheduler. The HP scheduler patch allows the current scheduler > to be replaced by another scheduler by loading a module in special > cases. HP also has a simple mq patch that is _not_ integrated into the pluggable scheduler framework, I have used it myself. Christoph -- Of course it doesn't work. We've performed a software upgrade. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Andrea Arcangeli wrote: > > On Wed, Apr 04, 2001 at 10:03:10AM -0400, Hubertus Franke wrote: > > I understand the dilemma that the Linux scheduler is in, namely satisfy > > the low end at all cost. [..] > > We can satisfy the low end by making the numa scheduler at compile time (that's > what I did in my patch at least). > > Andrea I fully agree with this approach. It would be very hard to design a scheduler that performs equally well on a UP machine running couple of processes and a NUMA machine. These two cases represent the two ends of spectrum. The two schedulers should be separate IMO and one of them should be selected at compile time. -- Khalid Khalid Aziz Linux Development Laboratory (970)898-9214Hewlett-Packard [EMAIL PROTECTED]Fort Collins, CO - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Hubertus Franke wrote: > > This is an important point that Mike is raising and it also addresses a > critique that Ingo issued yesterday, namely interactivity and fairness. > The HP scheduler completely separates the per-CPU runqueues and does > not take preemption goodness or alike into account. This can lead to > unfair proportionment of CPU cycles, strong priority inversion and a > potential > lack of interactivity. > > Our MQ scheduler does yield the same decision in most cases > (other than defined by some race condition on locks and counter members) > Let me stress that HP scheduler is not meant to be a replacement for the current scheduler. The HP scheduler patch allows the current scheduler to be replaced by another scheduler by loading a module in special cases. HP is providing three different loadable scheduler modules - Processor sets, Constant time scheduler, and Multi-runqueue scheduler. Each one of these is geared towards a specific requirement. I would not suggest using any of these for a generalized case. Processor sets scheduler is designed to make scheduling decisions on a per-cpu basis and not global basis. All we are trying to do is to make the current scheduler modular so we CAN load an alternate scheduling policy module in cases where the process mix requires a different scheduling policy or the site policy require a different scheduling policy. An example of a specific site processor allocation policy could be a site that runs a database server on an MP machine along with a few other processes and the administrator wants to guarantee that the database server process always gets x percent of processing time or one CPU be dedicated to just the database server. A policy like this is not meant to be fair and of course, not a policy we want to impose upon others. The only HP changes I would put in the kernel sources for general release would be the changes to scheduler to allow such alternate (not necessarily fair or the fastest for benchmarks, general process mix or 1000's of processes) policies to be loaded. When a policy module is not loaded, scheduler works exactly like the current scheduler even after HP patches. There are people who could benefit from being able to load alternate policy schedules. Fabio Ricardi happens to be one of them :-) Anyone who does not want to even allow an alternate scheduler module to be loaded can simply compile the alternate scheduler support out and that is how I would expect most kernels to be compiled, especially the ones that ship with various distributions. I would like the decision to include support for alternate scheduler to be made by sys admins themselves for their individual cases. -- Khalid Khalid Aziz Linux Development Laboratory (970)898-9214Hewlett-Packard [EMAIL PROTECTED]Fort Collins, CO - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
You imply that high end means thousands of processes, simply because we have shown that in our graphs as an asymptotic end. No, it could mean 5*#cpus and that is not all that absurd. This could happen with a spike in demand. TUX is not the greatest example to use, because it does static webpage serving and is hence tied into the file cache. If you move up the food chain, where middleware is active and things are a bit more complicated than sending stuff out of the filecache, having a bunch of threads hanging around to deal with the spike in demand is common practive, although you think its bad. Now coming again to MQ (forget about priority list from now on). When we scan either the local or the realtime queues we do use goodness value. So we have the same flexibility as the current scheduler if it comes to goodness() flexibility and future improvements. For remote stealing we do use na_goodness to compare for a better process to steal. Hence we would loose the "+1" information here, nevertheless, once a decision has been made, we still use preemption verification with goodness. Eitherway, being off by "+1" for regular tasks once in a while is no big deal, because this problem already exists today. While walking the runqueue, another processor can either update the counter value of task (ok that's covered by can_schedule) or can run recalculate, which makes the decision that one is about to make wrong from the point of always running the best. But that's ok, because counter, nice etc. are approximations anyway. Through in PROC_CHANGE_PENALTY and you have a few knobs that are used to control interactivity and throughput. If you look at some of the results with our reflex benchmark. For the low thread count we basically show improved performance on the 2,4,5,6,7,8 way system if #threads < #cpus, they all show improvements. Look at the following numbers running the reflex benchmark, left most columns are number of threads, with typically 1/2 of them runnable. You can clearly see that the priority list suffers from overhead, but MQ is beating vanilla pretty much everywhere. Again, this is due because of rapid scheduler invocation and resulting lock contention. 2-way 2.4.1 2.4.1-mq1 2.4.1-prbit 46.725 4.691 7.387 86.326 4.766 6.421 12 6.838 5.233 6.431 16 7.13 5.415 7.29 4-way 2.4.1 2.4.1-mq1 2.4.1-prbit 49.42 7.873 10.592 88.143 3.799 8.691 12 7.877 3.537 8.101 16 7.688 2.953 7.144 6-way 2.4.1 2.4.1-mq1 2.4.1-prbit 49.595 7.88 10.358 89.703 7.278 10.523 12 10.0164.652 10.985 16 9.882 3.629 10.525 8-way 2.4.1 2.4.1-mq1 2.4.1-prbit 49.804 8.033 10.548 810.4365.783 11.475 12 10.9256.787 11.646 16 11.4265.048 11.877 20 11.4383.895 11.633 24 11.4573.304 11.347 28 11.4953.073 11.09 32 11.53 2.944 10.898 Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Ingo Molnar <[EMAIL PROTECTED]>@elte.hu> on 04/04/2001 09:23:34 AM Please respond to <[EMAIL PROTECTED]> Sent by: <[EMAIL PROTECTED]> To: Hubertus Franke/Watson/IBM@IBMUS cc: Mike Kravetz <[EMAIL PROTECTED]>, Fabio Riccardi <[EMAIL PROTECTED]>, Linux Kernel List <[EMAIL PROTECTED]> Subject: Re: a quest for a better scheduler On Wed, 4 Apr 2001, Hubertus Franke wrote: > I understand the dilemma that the Linux scheduler is in, namely > satisfy the low end at all cost. [...] nope. The goal is to satisfy runnable processes in the range of NR_CPUS. You are playing word games by suggesting that the current behavior prefers 'low end'. 'thousands of runnable processes' is not 'high end' at all, it's 'broken end'. Thousands of runnable processes are the sign of a broken application design, and 'fixing' the scheduler to perform better in that case is just fixing the symptom. [changing the scheduler to perform better in such situations is possible too, but all solutions proposed so far had strings attached.] Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Yes, Andrea. We actually already went a step further. We treat the scheduler as a single entity, rather than splitting it up. Based on the MQ scheduler we do the balancing across all nodes at reschedule_idle time. We experimented to see whether only looking for idle tasks remotely is a good idea, but it bloats the code. Local scheduling decisions are limited to a set of cpus, which could coincide with the cpus of one node, or if desirable on smaller granularities. In addition we implemented a global load balancing scheme that ensures that load is equally distributed across all run queues. This is made a loadable module, so you can plug in what ever you want. I grant in NUMA it might actually be desirable to separate schedulers completely (we can do that trivially in reschedule_idle), but you need loadbalancing at some point. Here is the list of patches: MultiQueue Scheduler: http://lse.sourceforge.net/scheduling/2.4.1.mq1-sched Pooling Extension:http://lse.sourceforge.net/scheduling/LB/2.4.1-MQpool LoadBalancing: http://lse.sourceforge.net/scheduling/LB/loadbalance.c Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) , OS-PIC (Chair) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Andrea Arcangeli <[EMAIL PROTECTED]> on 04/04/2001 11:08:47 AM To: Ingo Molnar <[EMAIL PROTECTED]> cc: Hubertus Franke/Watson/IBM@IBMUS, Mike Kravetz <[EMAIL PROTECTED]>, Fabio Riccardi <[EMAIL PROTECTED]>, Linux Kernel List <[EMAIL PROTECTED]>, [EMAIL PROTECTED] Subject: Re: a quest for a better scheduler On Wed, Apr 04, 2001 at 03:34:22PM +0200, Ingo Molnar wrote: > > On Wed, 4 Apr 2001, Hubertus Franke wrote: > > > Another point to raise is that the current scheduler does a exhaustive > > search for the "best" task to run. It touches every process in the > > runqueue. this is ok if the runqueue length is limited to a very small > > multiple of the #cpus. [...] > > indeed. The current scheduler handles UP and SMP systems, up to 32 > (perhaps 64) CPUs efficiently. Agressively NUMA systems need a different > approach anyway in many other subsystems too, Kanoj is doing some > scheduler work in that area. I didn't seen anything from Kanoj but I did something myself for the wildfire: ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1 this is mostly an userspace issue, not really intended as a kernel optimization (however it's also partly a kernel optimization). Basically it splits the load of the numa machine into per-node load, there can be unbalanced load across the nodes but fairness is guaranteed inside each node. It's not extremely well tested but benchmarks were ok and it is at least certainly stable. However Ingo consider that in a 32-way if you don't have at least 32 tasks running all the time _always_ you're really stupid paying such big money for nothing ;). So the fact the scheduler is optimized for 1/2 tasks running all the time is not nearly enough for those machines (and of course also the scheduling rate automatically increases linearly with the increase of the number of cpus). Now it's perfectly fine that we don't ask the embedded and desktop guys to pay for that, but a kernel configuration option to select an algorithm that scales would be a good idea IMHO. The above patch just adds a CONFIG_NUMA_SCHED. The scalable algorithm can fit into it and nobody will be hurted by that (CONFIG_NUMA_SCHED cannot even be selected by x86 compiles). Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 10:03:10AM -0400, Hubertus Franke wrote: > I understand the dilemma that the Linux scheduler is in, namely satisfy > the low end at all cost. [..] We can satisfy the low end by making the numa scheduler at compile time (that's what I did in my patch at least). Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Wed, Apr 04, 2001 at 03:34:22PM +0200, Ingo Molnar wrote: > > On Wed, 4 Apr 2001, Hubertus Franke wrote: > > > Another point to raise is that the current scheduler does a exhaustive > > search for the "best" task to run. It touches every process in the > > runqueue. this is ok if the runqueue length is limited to a very small > > multiple of the #cpus. [...] > > indeed. The current scheduler handles UP and SMP systems, up to 32 > (perhaps 64) CPUs efficiently. Agressively NUMA systems need a different > approach anyway in many other subsystems too, Kanoj is doing some > scheduler work in that area. I didn't seen anything from Kanoj but I did something myself for the wildfire: ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.3aa1/10_numa-sched-1 this is mostly an userspace issue, not really intended as a kernel optimization (however it's also partly a kernel optimization). Basically it splits the load of the numa machine into per-node load, there can be unbalanced load across the nodes but fairness is guaranteed inside each node. It's not extremely well tested but benchmarks were ok and it is at least certainly stable. However Ingo consider that in a 32-way if you don't have at least 32 tasks running all the time _always_ you're really stupid paying such big money for nothing ;). So the fact the scheduler is optimized for 1/2 tasks running all the time is not nearly enough for those machines (and of course also the scheduling rate automatically increases linearly with the increase of the number of cpus). Now it's perfectly fine that we don't ask the embedded and desktop guys to pay for that, but a kernel configuration option to select an algorithm that scales would be a good idea IMHO. The above patch just adds a CONFIG_NUMA_SCHED. The scalable algorithm can fit into it and nobody will be hurted by that (CONFIG_NUMA_SCHED cannot even be selected by x86 compiles). Andrea - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Wed, 4 Apr 2001, Hubertus Franke wrote: > It is not clear that yielding the same decision as the current > scheduler is the ultimate goal to shoot for, but it allows > comparision. obviously the current scheduler is not cast into stone, it never was, never will be. but determining whether the current behavior is possible in a different scheduler design is sure a good metric of how flexible that different scheduler design is. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Wed, 4 Apr 2001, Hubertus Franke wrote: > I understand the dilemma that the Linux scheduler is in, namely > satisfy the low end at all cost. [...] nope. The goal is to satisfy runnable processes in the range of NR_CPUS. You are playing word games by suggesting that the current behavior prefers 'low end'. 'thousands of runnable processes' is not 'high end' at all, it's 'broken end'. Thousands of runnable processes are the sign of a broken application design, and 'fixing' the scheduler to perform better in that case is just fixing the symptom. [changing the scheduler to perform better in such situations is possible too, but all solutions proposed so far had strings attached.] Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
I grant you that the code is not as clean as the current scheduler, so maybe you missed that part. For the priority scheduler: Yes the task_to_qid assumes a NON-affinity (no cpu, no mm) to determine the list index to where the task has to be enqueued. However, if you wonder down to the search_range section of the scheduler, you see that we do add the "+1" for equal mm. All I did here was take the goodness() function a part for search_range in order to insert some extra code that speeds up the code. Again, I don't want to lean to hard on the priority scheduler, because we only did this to have a reference point regarding lock contention and lock hold. This is stuff that many operating systems did years ago, most of which have moved on to add MQ to that. So that is what the LSE scheduling team is pushing. I understand the dilemma that the Linux scheduler is in, namely satisfy the low end at all cost. But I believe that in order for Linux to push into the high end we need to address the scalability of the current scheduler. Simply handwaving and declaring that lots of running tasks is a stupid idea doesn't get us there. If indeed you assume that there #running-tasks ~ #cpus then each task should alot a reasonable amount of work and any small overhead incurred should be amortizable. However as we contend if #running-tasks >> #cpus is a situation we need to deal with, the living with the current lock contention can really drag us down. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Ingo Molnar <[EMAIL PROTECTED]>@elte.hu> on 04/04/2001 02:28:31 AM Please respond to <[EMAIL PROTECTED]> Sent by: <[EMAIL PROTECTED]> To: Mike Kravetz <[EMAIL PROTECTED]> cc: Hubertus Franke/Watson/IBM@IBMUS, Fabio Riccardi <[EMAIL PROTECTED]>, Linux Kernel List <[EMAIL PROTECTED]> Subject: Re: a quest for a better scheduler On Tue, 3 Apr 2001, Mike Kravetz wrote: > Our 'priority queue' implementation uses almost the same goodness > function as the current scheduler. The main difference between our > 'priority queue' scheduler and the current scheduler is the structure > of the runqueue. We break up the single runqueue into a set of > priority based runqueues. The 'goodness' of a task determines what > sub-queue a task is placed in. Tasks with higher goodness values are > placed in higher priority queues than tasks with lower goodness > values. [...] we are talking about the same thing, re-read my mail. this design assumes that 'goodness' is constant in the sense that it does not depend on the previous process. and no, your are not using the same goodness() function, you omitted the prev->mm == next->mm component to goodness(), due to this design limitation. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
This is an important point that Mike is raising and it also addresses a critique that Ingo issued yesterday, namely interactivity and fairness. The HP scheduler completely separates the per-CPU runqueues and does not take preemption goodness or alike into account. This can lead to unfair proportionment of CPU cycles, strong priority inversion and a potential lack of interactivity. Our MQ scheduler does yield the same decision in most cases (other than defined by some race condition on locks and counter members) It is not clear that yielding the same decision as the current scheduler is the ultimate goal to shoot for, but it allows comparision. Another point to raise is that the current scheduler does a exhaustive search for the "best" task to run. It touches every process in the runqueue. this is ok if the runqueue length is limited to a very small multiple of the #cpus. But that is not what high end server systems encounter. With the rising number of processors, lock contention can quickly become a bottleneck. If we assume that load (#running-task) increases somewhat linear with #cpus, the problem gets even worth because not only have I increased the number of clients but also the lock hold time. Clinging on to the statement that #running-tasks ~ #cpus, ofcourse saves you from that dilemma, but not everybody is signing on to this limitation. MQ and priority-list help in 2 ways. MQ reduces the average lock holdtime because on average it only inspects #running-tasks/#cpus tasks to make a local decision. It then goes on to inspect (#cpus-1) datastructures representing the next best to run tasks on those remote cpus all without holding the lock, thus substantially reducing lock contention. Note we still search the entire set of runnable tasks, however we do it in a distributed collaborative manner. The only time we deviate from the current scheduler decision is in the case when two cpus have identified the same remote task as a target for remote stealing. In that case one will win and the other cpu will continue looking somewhere else, although there might have been another tasks on that cpu to steal. priority list schedulers (PRS) only helps in reducing lock hold time, which can result in some relieve wrt lock contention, but not a whole lot. PRS can limit the lists it has to search based on the PROC_CHANGE_PENALTY. It also keeps 0-counter in a list that is never inspected. One can even go further and put YIELD tasks in a separate list, given that the sys_sched_yield already does some optimizations. The older version (12/00) posted on LSE is functionally equivalent to the current scheduler. I will put up another version this week that is based on a bitmask and which is a bit more agressive in that it ignores the MM goodness boost of 1 which in my books is merely a tie breaker between two task of equal goodness. Beyond that we have done some work on cpu pooling, which is to identify a set of cpus that form a scheduling set. We still consider in reschedule_idle all cpus for preemption but in schedule it is sufficient to only schedule within the own set. That again can limit lock hold time with MQ and we have seen some improvements. We also deploy load balacing. To summarize, we have taken great care of trying to preserve the current scheduler semantics and have laid out a path to relax some of the semantics for further improvements. I don't believe that the HP scheduler is sufficient since it is lacking load balacing, which naturally occurs in our MQ scheduler, and it lacks the interactivity requirements that Ingo pointed out. Most of these things are discussed in great detail in the writeups under lse.sourceforge.net/scheduling. I also believe we show there that the MQ performance for low thread counts is also matching the vanilla case. I further don't understand the obsession of keeping the scheduler simple. If there are improvements and I don't believe the MQ is all that complicated and its well documented, why not put it in. Hubertus Franke Enterprise Linux Group (Mgr), Linux Technology Center (Member Scalability) email: [EMAIL PROTECTED] (w) 914-945-2003(fax) 914-945-4425 TL: 862-2003 Mike Kravetz <[EMAIL PROTECTED]> on 04/03/2001 10:47:00 PM To: Fabio Riccardi <[EMAIL PROTECTED]> cc: Mike Kravetz <[EMAIL PROTECTED]>, Ingo Molnar <[EMAIL PROTECTED]>, Hubertus Franke/Watson/IBM@IBMUS, Linux Kernel List <[EMAIL PROTECTED]>, Alan Cox <[EMAIL PROTECTED]> Subject: Re: a quest for a better scheduler On Tue, Apr 03, 2001 at 05:18:03PM -0700, Fabio Riccardi wrote: > > I have measured the HP and not the "scalability" patch because the two do more > or less the same thing and give me the same performance advantages, but the > former is a lot simpler and I could port it with no effort on any recent > kernel. Actually, there is a significant difference between the HP patch a
Re: a quest for a better scheduler
On Wed, 4 Apr 2001, Alan Cox wrote: > The problem has always been - alternative scheduler, crappier > performance for 2 tasks running (which is most boxes). [...] it's not only the 2-task case, but also less flexibility or lost semantics. > Indeed. I'd love to see you beat tux entirely in userspace. It proves > the rest of the API for the kernel is right well, until the cost of entry into the kernel is eliminated, this is not possible - unless there are performance bugs in TUX :-) but yes, getting a userspace solution that gets 'close enough' in eg. SPECweb99 benchmarks (which is complex enough to be trusted as a generic performance metric) would be a nice thing to have. There are existing SIGIO based, multithreaded solutions (eg. phttpd), with varying success. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, 3 Apr 2001, Fabio Riccardi wrote: > I agree that a better threading model would surely help in a web > server, but to me this is not an excuse to live up with a broken > scheduler. believe me, there are many other parts of the kernel that are not optimized for the nutcase. In this case it's the scheduler that punishes you first. Do not shoot the messenger. > The X15 server I'm working on now is a sort of user-space TUX, it uses > only 8 threads per CPU and it achieves the same level of performance > of the kernel space TUX. Even in this case the performance advantage > of the multiqueue scheduler is remarkable, especially on a multi-CPU > (> 2) architecture. then your design is still bad. TUX has no problems with the scheduler, and TUX doesnt have many runnable processes at once. And this has nothing to do with TUX being within the kernel. > To achieve some decent disk/CPU/network overlapping with the current > linux blocking disk IO limitations there is no way to avoid a "bunch > of server threads". [...] false. > [...] I've (experimentally) figured out that 8-16 threads per CPU can > assure some reasonable overlapping, depending on the memory size and > disk subsystem speed. On a 8-way machine this means 64-128 active > tasks, [...] if they are doing IO *only* then there wont be 64-128 active tasks. This is how TUX does async IO: helper threads do the IO and *only* the IO. This way cache locality is maximized. (the scheduler is irrelevant in this case.) Again you are blaming the scheduler - the poor scheduler is simply the first kernel subsystem that tells you: your design still sucks. (if you have 64-128 active tasks then you already pay a very big cache footprint price as well.) [I'm wondering how your solution was just as fast as TUX, although you claim that the scheduler was already killing things. Perhaps your test was done over localhost or was saturating network bandwidth?] > Unless we want to maintain the position tha the only way to achieve > good performance is to embed server applications in the kernel, [...] FYI, TUX related functionality is constantly being pushed out and made accessible to userspace as well. Witness zerocopy, IRQ-affinity, and tons of generic patches such as pagecache-scalability and timer-scalability. (and soon the remaining, TUX-private improvements too.) TUX is intended to be a testbed for kernel subsystems, where performance optimizations and APIs can be tested without having to export them to userspace. (exporting to userspace cannot be done lightly and makes things less flexible.) Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, 3 Apr 2001, Fabio Riccardi wrote: > I've spent my afternoon running some benchmarks to see if MQ patches > would degrade performance in the "normal case". no doubt priority-queue can run almost as fast as the current scheduler. What i'm worried about is the restriction of the 'priority' of processes, it cannot depend on previous processes (and other 'current state') anymore. to so we have two separate issues: #1: priority-queue: has the fundamental goodness() design limitation. #2: per-CPU-runqueues: changes semantics, makes scheduler less effective due to nonglobal decisions. about #1: while right now the prev->mm rule appears to be a tiny issue (it might not affect performance significantly), but forbidding it by hardcoding the assumption into data structures is a limitation of *future* goodness() functions. Eg. with the possible emergence of CPU-level threading and other, new multiprocessing technologies, this could be a *big* mistake. the Linux scheduler is not designed for the case of 1000 runnable processes. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, 3 Apr 2001, Mike Kravetz wrote: > Our 'priority queue' implementation uses almost the same goodness > function as the current scheduler. The main difference between our > 'priority queue' scheduler and the current scheduler is the structure > of the runqueue. We break up the single runqueue into a set of > priority based runqueues. The 'goodness' of a task determines what > sub-queue a task is placed in. Tasks with higher goodness values are > placed in higher priority queues than tasks with lower goodness > values. [...] we are talking about the same thing, re-read my mail. this design assumes that 'goodness' is constant in the sense that it does not depend on the previous process. and no, your are not using the same goodness() function, you omitted the prev->mm == next->mm component to goodness(), due to this design limitation. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
If we are facing these problems for "normal case" then hope the Solaris is handling it !! Amol Fabio Riccardi <[EMAIL PROTECTED]> on 04/04/2001 07:03:57 AM To: Alan Cox <[EMAIL PROTECTED]> cc: [EMAIL PROTECTED] (bcc: Amol Lad/HSS) Subject: Re: a quest for a better scheduler Alan, for the "normal case" performance see my other message. I agree that a better threading model would surely help in a web server, but to me this is not an excuse to live up with a broken scheduler. The X15 server I'm working on now is a sort of user-space TUX, it uses only 8 threads per CPU and it achieves the same level of performance of the kernel space TUX. Even in this case the performance advantage of the multiqueue scheduler is remarkable, especially on a multi-CPU (> 2) architecture. To achieve some decent disk/CPU/network overlapping with the current linux blocking disk IO limitations there is no way to avoid a "bunch of server threads". I've (experimentally) figured out that 8-16 threads per CPU can assure some reasonable overlapping, depending on the memory size and disk subsystem speed. On a 8-way machine this means 64-128 active tasks, a total disaster with the current scheduler. Unless we want to maintain the position tha the only way to achieve good performance is to embed server applications in the kernel, some minimal help should be provided to goodwilling user applications :) TIA, ciao, - Fabio Alan Cox wrote: > > Is there any special reason why any of those patches didn't make it to > > the mainstream kernel code? > > All of them are worse for the normal case. Also 1500 running apache's isnt > a remotely useful situation, you are thrashing the cache even if you are now > not thrashing the scheduler. Use an httpd designed for that situation. Then > you can also downgrade to a cheap pentium class box for the task ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
I was actually suspecting that the extra lines in your patch were there for a reason :) A few questions: What is the real impact of a (slight) change in scheduling semantics? Under which situation one should notice a difference? As you state in your papers the global decision comes with a cost, is it worth it? Could you make a port of your thing on recent kernels? I tried and I failed and I don't have enough time to figure out why, that should be trivial for you though. TIA, ciao, - Fabio Mike Kravetz wrote: > On Tue, Apr 03, 2001 at 05:18:03PM -0700, Fabio Riccardi wrote: > > > > I have measured the HP and not the "scalability" patch because the two do more > > or less the same thing and give me the same performance advantages, but the > > former is a lot simpler and I could port it with no effort on any recent > > kernel. > > Actually, there is a significant difference between the HP patch and > the one I developed. In the HP patch, if there is a schedulable task > on the 'local' (current CPU) runqueue it will ignore runnable tasks on > other (remote) runqueues. In the multi-queue patch I developed, the > scheduler always attempts to make the same global scheduling decisions > as the current scheduler. > > -- > Mike Kravetz [EMAIL PROTECTED] > IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, Apr 03, 2001 at 05:18:03PM -0700, Fabio Riccardi wrote: > > I have measured the HP and not the "scalability" patch because the two do more > or less the same thing and give me the same performance advantages, but the > former is a lot simpler and I could port it with no effort on any recent > kernel. Actually, there is a significant difference between the HP patch and the one I developed. In the HP patch, if there is a schedulable task on the 'local' (current CPU) runqueue it will ignore runnable tasks on other (remote) runqueues. In the multi-queue patch I developed, the scheduler always attempts to make the same global scheduling decisions as the current scheduler. -- Mike Kravetz [EMAIL PROTECTED] IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
--On Tuesday, April 03, 2001 18:17:30 -0700 Fabio Riccardi <[EMAIL PROTECTED]> wrote: > Alan Cox wrote: > Indeed, I'm using RT sigio/sigwait event scheduling, bare clone threads > and zero-copy io. Fabio, I'm working on a similar solution, although I'm experimenting with SGI's KAIO patch to see what it can do. I've had to patch the kernel to implement POSIX style signal dispatch symantics (so that the thread which posted an I/O request doesn't have to be the one which catches the signal). Are you taking a similar approach, or is the lack of this behavior the reason you are using so many threads? --Chris - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Alan Cox wrote: > > for the "normal case" performance see my other message. > > I did - and with a lot of interest thanks! :) > > I agree that a better threading model would surely help in a web server, but to > > me this is not an excuse to live up with a broken scheduler. > > The problem has always been - alternative scheduler, crappier performance for > 2 tasks running (which is most boxes). If your numbers are right then the > HP patch is working as well for 1 or 2 tasks too Please verify them if you have a couple of spare hours. BTW: I measured similar results for the "scalability" patches on a 2.4.1 kernel, it would be worth the effort to seriously compare them from an architectural point of view, but I don't have the time right now... > > Unless we want to maintain the position tha the only way to achieve good > > performance is to embed server applications in the kernel, some minimal help > > should be provided to goodwilling user applications :) > > Indeed. I'd love to see you beat tux entirely in userspace. It proves the > rest of the API for the kernel is right Indeed, I'm using RT sigio/sigwait event scheduling, bare clone threads and zero-copy io. If only I had a really asynchronous sendfile, or a smarter madvise that wouldn't require to map files :) My server cannot execute dynamic stuff yet, it relies on Apache for that. Running X15 and TUX in the same conditions (i.e. dynamic code in Apache) I get exactly the same score in both cases. I'm adding a TUX-like dynamic interface, I hope to get it to work by next week, then I'll make a real confrontation. Regards, ciao, - Fabio - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
> for the "normal case" performance see my other message. I did - and with a lot of interest > I agree that a better threading model would surely help in a web server, but to > me this is not an excuse to live up with a broken scheduler. The problem has always been - alternative scheduler, crappier performance for 2 tasks running (which is most boxes). If your numbers are right then the HP patch is working as well for 1 or 2 tasks too > Unless we want to maintain the position tha the only way to achieve good > performance is to embed server applications in the kernel, some minimal help > should be provided to goodwilling user applications :) Indeed. I'd love to see you beat tux entirely in userspace. It proves the rest of the API for the kernel is right - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Alan, for the "normal case" performance see my other message. I agree that a better threading model would surely help in a web server, but to me this is not an excuse to live up with a broken scheduler. The X15 server I'm working on now is a sort of user-space TUX, it uses only 8 threads per CPU and it achieves the same level of performance of the kernel space TUX. Even in this case the performance advantage of the multiqueue scheduler is remarkable, especially on a multi-CPU (> 2) architecture. To achieve some decent disk/CPU/network overlapping with the current linux blocking disk IO limitations there is no way to avoid a "bunch of server threads". I've (experimentally) figured out that 8-16 threads per CPU can assure some reasonable overlapping, depending on the memory size and disk subsystem speed. On a 8-way machine this means 64-128 active tasks, a total disaster with the current scheduler. Unless we want to maintain the position tha the only way to achieve good performance is to embed server applications in the kernel, some minimal help should be provided to goodwilling user applications :) TIA, ciao, - Fabio Alan Cox wrote: > > Is there any special reason why any of those patches didn't make it to > > the mainstream kernel code? > > All of them are worse for the normal case. Also 1500 running apache's isnt > a remotely useful situation, you are thrashing the cache even if you are now > not thrashing the scheduler. Use an httpd designed for that situation. Then > you can also downgrade to a cheap pentium class box for the task ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
Dear all, I've spent my afternoon running some benchmarks to see if MQ patches would degrade performance in the "normal case". To measure performance I've used the latest lmbench and I have mesured the kernel compile times on a dual pentium III box runing at 1GHz with an 133MHz bus. Results (attached) show that there is no measurable difference in performace between a vanilla scheduler and a multiqueue scheduler when running only few processes (the compilation benchmark runs essentially two processes, one per CPU). I have measured the HP and not the "scalability" patch because the two do more or less the same thing and give me the same performance advantages, but the former is a lot simpler and I could port it with no effort on any recent kernel. It is indeed interesting to see that this patch was originally designed for a 2.4.0-test10 kernel, and still works fine on the latest kernels, only a minor change (one line) was required. A version of the patch for the 2.4.2-ac26 kernel is attached to this message. Given the zero impact on "normal case" performance and the huge positive impact (> 20%) in the heavy load case (70-80 runnable processess on a load of about 1400 total) I don't see why such a thing shouldn't be accepted in the mainstream scheduler. - Fabio --- sched.c.origTue Mar 27 17:30:58 2001 +++ sched.c Tue Apr 3 16:45:21 2001 @@ -34,6 +34,7 @@ extern void timer_bh(void); extern void tqueue_bh(void); extern void immediate_bh(void); +static inline void hop_queues(struct task_struct *, int); /* * scheduler variables @@ -90,7 +91,8 @@ spinlock_t runqueue_lock __cacheline_aligned = SPIN_LOCK_UNLOCKED; /* inner */ rwlock_t tasklist_lock __cacheline_aligned = RW_LOCK_UNLOCKED; /* outer */ -static LIST_HEAD(runqueue_head); +static struct list_head runqueue_head[NR_CPUS] = { +LIST_HEAD_INIT((runqueue_head[0]))}; +static LIST_HEAD(rt_queue_head); /* * We align per-CPU scheduling data on cacheline boundaries, @@ -100,12 +102,15 @@ struct schedule_data { struct task_struct * curr; cycles_t last_schedule; + struct list_head runqueue_head; } schedule_data; char __pad [SMP_CACHE_BYTES]; -} aligned_data [NR_CPUS] __cacheline_aligned = { {{&init_task,0}}}; +} aligned_data [NR_CPUS] __cacheline_aligned = { {{&init_task,0, + LIST_HEAD_INIT((aligned_data[0].schedule_data.runqueue_head))}}}; #define cpu_curr(cpu) aligned_data[(cpu)].schedule_data.curr #define last_schedule(cpu) aligned_data[(cpu)].schedule_data.last_schedule +#define cpu_rq(cpu) (aligned_data[(cpu)].schedule_data.runqueue_head) struct kernel_stat kstat; @@ -199,6 +204,33 @@ return goodness(p, cpu, prev->active_mm) - goodness(prev, cpu, prev->active_mm); } + +static inline int other_goodness(struct task_struct * p, int this_cpu, struct +mm_struct *this_mm) +{ + int weight; + + /* +* select the current process after every other +* runnable process, but before the idle thread. +* Also, dont trigger a counter recalculation. +* +* Give the process a first-approximation goodness value +* according to the number of clock-ticks it has left. +* +* Don't do any other calculations if the time slice is +* over.. +*/ + weight = p->counter; + if (!weight) + goto out2; + + /* .. and a slight advantage to the current MM */ + if (p->mm == this_mm || !p->mm) + weight += 1; + weight += 20 - p->nice; +out2: + return weight; +} /* * This is ugly, but reschedule_idle() is very timing-critical. * We are called with the runqueue spinlock held and we must @@ -266,6 +298,10 @@ } else { if (oldest_idle == -1ULL) { int prio = preemption_goodness(tsk, p, cpu); + /* +* this will never be true for < 400 HZ non +* realtime. optimize this? SAR +*/ if (prio > max_prio) { max_prio = prio; @@ -277,6 +313,10 @@ tsk = target_tsk; if (tsk) { if (oldest_idle != -1ULL) { + /* push onto best queue */ + if (p->policy == SCHED_OTHER){ + hop_queues(p, tsk->processor); + } best_cpu = tsk->processor; goto send_now_idle; } @@ -306,20 +346,28 @@ */ static inline void add_to_runqueue(struct task_struct * p) { - list_add(&p->run_list, &runqueue_head); + if (p->policy == SCHED_OTHER){ + list_add(&p->run_list, &cpu_rq(p->processor)); + } else list_add(&p->run_list, &rt_queue_head);
Re: a quest for a better scheduler
On Tue, Apr 03, 2001 at 08:47:52PM +0200, Ingo Molnar wrote: > > this restriction (independence of the priority from the previous process) > is a fundamentally bad property, scheduling is nonlinear and affinity > decisions depend on the previous context. [i had a priority-queue SMP > scheduler running 2-3 years ago but dropped the idea due to this issue.] > IMO it's more important to have a generic and flexible scheduler, and > arbitrary, nonnatural restrictions tend to bite us later on. It seems like we may be talking about two different things. Our 'priority queue' implementation uses almost the same goodness function as the current scheduler. The main difference between our 'priority queue' scheduler and the current scheduler is the structure of the runqueue. We break up the single runqueue into a set of priority based runqueues. The 'goodness' of a task determines what sub-queue a task is placed in. Tasks with higher goodness values are placed in higher priority queues than tasks with lower goodness values. This allows us to limit the scan of runnable tasks to the highest priority sub-queue, as opposed to the entire runquue. When scanning the highest priority sub-queue we use almost the same goodness function as that which is used today (it does take CPU affinity into account). In fact, if we used the same goodness function I'm pretty sure we would exactly match the behavior of the existing scheduler. Hopefully, Hubertus Franke will speak up and provide more details, as he is much more familiar with this implementation than I am. In any case, I think we have almost reached the conclusion that our priority queue implementation may not be acceptable due to the extra overhead in low thread counts. > one issue regarding multiqueues is the ability of interactive processes to > preempt other, lower priority processes on other CPUs. These sort of > things tend to happen while using X. In a system where process priorities > are different, scheduling decisions cannot be localized per-CPU > (unfortunately), and 'the right to run' as such is a global thing. Agreed. This is why our multi-queue scheduler always attempts to make global decisions. It will preempt lower priority tasks on other CPUs. This implementation tends to make 'more localized' scheduling decisions as contention on the runqueue locks increase. However, at this point one could argue that we have moved away from a 'realistic' low task count system load. > lmbench's lat_ctx for example, and other tools in lmbench trigger various > scheduler workloads as well. Thanks, I'll add these to our list. -- Mike Kravetz [EMAIL PROTECTED] IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, 3 Apr 2001, Mike Kravetz wrote: > [...] Currently, in this implementation we only deviate from the > current scheduler in a small number of cases where tasks get a boost > due to having the same memory map. thread-thread-affinity pretty much makes it impossible to use a priority queue. This 'small number of cases' is actually a fundamental issue: can the 'goodness()' function be an arbitrary function of: goodness(process_prev,process_next) := f(process_prev,process_next) , or is the queue design restricting the choice of goodness() functions? Priority queues for example restrict the choice of the goodness() function to subset of functions: goodness(process_prev,process_next) := f(process_next); this restriction (independence of the priority from the previous process) is a fundamentally bad property, scheduling is nonlinear and affinity decisions depend on the previous context. [i had a priority-queue SMP scheduler running 2-3 years ago but dropped the idea due to this issue.] IMO it's more important to have a generic and flexible scheduler, and arbitrary, nonnatural restrictions tend to bite us later on. one issue regarding multiqueues is the ability of interactive processes to preempt other, lower priority processes on other CPUs. These sort of things tend to happen while using X. In a system where process priorities are different, scheduling decisions cannot be localized per-CPU (unfortunately), and 'the right to run' as such is a global thing. > Can someone tell me what a good workload/benchmark would be to examine > 'low thread count' performance? [...] lmbench's lat_ctx for example, and other tools in lmbench trigger various scheduler workloads as well. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Tue, Apr 03, 2001 at 10:55:12AM +0200, Ingo Molnar wrote: > > you can easily make the scheduler fast in the many-processes case by > sacrificing functionality in the much more realistic, few-processes case. > None of the patch i've seen so far maintained the current scheduler's > few-processes logic. But i invite you to improve the current scheduler's > many-processes behavior, without hurting its behavior in the few-processes > case. > Maintaining the current scheduler's logic is exactly what we are trying to do in the projects at: http://lse.sourceforge.net/scheduling/ A common design goal for the the two alternative scheduler implementations at this site is to maintain the current scheduler's behavior/scheduling decisions. In the case of the priority queue scheduler, we have actually used a copy of the existing scheduler running in parallel (in the same kernel) to determine if we are making the same scheduling decisions. Currently, in this implementation we only deviate from the current scheduler in a small number of cases where tasks get a boost due to having the same memory map. The multi-queue implementation is more interesting. It is also designed to maintain the behavior of the current scheduler. However, as the runqueues get longer (and we start getting contention on the runqueue locks) it starts to deviate from existing scheduler behavior and make more local scheduling decisions. Ideally, this implementation will exhibit the behavior of the current scheduler at low thread counts and make more localized decisions as pressure on the scheduler is increased. Neither of these implementations are at a point where I would advocate their adoption; yet. Can someone tell me what a good workload/benchmark would be to examine 'low thread count' performance? In the past people have used the 'spinning on sched_yield' benchmark. However, this now makes little sense with the sched_yield optimizations introduced in 2.4. In addition, such a benchmark mostly ignores the 'reschedule_idle' component of the scheduler. We have developed a 'token passing' benchmark which attempts to address these issues (called reflex at the above site). However, I would really like to get a pointer to a community acceptable workload/benchmark for these low thread cases. -- Mike Kravetz [EMAIL PROTECTED] IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
> Is there any special reason why any of those patches didn't make it to > the mainstream kernel code? All of them are worse for the normal case. Also 1500 running apache's isnt a remotely useful situation, you are thrashing the cache even if you are now not thrashing the scheduler. Use an httpd designed for that situation. Then you can also downgrade to a cheap pentium class box for the task ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: a quest for a better scheduler
On Mon, 2 Apr 2001, Fabio Riccardi wrote: > I sent a message a few days ago about some limitations I found in the > linux scheduler. > > In servers like Apache where a large (> 1000) number of processes can > be running at the same time and where many of them are runnable at the > same time, the default Linux scheduler just starts trashing and the > machine becomes very rapidly unusable. it is *not* a scheduler problem. This is an application design problem. Do not expect > 1000 runnable processes to perform well. Apache's one-process-per-parallel-client-connection application model is especially bad for such kind of loads. The scheduler has more important priorities to worry about than to fix application design problems. > Indeed I've tried the patches available on the sites for the > multi-queue scheduler and I was amazed by the performance improvement > that I got. Both patches allow me to get to a 100% real CPU > utilization on a two way machine running ~1500 processes. There were many promises along the years, and none as of now met all the requirements the scheduler has to fulfill. 'many runnable processes' is not on the top of the list - if we are scheduling like mad then we have lost lots of performance already. even with such 'improvements', the many-parallel-clients performance of one-process-per-request HTTP server designs is an order slower than what is possible. having said that, improving this workload is still a useful task and we added improvements to the scheduler to improve this case. But those patches sacrifice other functionality just to get better many-runnable-processes performance, which is unacceptable. > What those patches do is quite simple, instead of having the single > global process queue present in the normal Linux scheduler, they add > multiple queues (one per CPU). In this way the scheduling decision can > be greatly simplified and almost made local to each CPU. No hotspots, > no global locks (well, almost). the problem is that the *real* scheduling complexity (and needed functionality) shows when the number of runnable processes is in the neighborhood of the number of processors. Running many processes just masks eg. load-balancing deficiencies in schedulers, so obviously the simplest and most localized scheduler 'wins'. So comparing schedulers based on many-runnable-processes load is like comparing cars solely based on the size of their fuel tanks. > Although some scalability problems are still there (there still is a > global decision to make), the performance improvement obtained and the > simplicity of the solution are remarkable. you can easily make the scheduler fast in the many-processes case by sacrificing functionality in the much more realistic, few-processes case. None of the patch i've seen so far maintained the current scheduler's few-processes logic. But i invite you to improve the current scheduler's many-processes behavior, without hurting its behavior in the few-processes case. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/