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Date: Wed, 25 May 2016 10:10:12 +0800 From: Yang Zhang <yang.zhang.wz@...il.com> To: David Matlack <dmatlack@...gle.com>, Wanpeng Li <kernellwp@...il.com> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, kvm list <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>, Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, Christian Borntraeger <borntraeger@...ibm.com> Subject: Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers On 2016/5/25 7:37, David Matlack wrote: > On Tue, May 24, 2016 at 4:11 PM, Wanpeng Li <kernellwp@...il.com> wrote: >> 2016-05-25 6:38 GMT+08:00 David Matlack <dmatlack@...gle.com>: >>> On Tue, May 24, 2016 at 12:57 AM, Wanpeng Li <kernellwp@...il.com> wrote: >>>> From: Wanpeng Li <wanpeng.li@...mail.com> >>>> >>>> If an emulated lapic timer will fire soon(in the scope of 10us the >>>> base of dynamic halt-polling, lower-end of message passing workload >>>> latency TCP_RR's poll time < 10us) we can treat it as a short halt, >>>> and poll to wait it fire, the fire callback apic_timer_fn() will set >>>> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll. >>>> This can avoid context switch overhead and the latency which we wake >>>> up vCPU. >>>> >>>> This feature is slightly different from current advance expiration >>>> way. Advance expiration rely on the vCPU is running(do polling before >>>> vmentry). But in some cases, the timer interrupt may be blocked by >>>> other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to >>>> run immediately. So even advance the timer early, vCPU may still see >>>> the latency. But polling is different, it ensures the vCPU to aware >>>> the timer expiration before schedule out. >>>> >>>> echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests. >>>> >>>> Context switching - times in microseconds - smaller is better >>>> ------------------------------------------------------------------------- >>>> Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K >>>> ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw >>>> --------- ------------- ------ ------ ------ ------ ------ ------- ------- >>>> kernel Linux 4.6.0+ 7.9800 11.0 10.8 14.6 9.4300 13.0 10.2 vanilla >>>> kernel Linux 4.6.0+ 15.3 13.6 10.7 12.5 9.0000 12.8 7.38000 poll >>> >>> These results aren't very compelling. Sometimes polling is faster, >>> sometimes vanilla is faster, sometimes they are about the same. >> >> More processes and bigger cache footprints can get benefit from the >> result since I open the hrtimer for the precision preemption. > > The VCPU is halted (idle), so the timer interrupt is not preempting > anything. Also I would not expect any preemption in a context > switching benchmark, the threads should be handing off execution to > one another. > > I'm confused why timers would play any role in the performance of this > benchmark. Any idea why there's a speedup in the 8p/16K and 16p/64K > runs? > >> Actually >> I try to emulate Yang's workload, https://lkml.org/lkml/2016/5/22/162. >> And his real workload can get more benefit as he mentioned, >> https://lkml.org/lkml/2016/5/19/667. >> >>> I imagine there are hyper sensitive workloads which cannot tolerate a >>> long tail in timer latency (e.g. realtime workloads). I would expect a >>> patch like this to provide a "smoothing effect", reducing that tail. >>> But for cloud/server workloads, I would not expect any sensitivity to >>> jitter in timer latency (especially while the VCPU is halted). >> >> Yang's is real cloud workload. > > I have 2 issues with optimizing for Yang's workload. Yang, please > correct me if I am mis-characterizing it. > 1. The delay in timer interrupts is caused by something disabling the > interrupts on the CPU for more than a millisecond. It seems that is > the real issue. I'm wary of using polling as a workaround. Yes, this is the most likely case. > 2. The delay is caused by a separate task. Halt-polling would not help > in that scenario, it would yield the CPU to that task. In some cases, the separate task is migrated from other CPU after CPU enter idle state. So Halt-polling may still help. And the delay is caused by two context switches(VCPU schedule out and migrate VCPU to another idle CPU). > >> >>> >>> Note that while halt-polling happens when the CPU is idle, it's still >>> not free. It constricts the scheduler's cpu load balancer, because the >>> CPU appears to be busy. In KVM's default configuration, I'd prefer to >>> only add more polling when the gain is clear. If there are guest >>> workloads that want this patch, I'd suggest polling for timers be >>> default-off. At minimum, there should be a module parameter to control >>> it (like Christian Borntraeger suggested). >> >> Yeah, I will add the module parameter in order to enable/disable. >> >> Regards, >> Wanpeng Li -- best regards yang
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