linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

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Message-ID: <4FFD874B.4090606@linux.vnet.ibm.com>
Date:	Wed, 11 Jul 2012 19:31:47 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	habanero@...ux.vnet.ibm.com, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

On 07/11/2012 07:29 PM, Raghavendra K T wrote:
> On 07/11/2012 02:30 PM, Avi Kivity wrote:
>> On 07/10/2012 12:47 AM, Andrew Theurer wrote:
>>>
>>> For the cpu threads in the host that are actually active (in this case
>>> 1/2 of them), ~50% of their time is in kernel and ~43% in guest. This
>>> is for a no-IO workload, so that's just incredible to see so much cpu
>>> wasted. I feel that 2 important areas to tackle are a more scalable
>>> yield_to() and reducing the number of pause exits itself (hopefully by
>>> just tuning ple_window for the latter).
>>
>> One thing we can do is autotune ple_window. If a ple exit fails to wake
>> anybody (because all vcpus are either running, sleeping, or in ple
>> exits) then we deduce we are not overcommitted and we can increase the
>> ple window. There's the question of how to decrease it again though.
>>
>
> I see some problem here, If I interpret situation correctly. What
> happens if we have two guests with one VM having no over-commit and
> other with high over-commit. (except when we have gang scheduling).
>
Sorry, I meant less load and high load inside the guest.

> Rather we should have something tied to VM rather than rigid PLE
> window.

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