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

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Message-ID: <5061B437.8070300@linux.vnet.ibm.com>
Date:	Tue, 25 Sep 2012 19:10:07 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler

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).





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