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Message-ID: <20130626161130.GB18152@redhat.com> Date: Wed, 26 Jun 2013 19:11:30 +0300 From: Gleb Natapov <gleb@...hat.com> To: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> Cc: habanero@...ux.vnet.ibm.com, Andrew Jones <drjones@...hat.com>, mingo@...hat.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, ouyang@...pitt.edu, 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 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? -- 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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