linux-kernel - Re: getting processor numbers

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Message-ID: <4612ABD7.3000201@redhat.com>
Date:	Tue, 03 Apr 2007 12:32:39 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: getting processor numbers

Davide Libenzi wrote:
> It sucks when seen from a micro-bench POV, but does it really matter 
> overall? The vast majority of software usually calls 
> sysconf(_SC_NPROCESSORS_*) with very little frequency (mostly once at 
> initialization time) anyway. That's what 50us / call?

This is not today's situation.  Yes, 10 years ago when I added the
support to glibc it wasn't much of a problem.  But times change.  As I
said before in this thread, OpenMP by default scales the number of
threads used for a parallel loops depending on the number of available
processors/cores and therefore the number must be retrieved every time
(with perhaps minimal caching of a few secs, but this requires
gettimeofday calls...).  All of a sudden this is not micro benchmark
anymore.  It's a real issue which we only became aware of because it is
noticeable in real life.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

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