linux-kernel - Re: [kvm-devel] [PATCH 0/2][KVM] guest time accounting

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Date:	Mon, 13 Aug 2007 10:13:28 +0200
From:	Laurent Vivier <Laurent.Vivier@...l.net>
To:	Avi Kivity <avi@...ranet.com>
Cc:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [kvm-devel] [PATCH 0/2][KVM] guest time accounting

Avi Kivity wrote:
> Laurent Vivier wrote:
>> The aim of these two patches is to measure the CPU time used by a virtual
>> machine. All comments are welcome... I'm not sure it's the good way to
>> do that.
>>
>> [PATCH 1/2] introduce a new field, "guest", in cpustat to store the
>> time used by
>> the CPU to run virtual CPU. Modify /proc/stat to display this new field.
>>
>> [PATCH 2/2] modify account_system_time() to add cputime to
>> cpustat->guest if we
>> are running a VCPU. We add this cputime to  cpustat->user instead of
>> cpustat->system because this part of KVM code is in fact user code
>> although it
>> is executed in the kernel. We duplicate VCPU time between guest and
>> user to
>> allow an unmodified "top(1)" to display correct value. A modified
>> "top(1)" is
>> able to display good cpu user time and cpu guest time by subtracting
>> cpu guest
>> time from cpu user time.
>>   
> 
> [copying Ingo and Rusty]
> 
> The patches look good.  A couple of comments:
> 
> - perhaps the new fields should be guarded by a #ifdef CONFIG_HYPERVISOR
> (selected by CONFIG_KVM)?  that way the (minor) additional overhead is
> only incurred if it can possibly be used.  I imagine that our canine
> cousin will want to use this as well.

There is also a CONFIG_VIRTUALIZATION and a CONFIG_VIRT_CPU_ACCOUNTING (from
s390 and powerpc) Which one to use ?

I'm wondering if we can have a more accurate accounting:

- For the moment we add all system time since the previous entering to the VCPU
to the guest time (and I guess there is some real system time in it ???)

- Perhaps we can sum nanoseconds spent in the VCPU and add it to cpustat when
these ns are greater than 1 ms ? (I'm trying to make something in this way)

> - I think that there is per-task accounting of user time and system
> time; that should be extended as well.

it should be easy to do too...

Laurent
-- 
------------- Laurent.Vivier@...l.net  --------------
          "Software is hard" - Donald Knuth


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