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

From: Paul Mackerras
Date: Fri Dec 05 2008 - 04:34:48 EST

Peter Zijlstra writes:

> On Fri, 2008-12-05 at 18:57 +1100, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> >
> > > So, while most people would not consider two consecutive read() ops to
> > > be close or near the same time, due to preemption and such, that is
> > > taken away by the fact that the counters are task local time based - so
> > > preemption doesn't affect thing. Right?
> >
> > I'm sorry, I don't follow the argument here. What do you mean by
> > "task local time based"?
> time only flows when the task is running.

Right - but the monitored task is running while the monitoring task is
running. So time is flowing for the monitored task between the two
reads done by the monitoring task, meaning that you can't actually
relate the two values you read with any precision.

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