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

[Marcelo Tosatti <mtosatti@redhat.com>]

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@redhat.com>