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SubjectRe: [PATCH v2 0/3] KVM: LAPIC: Implement Exitless Timer
On Thu, 6 Jun 2019 at 13:31, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> Dedicated instances are currently disturbed by unnecessary jitter due
> to the emulated lapic timers fire on the same pCPUs which vCPUs resident.
> There is no hardware virtual timer on Intel for guest like ARM. Both
> programming timer in guest and the emulated timer fires incur vmexits.
> This patchset tries to avoid vmexit which is incurred by the emulated
> timer fires in dedicated instance scenario.
>
> When nohz_full is enabled in dedicated instances scenario, the unpinned
> timer will be moved to the nearest busy housekeepers after commit 444969223c8
> ("sched/nohz: Fix affine unpinned timers mess"). However, KVM always makes
> lapic timer pinned to the pCPU which vCPU residents, the reason is explained
> by commit 61abdbe0 (kvm: x86: make lapic hrtimer pinned). Actually, these
> emulated timers can be offload to the housekeeping cpus since APICv
> is really common in recent years. The guest timer interrupt is injected by
> posted-interrupt which is delivered by housekeeping cpu once the emulated
> timer fires.
>
> The host admin should fine tuned, e.g. dedicated instances scenario w/
> nohz_full cover the pCPUs which vCPUs resident, several pCPUs surplus
> for housekeeping, disable mwait/hlt/pause vmexits to occupy the pCPUs,
> fortunately preemption timer is disabled after mwait is exposed to
> guest which makes emulated timer offload can be possible.
> 3%~5% redis performance benefit can be observed on Skylake server.

w/o patchset:

VM-EXIT Samples Samples% Time%
Min Time Max Time Avg time

EXTERNAL_INTERRUPT 42916 49.43% 39.30% 0.47us
106.09us 0.71us ( +- 1.09% )

w/ patchset:

VM-EXIT Samples Samples% Time%
Min Time Max Time Avg time

EXTERNAL_INTERRUPT 6871 9.29% 2.96% 0.44us
57.88us 0.72us ( +- 4.02% )

Regards,
Wanpeng Li

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