Re: [patch 0/3] [Announcement] Performance Counters for Linux

From: Ingo Molnar
Date: Fri Dec 05 2008 - 07:13:35 EST

* David Miller <davem@xxxxxxxxxxxxx> wrote:

> From: Ingo Molnar <mingo@xxxxxxx>
> Date: Fri, 5 Dec 2008 09:42:33 +0100
> > Please let me repeat: it's a _fundamental_ thesis of performance
> > instrumentation to not disturb the monitored context. Your insistence
> > on _stopping_ the monitored task breaks that fundamental axiom!
> This is only a problem if you make your measurement quantums too small.

But if you make the measurement long enough - say we make it 100,000
usecs, then 0.2 usecs of delay between two read()s is insignificant
statistically, right? It's a 1:500,000 ratio.

Scheduling out a task and back is far more drastic of an effect than any
new events in 0.2 usecs.

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