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SubjectRe: [RFC][PATCH v2 4/7] taskstats: Add per task steal time accounting
On Mon, 15 Nov 2010 18:50:41 +0100
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Mon, 2010-11-15 at 18:42 +0100, Martin Schwidefsky wrote:
> > The steal time of a task tells us how much more progress a task could have
> > done if the hypervisor would not steal cpu. Now you could argue that the
> > steal time for a cpu is good enough for that purpose but steal time is not
> > necessarily uniform over all tasks. And we already do calculate this number,
> > we just do not store it right now.
>
> If you make the scheduler take steal time into account like Jeremy
> proposed then you schedule on serviced time and the steal time gain is
> proportional to the existing service distribution.
>
> Still, then you know, then what are you going to do about it? Are you
> going to avoid the hypervisor from scheduling when that one task is
> running?
>
> What good is knowing something you cannot do anything about.

Steal time per task is at least good for performance problem analysis.
Sometimes knowing what is not the cause of a performance problem can help you
tremendously. If a task is slow and has no steal time, well then the hypervisor
is likely not the culprit. On the other hand if you do see lots of steal time
for a task while the rest of the system doesn't cause any steal time can tell
you something as well. That task might hit a specific function which causes
hypervisor overhead. The usefulness depends on the situation, it is another
data point which may or may not help you.

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.



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