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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 <> 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,

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

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