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Date: Wed, 11 Jul 2012 19:29:42 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Avi Kivity <avi@redhat.com>
CC: habanero@linux.vnet.ibm.com, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Rik van Riel <riel@redhat.com>, S390 <linux-s390@vger.kernel.org>,
        Carsten Otte <cotte@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        KVM <kvm@vger.kernel.org>, chegu vinod <chegu_vinod@hp.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 <x86@kernel.org>,
        Gleb Natapov <gleb@redhat.com>, linux390@de.ibm.com,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Joerg Roedel <joerg.roedel@amd.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
References: <20120709062012.24030.37154.sendpatchset@codeblue> <1341870457.2909.27.camel@oc2024037011.ibm.com> <4FFD4091.8040804@redhat.com>
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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).

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