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

[Gleb Natapov <gleb@redhat.com>]

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.