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Date: Thu, 04 Oct 2012 09:41:03 -0500 From: Andrew Theurer <habanero@...ux.vnet.ibm.com> To: Avi Kivity <avi@...hat.com> Cc: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>, Rik van Riel <riel@...hat.com>, Peter Zijlstra <peterz@...radead.org>, "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>, Srikar <srikar@...ux.vnet.ibm.com>, "Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>, KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>, chegu vinod <chegu_vinod@...com>, LKML <linux-kernel@...r.kernel.org>, Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>, Gleb Natapov <gleb@...hat.com> Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler On Thu, 2012-10-04 at 14:41 +0200, Avi Kivity wrote: > On 10/04/2012 12:49 PM, Raghavendra K T wrote: > > On 10/03/2012 10:35 PM, Avi Kivity wrote: > >> On 10/03/2012 02:22 PM, Raghavendra K T wrote: > >>>> So I think it's worth trying again with ple_window of 20000-40000. > >>>> > >>> > >>> Hi Avi, > >>> > >>> I ran different benchmarks increasing ple_window, and results does not > >>> seem to be encouraging for increasing ple_window. > >> > >> Thanks for testing! Comments below. > >> > >>> Results: > >>> 16 core PLE machine with 16 vcpu guest. > >>> > >>> base kernel = 3.6-rc5 + ple handler optimization patch > >>> base_pleopt_8k = base kernel + ple window = 8k > >>> base_pleopt_16k = base kernel + ple window = 16k > >>> base_pleopt_32k = base kernel + ple window = 32k > >>> > >>> > >>> Percentage improvements of benchmarks w.r.t base_pleopt with > >>> ple_window = 4096 > >>> > >>> base_pleopt_8k base_pleopt_16k base_pleopt_32k > >>> ----------------------------------------------------------------- > >>> > >>> kernbench_1x -5.54915 -15.94529 -44.31562 > >>> kernbench_2x -7.89399 -17.75039 -37.73498 > >> > >> So, 44% degradation even with no overcommit? That's surprising. > > > > Yes. Kernbench was run with #threads = #vcpu * 2 as usual. Is it > > spending 8 times the original ple_window cycles for 16 vcpus > > significant? > > A PLE exit when not overcommitted cannot do any good, it is better to > spin in the guest rather that look for candidates on the host. In fact > when we benchmark we often disable PLE completely. Agreed. However, I really do not understand why the kernbench regressed with bigger ple_window. It should stay the same or improve. Raghu, do you have perf data for the kernbench runs? > > > > >> > >>> I also got perf top output to analyse the difference. Difference comes > >>> because of flushtlb (and also spinlock). > >> > >> That's in the guest, yes? > > > > Yes. Perf is in guest. > > > >> > >>> > >>> Ebizzy run for 4k ple_window > >>> - 87.20% [kernel] [k] arch_local_irq_restore > >>> - arch_local_irq_restore > >>> - 100.00% _raw_spin_unlock_irqrestore > >>> + 52.89% release_pages > >>> + 47.10% pagevec_lru_move_fn > >>> - 5.71% [kernel] [k] arch_local_irq_restore > >>> - arch_local_irq_restore > >>> + 86.03% default_send_IPI_mask_allbutself_phys > >>> + 13.96% default_send_IPI_mask_sequence_phys > >>> - 3.10% [kernel] [k] smp_call_function_many > >>> smp_call_function_many > >>> > >>> > >>> Ebizzy run for 32k ple_window > >>> > >>> - 91.40% [kernel] [k] arch_local_irq_restore > >>> - arch_local_irq_restore > >>> - 100.00% _raw_spin_unlock_irqrestore > >>> + 53.13% release_pages > >>> + 46.86% pagevec_lru_move_fn > >>> - 4.38% [kernel] [k] smp_call_function_many > >>> smp_call_function_many > >>> - 2.51% [kernel] [k] arch_local_irq_restore > >>> - arch_local_irq_restore > >>> + 90.76% default_send_IPI_mask_allbutself_phys > >>> + 9.24% default_send_IPI_mask_sequence_phys > >>> > >> > >> Both the 4k and the 32k results are crazy. Why is > >> arch_local_irq_restore() so prominent? Do you have a very high > >> interrupt rate in the guest? > > > > How to measure if I have high interrupt rate in guest? > > From /proc/interrupt numbers I am not able to judge :( > > 'vmstat 1' > > > > > I went back and got the results on a 32 core machine with 32 vcpu guest. > > Strangely, I got result supporting the claim that increasing ple_window > > helps for non-overcommitted scenario. > > > > 32 core 32 vcpu guest 1x scenarios. > > > > ple_gap = 0 > > kernbench: Elapsed Time 38.61 > > ebizzy: 7463 records/s > > > > ple_window = 4k > > kernbench: Elapsed Time 43.5067 > > ebizzy: 2528 records/s > > > > ple_window = 32k > > kernebench : Elapsed Time 39.4133 > > ebizzy: 7196 records/s > > So maybe something was wrong with the first measurement. OK, this is more in line with what I expected for kernbench. FWIW, in order to show an improvement for a larger ple_window, we really need a workload which we know has a longer lock holding time (without factoring in LHP). We have noticed this on IO based locks mostly. We saw it with a massive disk IO test (qla2xxx lock), and also with a large web serving test (some vfs related lock, but I forget what exactly it was). > > > > > > > perf top for ebizzy for above: > > ple_gap = 0 > > - 84.74% [kernel] [k] arch_local_irq_restore > > - arch_local_irq_restore > > - 100.00% _raw_spin_unlock_irqrestore > > + 50.96% release_pages > > + 49.02% pagevec_lru_move_fn > > - 6.57% [kernel] [k] arch_local_irq_restore > > - arch_local_irq_restore > > + 92.54% default_send_IPI_mask_allbutself_phys > > + 7.46% default_send_IPI_mask_sequence_phys > > - 1.54% [kernel] [k] smp_call_function_many > > smp_call_function_many > > Again the numbers are ridiculously high for arch_local_irq_restore. > Maybe there's a bad perf/kvm interaction when we're injecting an > interrupt, I can't believe we're spending 84% of the time running the > popf instruction. I do have a feeling that ebizzy just has too many variables and LHP is just one of many problems. However, am I curious what perf kvm from host shows as Avi suggested below. > > > > > ple_window = 32k > > - 84.47% [kernel] [k] arch_local_irq_restore > > + arch_local_irq_restore > > - 6.46% [kernel] [k] arch_local_irq_restore > > - arch_local_irq_restore > > + 93.51% default_send_IPI_mask_allbutself_phys > > + 6.49% default_send_IPI_mask_sequence_phys > > - 1.80% [kernel] [k] smp_call_function_many > > - smp_call_function_many > > + 99.98% native_flush_tlb_others > > > > > > ple_window = 4k > > - 91.35% [kernel] [k] arch_local_irq_restore > > - arch_local_irq_restore > > - 100.00% _raw_spin_unlock_irqrestore > > + 53.19% release_pages > > + 46.81% pagevec_lru_move_fn > > - 3.90% [kernel] [k] smp_call_function_many > > smp_call_function_many > > - 2.94% [kernel] [k] arch_local_irq_restore > > - arch_local_irq_restore > > + 93.12% default_send_IPI_mask_allbutself_phys > > + 6.88% default_send_IPI_mask_sequence_phys > > > > Let me know if I can try something here.. > > /me confused :( > > > > I'm even more confused. Please try 'perf kvm' from the host, it does > fewer dirty tricks with the PMU and so may be more accurate. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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