Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

[Avi Kivity <avi@redhat.com>]

On 09/25/2012 03:40 PM, Raghavendra K T wrote:
> On 09/24/2012 07:46 PM, Raghavendra K T wrote:
>> On 09/24/2012 07:24 PM, Peter Zijlstra wrote:
>>> On Mon, 2012-09-24 at 18:59 +0530, Raghavendra K T wrote:
>>>> However Rik had a genuine concern in the cases where runqueue is not
>>>> equally distributed and lockholder might actually be on a different run
>>>> queue but not running.
>>>
>>> Load should eventually get distributed equally -- that's what the
>>> load-balancer is for -- so this is a temporary situation.
>>>
>>> We already try and favour the non running vcpu in this case, that's what
>>> yield_to_task_fair() is about. If its still not eligible to run, tough
>>> luck.
>>
>> Yes, I agree.
>>
>>>
>>>> Do you think instead of using rq->nr_running, we could get a global
>>>> sense of load using avenrun (something like avenrun/num_onlinecpus)
>>>
>>> To what purpose? Also, global stuff is expensive, so you should try and
>>> stay away from it as hard as you possibly can.
>>
>> Yes, that concern only had made me to fall back to rq->nr_running.
>>
>> Will come back with the result soon.
> 
> Got the result with the patches:
> So here is the result,
> 
> Tried this on a 32 core ple box with HT disabled. 32 guest vcpus with
> 1x and 2x overcommits
> 
> Base = 3.6.0-rc5 + ple handler optimization patches
> A = Base + checking rq_running in vcpu_on_spin() patch
> B = Base + checking rq->nr_running in sched/core
> C = Base - PLE
> 
> ---+-----------+-----------+-----------+-----------+
>    |    Ebizzy result (rec/sec higher is better)   |
> ---+-----------+-----------+-----------+-----------+
>    |    Base   |     A     |      B    |     C     |
> ---+-----------+-----------+-----------+-----------+
> 1x | 2374.1250 | 7273.7500 | 5690.8750 |  7364.3750|
> 2x | 2536.2500 | 2458.5000 | 2426.3750 |    48.5000|
> ---+-----------+-----------+-----------+-----------+
> 
>    % improvements w.r.t BASE
> ---+------------+------------+------------+
>    |      A     |    B       |     C      |
> ---+------------+------------+------------+
> 1x | 206.37603  |  139.70410 |  210.19323 |
> 2x | -3.06555   |  -4.33218  |  -98.08773 |
> ---+------------+------------+------------+
> 
> we are getting the benefit of almost PLE disabled case with this
> approach. With patch B, we have dropped a bit in gain.
> (because we still would iterate vcpus until we decide to do a directed
> yield).

This gives us a good case for tracking preemption on a per-vm basis.  As
long as we aren't preempted, we can keep the PLE window high, and also
return immediately from the handler without looking for candidates.


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