Re: [PATCH RFC 0/2] kvm: Better yield_to candidate using preemptionnotifiers

From: Marcelo Tosatti
Date: Thu Mar 07 2013 - 14:11:34 EST


On Mon, Mar 04, 2013 at 11:31:46PM +0530, Raghavendra K T wrote:
> This patch series further filters better vcpu candidate to yield to
> in PLE handler. The main idea is to record the preempted vcpus using
> preempt notifiers and iterate only those preempted vcpus in the
> handler. Note that the vcpus which were in spinloop during pause loop
> exit are already filtered.
>
> Thanks Jiannan, Avi for bringing the idea and Gleb, PeterZ for
> precious suggestions during the discussion.
> Thanks Srikar for suggesting to avoid rcu lock while checking task state
> that has improved overcommit cases.
>
> There are basically two approches for the implementation.
>
> Method 1: Uses per vcpu preempt flag (this series).
>
> Method 2: We keep a bitmap of preempted vcpus. using this we can easily
> iterate over preempted vcpus.
>
> Note that method 2 needs an extra index variable to identify/map bitmap to
> vcpu and it also needs static vcpu allocation.
>
> I am also posting Method 2 approach for reference in case it interests.
>
> Result: decent improvement for kernbench and ebizzy.
>
> base = 3.8.0 + undercommit patches
> patched = base + preempt patches
>
> Tested on 32 core (no HT) mx3850 machine with 32 vcpu guest 8GB RAM
>
> --+-----------+-----------+-----------+------------+-----------+
> kernbench (exec time in sec lower is beter)
> --+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> --+-----------+-----------+-----------+------------+-----------+
> 1x 47.0383 4.6977 44.2584 1.2899 5.90986
> 2x 96.0071 7.1873 91.2605 7.3567 4.94401
> 3x 164.0157 10.3613 156.6750 11.4267 4.47561
> 4x 212.5768 23.7326 204.4800 13.2908 3.80888
> --+-----------+-----------+-----------+------------+-----------+
> no ple kernbench 1x result for reference: 46.056133
>
> --+-----------+-----------+-----------+------------+-----------+
> ebizzy (record/sec higher is better)
> --+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> --+-----------+-----------+-----------+------------+-----------+
> 1x 5609.2000 56.9343 6263.7000 64.7097 11.66833
> 2x 2071.9000 108.4829 2653.5000 181.8395 28.07085
> 3x 1557.4167 109.7141 1993.5000 166.3176 28.00043
> 4x 1254.7500 91.2997 1765.5000 237.5410 40.70532
> --+-----------+-----------+-----------+------------+-----------+
> no ple ebizzy 1x result for reference : 7394.9 rec/sec
>
> Please let me know if you have any suggestions and comments.
>
> Raghavendra K T (2):
> kvm: Record the preemption status of vcpus using preempt notifiers
> kvm: Iterate over only vcpus that are preempted

Reviewed-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

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