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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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).