Re: [PATCH V3 RESEND RFC 0/2] kvm: Improving undercommit scenarios

Gleb Natapov
2013-01-29T14:05:58Z

On Tue, Jan 22, 2013 at 01:08:54PM +0530, Raghavendra K T wrote:
> In some special scenarios like #vcpu <= #pcpu, PLE handler may
> prove very costly, because there is no need to iterate over vcpus
> and do unsuccessful yield_to burning CPU.
>
> The first patch optimizes all the yield_to by bailing out when there
> is no need to continue in yield_to (i.e., when there is only one task
> in source and target rq).
>
> Second patch uses that in PLE handler. Further when a yield_to fails
> we do not immediately go out of PLE handler instead we try thrice
> to have better statistical possibility of false return. Otherwise that
> would affect moderate overcommit cases.
>
> Result on 3.7.0-rc6 kernel shows around 140% improvement for ebizzy 1x and
> around 51% for dbench 1x with 32 core PLE machine with 32 vcpu guest.
>
>
> base = 3.7.0-rc6
> machine: 32 core mx3850 x5 PLE mc
>
> --+-----------+-----------+-----------+------------+-----------+
> ebizzy (rec/sec higher is beter)
> --+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> --+-----------+-----------+-----------+------------+-----------+
> 1x 2511.3000 21.5409 6051.8000 170.2592 140.98276
> 2x 2679.4000 332.4482 2692.3000 251.4005 0.48145
> 3x 2253.5000 266.4243 2192.1667 178.9753 -2.72169
> --+-----------+-----------+-----------+------------+-----------+
>
> --+-----------+-----------+-----------+------------+-----------+
> dbench (throughput in MB/sec. higher is better)
> --+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improve
> --+-----------+-----------+-----------+------------+-----------+
> 1x 6677.4080 638.5048 10098.0060 3449.7026 51.22643
> 2x 2012.6760 64.7642 2019.0440 62.6702 0.31639
> 3x 1302.0783 40.8336 1292.7517 27.0515 -0.71629
> --+-----------+-----------+-----------+------------+-----------+
>
> Here is the refernce of no ple result.
> ebizzy-1x_nople 7592.6000 rec/sec
> dbench_1x_nople 7853.6960 MB/sec
>
> The result says we can still improve by 60% for ebizzy, but overall we are
> getting impressive performance with the patches.
>
> Changes Since V2:
> - Dropped global measures usage patch (Peter Zilstra)
> - Do not bail out on first failure (Avi Kivity)
> - Try thrice for the failure of yield_to to get statistically more correct
> behaviour.
>
> Changes since V1:
> - Discard the idea of exporting nrrunning and optimize in core scheduler (Peter)
> - Use yield() instead of schedule in overcommit scenarios (Rik)
> - Use loadavg knowledge to detect undercommit/overcommit
>
> Peter Zijlstra (1):
> Bail out of yield_to when source and target runqueue has one task
>
> Raghavendra K T (1):
> Handle yield_to failure return for potential undercommit case
>
> Please let me know your comments and suggestions.
>
> Link for the discussion of V3 original:
> https://lkml.org/lkml/2012/11/26/166
>
> Link for V2:
> https://lkml.org/lkml/2012/10/29/287
>
> Link for V1:
> https://lkml.org/lkml/2012/9/21/168
>
> kernel/sched/core.c | 25 +++++++++++++++++++------
> virt/kvm/kvm_main.c | 26 ++++++++++++++++----------
> 2 files changed, 35 insertions(+), 16 deletions(-)
Applied, thanks.

--
Gleb.