linux-kernel - Re: Measuring cpu time using getrusage

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Message-ID: <Pine.LNX.4.63.0702171302570.20161@gockel.physik3.uni-rostock.de>
Date:	Sat, 17 Feb 2007 13:09:58 +0100 (CET)
From:	Tim Schmielau <tim@...sik3.uni-rostock.de>
To:	Johann Schuster <johann.schuster@...bw.de>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Measuring cpu time using getrusage

On Wed, 14 Feb 2007, Johann Schuster wrote:

> currently i'm trying to measure the cpu times for
> numerical algorithms written in C using two calls of
> getrusage() and then calculating the time difference.
> 
> According to the man page of getrusage i looked at the
> user time of my process. Surprisingly the measured user
> times vary strongly depending on the system load of the
> machine.
> A heavy system load nearly doubles the measured time
> for my routine.

Does your system have hyperthreading? (look at whether the siblings field 
in /proc/cpuinfo is greater than one)
If yes, there is no way to accurately measure runtimes because the 
(single-threaded) performance of your CPU depends on the other tasks 
running. Just disable hyperthreading for these measurements.

Tim
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