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Date: Wed, 10 Jul 2013 13:33:25 +0300 From: Gleb Natapov <gleb@...hat.com> To: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> Cc: Andrew Jones <drjones@...hat.com>, mingo@...hat.com, ouyang@...pitt.edu, habanero@...ux.vnet.ibm.com, jeremy@...p.org, x86@...nel.org, konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com, linux-doc@...r.kernel.org, xen-devel@...ts.xensource.com, peterz@...radead.org, mtosatti@...hat.com, stefano.stabellini@...citrix.com, andi@...stfloor.org, attilio.rao@...rix.com, gregkh@...e.de, agraf@...e.de, chegu_vinod@...com, torvalds@...ux-foundation.org, avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, stephan.diestelhorst@....com, riel@...hat.com, virtualization@...ts.linux-foundation.org, srivatsa.vaddagiri@...il.com Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks On Tue, Jul 09, 2013 at 02:41:30PM +0530, Raghavendra K T wrote: > On 06/26/2013 11:24 PM, Raghavendra K T wrote: > >On 06/26/2013 09:41 PM, Gleb Natapov wrote: > >>On Wed, Jun 26, 2013 at 07:10:21PM +0530, Raghavendra K T wrote: > >>>On 06/26/2013 06:22 PM, Gleb Natapov wrote: > >>>>On Wed, Jun 26, 2013 at 01:37:45PM +0200, Andrew Jones wrote: > >>>>>On Wed, Jun 26, 2013 at 02:15:26PM +0530, Raghavendra K T wrote: > >>>>>>On 06/25/2013 08:20 PM, Andrew Theurer wrote: > >>>>>>>On Sun, 2013-06-02 at 00:51 +0530, Raghavendra K T wrote: > >>>>>>>>This series replaces the existing paravirtualized spinlock > >>>>>>>>mechanism > >>>>>>>>with a paravirtualized ticketlock mechanism. The series provides > >>>>>>>>implementation for both Xen and KVM. > >>>>>>>> > >>>>>>>>Changes in V9: > >>>>>>>>- Changed spin_threshold to 32k to avoid excess halt exits that are > >>>>>>>> causing undercommit degradation (after PLE handler > >>>>>>>>improvement). > >>>>>>>>- Added kvm_irq_delivery_to_apic (suggested by Gleb) > >>>>>>>>- Optimized halt exit path to use PLE handler > >>>>>>>> > >>>>>>>>V8 of PVspinlock was posted last year. After Avi's suggestions > >>>>>>>>to look > >>>>>>>>at PLE handler's improvements, various optimizations in PLE > >>>>>>>>handling > >>>>>>>>have been tried. > >>>>>>> > >>>>>>>Sorry for not posting this sooner. I have tested the v9 > >>>>>>>pv-ticketlock > >>>>>>>patches in 1x and 2x over-commit with 10-vcpu and 20-vcpu VMs. I > >>>>>>>have > >>>>>>>tested these patches with and without PLE, as PLE is still not > >>>>>>>scalable > >>>>>>>with large VMs. > >>>>>>> > >>>>>> > >>>>>>Hi Andrew, > >>>>>> > >>>>>>Thanks for testing. > >>>>>> > >>>>>>>System: x3850X5, 40 cores, 80 threads > >>>>>>> > >>>>>>> > >>>>>>>1x over-commit with 10-vCPU VMs (8 VMs) all running dbench: > >>>>>>>---------------------------------------------------------- > >>>>>>> Total > >>>>>>>Configuration Throughput(MB/s) Notes > >>>>>>> > >>>>>>>3.10-default-ple_on 22945 5% CPU in host > >>>>>>>kernel, 2% spin_lock in guests > >>>>>>>3.10-default-ple_off 23184 5% CPU in host > >>>>>>>kernel, 2% spin_lock in guests > >>>>>>>3.10-pvticket-ple_on 22895 5% CPU in host > >>>>>>>kernel, 2% spin_lock in guests > >>>>>>>3.10-pvticket-ple_off 23051 5% CPU in host > >>>>>>>kernel, 2% spin_lock in guests > >>>>>>>[all 1x results look good here] > >>>>>> > >>>>>>Yes. The 1x results look too close > >>>>>> > >>>>>>> > >>>>>>> > >>>>>>>2x over-commit with 10-vCPU VMs (16 VMs) all running dbench: > >>>>>>>----------------------------------------------------------- > >>>>>>> Total > >>>>>>>Configuration Throughput Notes > >>>>>>> > >>>>>>>3.10-default-ple_on 6287 55% CPU host > >>>>>>>kernel, 17% spin_lock in guests > >>>>>>>3.10-default-ple_off 1849 2% CPU in host > >>>>>>>kernel, 95% spin_lock in guests > >>>>>>>3.10-pvticket-ple_on 6691 50% CPU in host > >>>>>>>kernel, 15% spin_lock in guests > >>>>>>>3.10-pvticket-ple_off 16464 8% CPU in host > >>>>>>>kernel, 33% spin_lock in guests > >>>>>> > >>>>>>I see 6.426% improvement with ple_on > >>>>>>and 161.87% improvement with ple_off. I think this is a very good > >>>>>>sign > >>>>>> for the patches > >>>>>> > >>>>>>>[PLE hinders pv-ticket improvements, but even with PLE off, > >>>>>>> we still off from ideal throughput (somewhere >20000)] > >>>>>>> > >>>>>> > >>>>>>Okay, The ideal throughput you are referring is getting around > >>>>>>atleast > >>>>>>80% of 1x throughput for over-commit. Yes we are still far away from > >>>>>>there. > >>>>>> > >>>>>>> > >>>>>>>1x over-commit with 20-vCPU VMs (4 VMs) all running dbench: > >>>>>>>---------------------------------------------------------- > >>>>>>> Total > >>>>>>>Configuration Throughput Notes > >>>>>>> > >>>>>>>3.10-default-ple_on 22736 6% CPU in host > >>>>>>>kernel, 3% spin_lock in guests > >>>>>>>3.10-default-ple_off 23377 5% CPU in host > >>>>>>>kernel, 3% spin_lock in guests > >>>>>>>3.10-pvticket-ple_on 22471 6% CPU in host > >>>>>>>kernel, 3% spin_lock in guests > >>>>>>>3.10-pvticket-ple_off 23445 5% CPU in host > >>>>>>>kernel, 3% spin_lock in guests > >>>>>>>[1x looking fine here] > >>>>>>> > >>>>>> > >>>>>>I see ple_off is little better here. > >>>>>> > >>>>>>> > >>>>>>>2x over-commit with 20-vCPU VMs (8 VMs) all running dbench: > >>>>>>>---------------------------------------------------------- > >>>>>>> Total > >>>>>>>Configuration Throughput Notes > >>>>>>> > >>>>>>>3.10-default-ple_on 1965 70% CPU in host > >>>>>>>kernel, 34% spin_lock in guests > >>>>>>>3.10-default-ple_off 226 2% CPU in host > >>>>>>>kernel, 94% spin_lock in guests > >>>>>>>3.10-pvticket-ple_on 1942 70% CPU in host > >>>>>>>kernel, 35% spin_lock in guests > >>>>>>>3.10-pvticket-ple_off 8003 11% CPU in host > >>>>>>>kernel, 70% spin_lock in guests > >>>>>>>[quite bad all around, but pv-tickets with PLE off the best so far. > >>>>>>> Still quite a bit off from ideal throughput] > >>>>>> > >>>>>>This is again a remarkable improvement (307%). > >>>>>>This motivates me to add a patch to disable ple when pvspinlock is > >>>>>>on. > >>>>>>probably we can add a hypercall that disables ple in kvm init patch. > >>>>>>but only problem I see is what if the guests are mixed. > >>>>>> > >>>>>> (i.e one guest has pvspinlock support but other does not. Host > >>>>>>supports pv) > >>>>> > >>>>>How about reintroducing the idea to create per-kvm ple_gap,ple_window > >>>>>state. We were headed down that road when considering a dynamic > >>>>>window at > >>>>>one point. Then you can just set a single guest's ple_gap to zero, > >>>>>which > >>>>>would lead to PLE being disabled for that guest. We could also revisit > >>>>>the dynamic window then. > >>>>> > >>>>Can be done, but lets understand why ple on is such a big problem. > >>>>Is it > >>>>possible that ple gap and SPIN_THRESHOLD are not tuned properly? > >>>> > >>> > >>>The one obvious reason I see is commit awareness inside the guest. for > >>>under-commit there is no necessity to do PLE, but unfortunately we do. > >>> > >>>atleast we return back immediately in case of potential undercommits, > >>>but we still incur vmexit delay. > >>But why do we? If SPIN_THRESHOLD will be short enough (or ple windows > >>long enough) to not generate PLE exit we will not go into PLE handler > >>at all, no? > >> > > > >Yes. you are right. dynamic ple window was an attempt to solve it. > > > >Probelm is, reducing the SPIN_THRESHOLD is resulting in excess halt > >exits in under-commits and increasing ple_window may be sometimes > >counter productive as it affects other busy-wait constructs such as > >flush_tlb AFAIK. > >So if we could have had a dynamically changing SPIN_THRESHOLD too, that > >would be nice. > > > > Gleb, Andrew, > I tested with the global ple window change (similar to what I posted > here https://lkml.org/lkml/2012/11/11/14 ), This does not look global. It changes PLE per vcpu. > But did not see good result. May be it is good to go with per VM > ple_window. > > Gleb, > Can you elaborate little more on what you have in mind regarding per > VM ple_window. (maintaining part of it as a per vm variable is clear > to > me), but is it that we have to load that every time of guest entry? > Only when it changes, shouldn't be to often no? > I 'll try that idea next. > > Ingo, Gleb, > > From the results perspective, Andrew Theurer, Vinod's test results are > pro-pvspinlock. > Could you please help me to know what will make it a mergeable > candidate?. > I need to spend more time reviewing it :) The problem with PV interfaces is that they are easy to add but hard to get rid of if better solution (HW or otherwise) appears. > I agree that Jiannan's Preemptable Lock idea is promising and we could > evaluate that approach, and make the best one get into kernel and also > will carry on discussion with Jiannan to improve that patch. That would be great. The work is stalled from what I can tell. > Experiments so far have been good for smaller machine but it is not > scaling for bigger machines. -- Gleb. -- 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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