Re: [Xen-devel] Performance evaluation and Questions: Eliminating Xen (RTDS) scheduler overhead on dedicated CPU
Hi Dario, 2015-04-23 8:48 GMT-04:00 Dario Faggioli dario.faggi...@citrix.com: Hey, guys, I know, I know, I'm s much late to the party! Sorry, I got trapped into a thing that I really needed to finish... :-/ I've got no intention to resurrect this old thread, just wanted to pointed out a few things. What you pointed out is very interesting! I will have a look at them and then show the numbers of the measurements. (I will also need/have to redo some experiments to see how the TSC may affect the results, which I didn't realize before. :-( ) On Wed, 2015-04-08 at 16:52 -0400, Meng Xu wrote: 2015-04-08 5:13 GMT-04:00 George Dunlap george.dun...@eu.citrix.com: On 04/07/2015 09:25 PM, Meng Xu wrote: Hi George, Dario and Konrad, I finished a prototype of the RTDS scheduler with the dedicated CPU feature and did some quick evaluation on this feature. Right now, I need to refactor the code (because it is kind of messy when I was exploring different approaches :() and will send out the clean patch later (this week or next week). But the design follows our discussion at http://lists.xenproject.org/archives/html/xen-devel/2015-03/msg02854.html. The idea of 'dedicated CPU' makes sense. It's also always been quite common, in the Linux community, to see it as a real-time oriented feature. I personally don't agree much, as real-time is about determinism and dedicating a CPU to a task (in our case, that would mean dedicating a pCPU to a vCPU and then, in the guest, that vCPU to a task) does not automatically gives you determinism. Sure, it cuts off some overhead and some sources of unpredictable behavior (e.g., scheduler code), but not all of them (what about, for instance, caches shared with non-isolated pCPUs). I'm working on eliminating the shared cache interference among guest domains on non-isolated pCPUs. I have a prototype that can statically partition the LLC into equal size partitions and assign it to guest domains based on domain's configuration file. I'm running some evaluations to show the effectiveness of the cache partitioning approach (via page coloring) and will send you guys a slide about this soon. :-) And in any case, as you say, it looks like the source of the overhead is the very frequent invocation of the RTDS scheduler. Exactly! I'd put it this way: there are more urgent and more useful optimization, in general, but especially in RTDS, to be done before thinking at something like this. Right. What I was expecting you to test, for the RTDS scheduler, was the wake-up latency. Have you looked at that at all? Indeed, that would be really interesting. Ah, I didn't realize this... Do you have any concrete evaluation plan for this? In my mind, I can issue hypercalls in domU to wake-up and sleep a vcpu and measure how long it takes to wake up a vcpu. Maybe you have some better idea in mind? (The wake up latency of a vcpu will depends on the priority of the vcpu and how heavy loaded the system is, in my speculation.) Yes, that is something that could (should?) be done, as the wakeup latency of a vcpu is a lower bound for the wakeup latency of in-guest workloads, so we really want to know where we stand wrt to that, if we need to improve things, and if yes how. It's priority and load dependant... yes, of course, but that's why we have real-time schedulers for, isn't it? :-P Jokes apart, for the actual 'lower bound', we're clearly interesting to measure a vcpu when running alone on a pCPU, or with top priority. On the other hand, to look at wakeup latency from within the guest, cyclictest is the way to go: https://rt.wiki.kernel.org/index.php/Cyclictest What we want is to run it inside a guest, under different host and guest load conditions (and using different schedulers, varying the scheduling parameters, etc), and see what happens... Ever looked at that? I think it would be interesting. I will have a look at the cyclictest and have some evaluation set up. I totally agree that wakeup latency (on highest priority vcpu) is very important for real-time applications. I will start a new thread about the cyclictest result once I get it. I've done something similar while preparing this talk: https://archive.fosdem.org/2014/schedule/event/virtiaas16/ But never got the chance to repeat the experiments (neither I did any further reasoning or investigation about how the timestamps are obtained, TSC emulation, etc., as George pointed out) That's all... Sorry again for chiming in only now. :-( No problem at all! :-) Your advice and information are very useful! Thanks and best regards, Meng --- Meng Xu PhD Student in Computer and Information Science University of Pennsylvania ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] Performance evaluation and Questions: Eliminating Xen (RTDS) scheduler overhead on dedicated CPU
Hey, guys, I know, I know, I'm s much late to the party! Sorry, I got trapped into a thing that I really needed to finish... :-/ I've got no intention to resurrect this old thread, just wanted to pointed out a few things. On Wed, 2015-04-08 at 16:52 -0400, Meng Xu wrote: 2015-04-08 5:13 GMT-04:00 George Dunlap george.dun...@eu.citrix.com: On 04/07/2015 09:25 PM, Meng Xu wrote: Hi George, Dario and Konrad, I finished a prototype of the RTDS scheduler with the dedicated CPU feature and did some quick evaluation on this feature. Right now, I need to refactor the code (because it is kind of messy when I was exploring different approaches :() and will send out the clean patch later (this week or next week). But the design follows our discussion at http://lists.xenproject.org/archives/html/xen-devel/2015-03/msg02854.html. The idea of 'dedicated CPU' makes sense. It's also always been quite common, in the Linux community, to see it as a real-time oriented feature. I personally don't agree much, as real-time is about determinism and dedicating a CPU to a task (in our case, that would mean dedicating a pCPU to a vCPU and then, in the guest, that vCPU to a task) does not automatically gives you determinism. Sure, it cuts off some overhead and some sources of unpredictable behavior (e.g., scheduler code), but not all of them (what about, for instance, caches shared with non-isolated pCPUs). No, IMO, if you want determinism, you should make the code deterministic, not get rid of it! :-D In fact, Linux has a feature similar the one Meng investigated, and that has traditionally been used (at least until I was involved with Linux scheduling) by HPC people, database engines and high frequency trading use cases (which are also often categorized as 'real-time workloads' but just aren't, IMO). It's called isolcpus. For sure there was a boot time parameter for it, and it looks like it is still there: http://wiki.linuxcnc.org/cgi-bin/wiki.pl?The_Isolcpus_Boot_Parameter_And_GRUB2 http://www.linuxtopia.org/online_books/linux_kernel/kernel_configuration/re46.html http://lxr.linux.no/linux+v3.19.1/Documentation/kernel-parameters.txt#L1530 I'm not sure whether they grew interfaces to setup this at runtime, but doubt it. For us, I'm not sure whether something like that would be useful. To be fruitfully used together with something similar to Linux's isolcpus, it need to look like how Meng is doing it, i.e., it ought to be possible to handle single vCPUs, not full domains. However... Secondly, adding an entirely new interface, as implementing the dedicated cpu would require, on the other hand, is a fairly significant cost. It's costly for users to learn and configure the new interface, it's costly to document, and once it's there we have to continue to support it perhaps for a long time to come; and the feature itself is also fairly complicated and increases the code maintenance. So the performance improvement you've shown so far I think is nowhere near high enough a benefit to outweigh this cost. ... I agree with George on this... OK. I see and agree. ... and I'm happy you also do! :-D And in any case, as you say, it looks like the source of the overhead is the very frequent invocation of the RTDS scheduler. Exactly! I'd put it this way: there are more urgent and more useful optimization, in general, but especially in RTDS, to be done before thinking at something like this. You could probably get the same kinds of benefits without adding any new interfaces by reducing the amount of time the scheduler gets invoked when there are no other tasks to run on that cpu. Exactly. And again, that is particularly relevant to RTDS, as numbers show. Looking again at Linux world, this (i.e., avoiding invoking the scheduler when there is only one task on a CPU) is also something they've introduced rather recently. It's called full dynticks: https://www.kernel.org/doc/Documentation/timers/NO_HZ.txt http://ertl.jp/~shinpei/conf/ospert13/slides/FredericWeisbecker.pdf https://lwn.net/Articles/549580/ http://thread.gmane.org/gmane.linux.kernel/1485210 [*] [*] check out Linus' replies... awesome as usual, he even managed to rant about virtualization, all by himself!! :-P That is IMO a line of action that may deserve some investigation. RTDS-wise, for sure... the Credit-s, are not at all bad from that perspective (as your numbers also show), but it might be possible to do better. Yes. This is what Dagaen (cc.ed) is doing right now. He had a RFC patch and sent to me last week. We are working on refining the patch before sending it out to the mailing list. I'll be super glad to see this! :-D What I was expecting you to test, for the RTDS scheduler, was the wake-up latency. Have you looked at that at all? Indeed, that would be really interesting. Ah, I didn't realize this... Do you have any concrete evaluation plan for this? In my mind, I can issue
Re: [Xen-devel] Performance evaluation and Questions: Eliminating Xen (RTDS) scheduler overhead on dedicated CPU
2015-04-08 5:13 GMT-04:00 George Dunlap george.dun...@eu.citrix.com: On 04/07/2015 09:25 PM, Meng Xu wrote: Hi George, Dario and Konrad, I finished a prototype of the RTDS scheduler with the dedicated CPU feature and did some quick evaluation on this feature. Right now, I need to refactor the code (because it is kind of messy when I was exploring different approaches :() and will send out the clean patch later (this week or next week). But the design follows our discussion at http://lists.xenproject.org/archives/html/xen-devel/2015-03/msg02854.html. In a nutshell of the design, when a CPU is marked as dedicated CPU, the scheduler on that CPU will return the dedicated VCPU on it with a negative time so that it disable the scheduler timer on that CPU and other CPUs will no longer send SCHEDULE_SOFTIRQ to the dedicated CPU. The scheduler on the dedicated CPU may still be invoked when the dedicated VCPU is blocked/unblocked by the domU. Once this situation occurs, the schedule go though a fast pass to just return an idle VCPU/the dedicated VCPU instead of go through the runq. I did the following evaluation to show the benefits of introducing this dedicated CPU feature: I created a simple cpu-intensive task which just do the multiplication for specified times: start = rdtsc(); while ( i++ cpu_measurement-multiply_times ) result += i * i; finish = rdtsc(); latencies[k] = finish - start; I run this task and measure the execution time of above piece of code on different environments: native Linux on bare metal, domU on Xen with RTDS scheduler and domU on Xen with RTDS scheduler with dedicated CPU feature, domU on Xen with Credit/Credit2 scheduler. The difference between the execution time in virtualization environment and the execution time on native linux on bare metal is the virtualization overhead introduced by Xen. I want to see that 1) The virtualization overhead decreases a lot after the dedicated CPU feature is employed for RTDS scheduler (because the execution of the task will no longer suffer the scheduler overhead any more). 2) The frequency of invoking the scheduler on the dedicated CPU becomes very low once the dedicated CPU feature is applied. The result is as follows: When the cpu-intensive task did the multiplication for 1024 times, the execution time of the piece of code is: 9264 cycles on native linux on bare metal; 10320 cycles on Xen RTDS scheduler with dedicated CPU feature; 10324 cycles on Xen RTDS scheduler without dedicated CPU feature; We didn't see the improvement of the dedicated CPU feature here because the execution time is too short and it may not experience the scheduler overhead yet. When the cpu-intensive task did the multiplication for 536870912 times, the execution time of the piece of code is: 4838016028 cycles on native linux on bare metal; 4839649567 cycles on Xen RTDS scheduler with dedicated CPU feature; 4855509977 cycles on Xen RTDS scheduler without dedicated CPU feature; Hey Meng! Thanks for looking at this. One thing: it's not entirely clear to me whether the numbers for without dedicated CPU feature are still with the equivalent of pinning' -- i.e., is it guaranteed that no other vcpu will be run on the same cpu as the test program? Yes. In the experiment, every VCPU is pinned to one isolated CPU. So no other VCPU will run on the same cpu as the test program. Assuming that's the case, the numbers you give above show a 0.3% improvement for the dedicated cpu for cpu-intensive workloads. Yes. This is the save when the cpu-intensive task did the multiplication for 536870912 times. Another thing to note is that the scheduling quantum of RTDS scheduler is = 1ms. If the scheduling quantum is smaller, the saving should increase, but won't be too much in my speculation. (So I agree with your conclusion that the benefits may not worth the complexity) We can see that the dedicated CPU feature did save time for the cpu-intensive task. Without the dedicated CPU feature, the hypervisor scheduler may steal time from the domU and delay the execution of the task inside domU. I did vary the number of multiplications of the above piece of code in cpu-intensive task, and draw a figure to show the relation of the overhead and the execution time of the task on native linux. The figure can be found at http://www.cis.upenn.edu/~mengxu/xen-ml/cpu-base-alone_multiply_0_0_100.virtOhVSwcetnative.pdf. Please note the x-axis is the log value of the execution time. So the overhead is actually linear to the execution time of the task. I'm not sure I can gain any useful information out of this graph. A more useful comparison would be to graph the execution time as an *overhead* compared to the Linux execution time. For instance, in the numbers above, you'd have Linux = 1, RTDS+dedicated = 1.000337, RTDS = 1.00361. Do you mean I should normalize the
Re: [Xen-devel] Performance evaluation and Questions: Eliminating Xen (RTDS) scheduler overhead on dedicated CPU
On 04/07/2015 09:25 PM, Meng Xu wrote: Hi George, Dario and Konrad, I finished a prototype of the RTDS scheduler with the dedicated CPU feature and did some quick evaluation on this feature. Right now, I need to refactor the code (because it is kind of messy when I was exploring different approaches :() and will send out the clean patch later (this week or next week). But the design follows our discussion at http://lists.xenproject.org/archives/html/xen-devel/2015-03/msg02854.html. In a nutshell of the design, when a CPU is marked as dedicated CPU, the scheduler on that CPU will return the dedicated VCPU on it with a negative time so that it disable the scheduler timer on that CPU and other CPUs will no longer send SCHEDULE_SOFTIRQ to the dedicated CPU. The scheduler on the dedicated CPU may still be invoked when the dedicated VCPU is blocked/unblocked by the domU. Once this situation occurs, the schedule go though a fast pass to just return an idle VCPU/the dedicated VCPU instead of go through the runq. I did the following evaluation to show the benefits of introducing this dedicated CPU feature: I created a simple cpu-intensive task which just do the multiplication for specified times: start = rdtsc(); while ( i++ cpu_measurement-multiply_times ) result += i * i; finish = rdtsc(); latencies[k] = finish - start; I run this task and measure the execution time of above piece of code on different environments: native Linux on bare metal, domU on Xen with RTDS scheduler and domU on Xen with RTDS scheduler with dedicated CPU feature, domU on Xen with Credit/Credit2 scheduler. The difference between the execution time in virtualization environment and the execution time on native linux on bare metal is the virtualization overhead introduced by Xen. I want to see that 1) The virtualization overhead decreases a lot after the dedicated CPU feature is employed for RTDS scheduler (because the execution of the task will no longer suffer the scheduler overhead any more). 2) The frequency of invoking the scheduler on the dedicated CPU becomes very low once the dedicated CPU feature is applied. The result is as follows: When the cpu-intensive task did the multiplication for 1024 times, the execution time of the piece of code is: 9264 cycles on native linux on bare metal; 10320 cycles on Xen RTDS scheduler with dedicated CPU feature; 10324 cycles on Xen RTDS scheduler without dedicated CPU feature; We didn't see the improvement of the dedicated CPU feature here because the execution time is too short and it may not experience the scheduler overhead yet. When the cpu-intensive task did the multiplication for 536870912 times, the execution time of the piece of code is: 4838016028 cycles on native linux on bare metal; 4839649567 cycles on Xen RTDS scheduler with dedicated CPU feature; 4855509977 cycles on Xen RTDS scheduler without dedicated CPU feature; Hey Meng! Thanks for looking at this. One thing: it's not entirely clear to me whether the numbers for without dedicated CPU feature are still with the equivalent of pinning' -- i.e., is it guaranteed that no other vcpu will be run on the same cpu as the test program? Assuming that's the case, the numbers you give above show a 0.3% improvement for the dedicated cpu for cpu-intensive workloads. We can see that the dedicated CPU feature did save time for the cpu-intensive task. Without the dedicated CPU feature, the hypervisor scheduler may steal time from the domU and delay the execution of the task inside domU. I did vary the number of multiplications of the above piece of code in cpu-intensive task, and draw a figure to show the relation of the overhead and the execution time of the task on native linux. The figure can be found at http://www.cis.upenn.edu/~mengxu/xen-ml/cpu-base-alone_multiply_0_0_100.virtOhVSwcetnative.pdf. Please note the x-axis is the log value of the execution time. So the overhead is actually linear to the execution time of the task. I'm not sure I can gain any useful information out of this graph. A more useful comparison would be to graph the execution time as an *overhead* compared to the Linux execution time. For instance, in the numbers above, you'd have Linux = 1, RTDS+dedicated = 1.000337, RTDS = 1.00361. But what it sounds like you're saying is that if you did such a graph, the overhead would be pretty flat. That's what I'd expect -- a fairly constant overhead, regardless of how long you were running the test. As to the frequence of invoking the RTDS scheduler with/without the dedicated CPU feature, I add some code to trace which event triggers the scheduler on the dedicated cpu and how frequent it is. Before we apply the dedicated CPU feature to the RTDS scheduler, the dedicated CPU 3 was invoked once every 3.5us in average. (XEN) cpu 3 has invoked 356805936 SCHED_SOFTIRQ (sched) within
[Xen-devel] Performance evaluation and Questions: Eliminating Xen (RTDS) scheduler overhead on dedicated CPU
Hi George, Dario and Konrad, I finished a prototype of the RTDS scheduler with the dedicated CPU feature and did some quick evaluation on this feature. Right now, I need to refactor the code (because it is kind of messy when I was exploring different approaches :() and will send out the clean patch later (this week or next week). But the design follows our discussion at http://lists.xenproject.org/archives/html/xen-devel/2015-03/msg02854.html. In a nutshell of the design, when a CPU is marked as dedicated CPU, the scheduler on that CPU will return the dedicated VCPU on it with a negative time so that it disable the scheduler timer on that CPU and other CPUs will no longer send SCHEDULE_SOFTIRQ to the dedicated CPU. The scheduler on the dedicated CPU may still be invoked when the dedicated VCPU is blocked/unblocked by the domU. Once this situation occurs, the schedule go though a fast pass to just return an idle VCPU/the dedicated VCPU instead of go through the runq. I did the following evaluation to show the benefits of introducing this dedicated CPU feature: I created a simple cpu-intensive task which just do the multiplication for specified times: start = rdtsc(); while ( i++ cpu_measurement-multiply_times ) result += i * i; finish = rdtsc(); latencies[k] = finish - start; I run this task and measure the execution time of above piece of code on different environments: native Linux on bare metal, domU on Xen with RTDS scheduler and domU on Xen with RTDS scheduler with dedicated CPU feature, domU on Xen with Credit/Credit2 scheduler. The difference between the execution time in virtualization environment and the execution time on native linux on bare metal is the virtualization overhead introduced by Xen. I want to see that 1) The virtualization overhead decreases a lot after the dedicated CPU feature is employed for RTDS scheduler (because the execution of the task will no longer suffer the scheduler overhead any more). 2) The frequency of invoking the scheduler on the dedicated CPU becomes very low once the dedicated CPU feature is applied. The result is as follows: When the cpu-intensive task did the multiplication for 1024 times, the execution time of the piece of code is: 9264 cycles on native linux on bare metal; 10320 cycles on Xen RTDS scheduler with dedicated CPU feature; 10324 cycles on Xen RTDS scheduler without dedicated CPU feature; We didn't see the improvement of the dedicated CPU feature here because the execution time is too short and it may not experience the scheduler overhead yet. When the cpu-intensive task did the multiplication for 536870912 times, the execution time of the piece of code is: 4838016028 cycles on native linux on bare metal; 4839649567 cycles on Xen RTDS scheduler with dedicated CPU feature; 4855509977 cycles on Xen RTDS scheduler without dedicated CPU feature; We can see that the dedicated CPU feature did save time for the cpu-intensive task. Without the dedicated CPU feature, the hypervisor scheduler may steal time from the domU and delay the execution of the task inside domU. I did vary the number of multiplications of the above piece of code in cpu-intensive task, and draw a figure to show the relation of the overhead and the execution time of the task on native linux. The figure can be found at http://www.cis.upenn.edu/~mengxu/xen-ml/cpu-base-alone_multiply_0_0_100.virtOhVSwcetnative.pdf. Please note the x-axis is the log value of the execution time. So the overhead is actually linear to the execution time of the task. As to the frequence of invoking the RTDS scheduler with/without the dedicated CPU feature, I add some code to trace which event triggers the scheduler on the dedicated cpu and how frequent it is. Before we apply the dedicated CPU feature to the RTDS scheduler, the dedicated CPU 3 was invoked once every 3.5us in average. (XEN) cpu 3 has invoked 356805936 SCHED_SOFTIRQ (sched) within 1267613845122 ns (XEN) tasklet_enqueue(0), do_tasklet(0), s_timer_fn(356789129), do_pool(18) (XEN) vcpu_yield(0), vcpu_block(10483) After we apply the dedicated CPU feature to the RTDS scheduler, the dedicated CPU 3 was invoked once every 136ms in average. And the scheduler was invoked because of vcpu_block/vcpu_unblock event. (We could modify Linux in domU as Konrad suggests to avoid the hypercall when vcpu is blocked/unblocked, but I'm unsure if it is better to do that since it involves the change in domU.) (XEN) cpu 3 has invoked 5396 SCHED_SOFTIRQ (nooh) within 736973916783 ns (XEN) tasklet_enqueue(0), do_tasklet(0), s_timer_fn(0), do_pool(0) (XEN) vcpu_yield(0), vcpu_block(2698) Here are some conclusions/observation we have: 1) Dedicated CPU feature can save the scheduler overhead from domU and thus reduce the virtualization overhead. 2) Scheduler overhead of the current RTDS scheduler in the view of application is higher than the scheduler overhead of the current credit/credit2 scheduler because the
