From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
Date: Thu, 12 Jul 2012 11:15:03 +0300
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To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.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>
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On 07/11/2012 05:01 PM, Raghavendra K T wrote:
> 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.

The problem occurs even with no overcommit at all.  One vcpu is in a
legitimately long pause loop.  All those exits accomplish nothing, since
all vcpus are scheduled.  Better to let it spin in guest mode.

-- 
error compiling committee.c: too many arguments to function