Re: Clearing linux buffers and caches
On Wed, 8 Jul 2009 03:00:27 +0200 Ivan Warren wrote: > cmm-1 (arch/s390/mm/cmm.c) uses SMSGs to communicate with the VMRM > service virtual machine. The VMRM will instruct the guest to release its > pages using good ole Diag 10.. nothing fancy here.. diag 10 actively tells z/VM that the page is free. > cmma (arch/s390/mm/page_states.c) uses the B9AB instruction to indicate > to CP which pages linux no longer needs.. It uses the standard Linux MM > mechanism to be instructed to do so. (page_free()).. The only diff > between Diag 10 and B9AB is that a page marked "Unused" with B9AB will > generate an Addressing Exception (PIC 5) if you attempt to access it, > while Diag 10 won't do that. The B9AB instruction will mark a page as free in the pgste. That does not cause a sie exit to z/VM which means z/VM doesn't know the page is free until it does a page scan for the guest. So you won't see an immediate effect in the z/VM page counts. If z/VM needs memory it will eventually find the unused pages and the page count will drop belately. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
Mark Post wrote: But that wasn't your comment/question. You asked if my original answer would apply to CMMA, and I said no, it would not apply to CMMA, since that is an architected interface between Linux and z/VM, implying that the drop cache /proc interface is not. And indeed it is not. Using CMM-1 (and possibly CMMA/CMM-2 if you're brave enough) will achieve real storage savings and, unless mis-handled by VMRM or the system administrator, not adversely affect performance of the Linux guest. Using the /proc interface to drop cache and buffers is not recommended in a production environment. Ok.. Just a communication problem then.. My comment addressed a specific portion of your statement : "Just because linux no longer treats those pages as being used doesn't mean z/VM sees them as free". My (verbatim) comment was : "Does this still hold true when using CMMA ?" This may have been the reason why we had a miscommunication - I wasn't specific enough about what portion of your statement I was referring to. I was making this comment since I was under the impression that CMMA's intent is to allow linux to indicate explicit page disposition to CP. Bah.; anyway.. no big deal ;) --Ivan -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 smime.p7s Description: S/MIME Cryptographic Signature
Re: Clearing linux buffers and caches
>>> On 7/7/2009 at 9:00 PM, Ivan Warren wrote: > To which I commented : Would CMMA change this ? To which you answered "no" > (with the implication that linux free page management is not connected to > z/VM's own free page management - but only a protocol to But that wasn't your comment/question. You asked if my original answer would apply to CMMA, and I said no, it would not apply to CMMA, since that is an architected interface between Linux and z/VM, implying that the drop cache /proc interface is not. And indeed it is not. Using CMM-1 (and possibly CMMA/CMM-2 if you're brave enough) will achieve real storage savings and, unless mis-handled by VMRM or the system administrator, not adversely affect performance of the Linux guest. Using the /proc interface to drop cache and buffers is not recommended in a production environment. Mark Post -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
Mark Post wrote: In the context of the original question, I stand by my response. Introducing all sort of hypothetical situations will change just about any answer. Mark, I don't understand.. The original question was : Will "sync; echo 3 > /proc/sys/vm/drop_caches" have any adverse effect ? Your answer was : "Yes, just because linux no longer treats those pages as being used doesn't mean z/VM sees them as free" - and your statement is that the only solution to ensure linux doesn't eat more memory than it needs to is to adjust the virtual machine's storage size. To which I commented : Would CMMA change this ? To which you answered "no" (with the implication that linux free page management is not connected to z/VM's own free page management - but only a protocol to After which, I looked into the source code and the relevant CP manual and disagree with your analysis since those information seem to dismiss your answer. What "hypothetical" situation are you talking about ? Using CMMA (but that was part of my question) ? Running linux under z/VM (which you already implied in your answer) ? or that flushing buffer/cache will call linux Memory Management free_page() function ? --Ivan -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 smime.p7s Description: S/MIME Cryptographic Signature
Re: Clearing linux buffers and caches
Rob van der Heij wrote: On Wed, Jul 8, 2009 at 1:26 AM, Ivan Warren wrote: What I am saying is that, on the contrary, when linux treats those pages as no longer being in use, z/VM *WILL* see them as free ! I think you have read too much glossy marketing PDFs :-) All we know is that VM *could* see it as free and we know it's not always looking for it at that place. And in fact, in this case Holy Page Management Assist is likely to do the thing. I second Mark that CMM-1 is the thing immediately frees the backing pages in z/VM. I use it on my test servers to reduce the footprint when I am done with my tests. It's not true that CMM-1 involves another virtual machine to do that (only if you choose to use VMRM to steer CMM). I haven't read any of the (glossy or not) marketing PDFs ! I'm reading the linux 2.6 source code and the z/VM CP System services manuals. cmm-1 (arch/s390/mm/cmm.c) uses SMSGs to communicate with the VMRM service virtual machine. The VMRM will instruct the guest to release its pages using good ole Diag 10.. nothing fancy here.. cmma (arch/s390/mm/page_states.c) uses the B9AB instruction to indicate to CP which pages linux no longer needs.. It uses the standard Linux MM mechanism to be instructed to do so. (page_free()).. The only diff between Diag 10 and B9AB is that a page marked "Unused" with B9AB will generate an Addressing Exception (PIC 5) if you attempt to access it, while Diag 10 won't do that. Ok.. maybe I'm confusing the terminology.. but that's what's in the linux source code.. --Ivan -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 smime.p7s Description: S/MIME Cryptographic Signature
Re: Clearing linux buffers and caches
On Wed, Jul 8, 2009 at 1:26 AM, Ivan Warren wrote: > What I am saying is that, on the contrary, when linux treats those pages > as no longer being in use, z/VM *WILL* see them as free ! I think you have read too much glossy marketing PDFs :-) All we know is that VM *could* see it as free and we know it's not always looking for it at that place. And in fact, in this case Holy Page Management Assist is likely to do the thing. I second Mark that CMM-1 is the thing immediately frees the backing pages in z/VM. I use it on my test servers to reduce the footprint when I am done with my tests. It's not true that CMM-1 involves another virtual machine to do that (only if you choose to use VMRM to steer CMM). Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
>>> On 7/7/2009 at 7:26 PM, Ivan Warren wrote: > You were stating : MP: - "Just because Linux no longer treats those pages as > being used doesn't mean z/VM sees them as free." What I am saying is that, on > the contrary, when linux treats those pages as no longer being in use, z/VM > *WILL* see them as free ! In the context of the original question, I stand by my response. Introducing all sort of hypothetical situations will change just about any answer. Mark Post -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
Mark Post wrote: Of course not. CMM and CMMA are designed interfaces between Linux and z/VM. Their whole purpose in life is to let the two work together on virtual memory management. Precisely ! if the linux kernel deems the page to be 'free' (and dropping a page from buffer/cache should do that - but then again, maybe not - and then I'll go back to my corner), and CMMA is available and functional (that is, activated and you have the right level of z/VM with the right PTFs), then the kernel Memory Manager (mm/page_alloc) should call the CMMA arch_free_page() In turn, arch/s390/mm/page_states.c[arch_free_page] (page_states.c IS CMMA) calls the z/VM EXTRACT_AND_SET_STORAGE_ATTRIBUTE (B9AB) instruction to indicate to CP that the page is no longer needed. arch_free_state sets the page to the 'Unused' state (as per 6.1.4.4 of the z/VM 5.4 CP Programing services) Setting the page to an 'Unused' state should make any physical page associated with the virtual page eligible to be reused for other purpose as well as free any associated paging slot as the guest operating system is declaring by that action that it won't be needing that page until it puts it back into a 'Stable' state. Thus achieving the intended result. You were stating : MP: - "Just because Linux no longer treats those pages as being used doesn't mean z/VM sees them as free." What I am saying is that, on the contrary, when linux treats those pages as no longer being in use, z/VM *WILL* see them as free ! ... CMM is different.. It uses a service virtual machine and is a completely different mechanism.. ... Then again, it's probably a better idea to let Linux and z/VM handle caching, buffering, paging and whatnot using their own heuristics than using a sledgehammer approach to flush the cache at regular intervals. --Ivan -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 smime.p7s Description: S/MIME Cryptographic Signature
Re: Clearing linux buffers and caches
On Wed, Jul 8, 2009 at 12:16 AM, Mark Post wrote: > Yes, it will hurt your Linux system's performance, while not doing anything > useful. It was meant as a way for benchmarks to start off with a clean cache so the first test does not spoil the next one. But if you want to do that on z/VM, you should also get rid of the real pages that back you (and be aware that the size of the available list makes a difference, and the other users, etc). It may also help to provide a starting point for how much memory you can take away with CMM Rob van der Heij Velocity Software http://www.velocitysoftware.com/ -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
>>> On 7/7/2009 at 6:37 PM, Ivan Warren wrote: > Does that still hold true when using CMMA ? Of course not. CMM and CMMA are designed interfaces between Linux and z/VM. Their whole purpose in life is to let the two work together on virtual memory management. Mark Post -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
Re: Clearing linux buffers and caches
Mark Post wrote: Yes, it will hurt your Linux system's performance, while not doing anything useful. Just because Linux no longer treats those pages as being used doesn't mean z/VM sees them as free. If you want to make sure Linux "doesn't eat up any more memory than it needs to" then shrink the size of the guest. (Or the LPAR if you're unfortunate enough to not have z/VM.) Does that still hold true when using CMMA ? --Ivan -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 smime.p7s Description: S/MIME Cryptographic Signature
Re: Clearing linux buffers and caches
>>> On 7/7/2009 at 6:02 PM, Deric Abel wrote: -snip- > Is there any harm in running this command, and if not, what about setting it > to run every hour or so in cron, therefore making sure that linux doesn't eat > up any more memory then it needs to? Yes, it will hurt your Linux system's performance, while not doing anything useful. Just because Linux no longer treats those pages as being used doesn't mean z/VM sees them as free. If you want to make sure Linux "doesn't eat up any more memory than it needs to" then shrink the size of the guest. (Or the LPAR if you're unfortunate enough to not have z/VM.) Mark Post -- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
