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

[Chegu Vinod <chegu_vinod@hp.com>]

On 11/26/2012 4:07 AM, 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
> 4x   1784.3750   102.2699    2018.7500   187.5723    13.13485
> --+-----------+-----------+-----------+------------+-----------+
>
> --+-----------+-----------+-----------+------------+-----------+
>          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
> 4x  3043.1725  3243.7281    4664.4662   5946.5741      53.27643
> --+-----------+-----------+-----------+------------+-----------+
>
> 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 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(-)
>
> .
>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>