linux-kernel - Re: getting processor numbers

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Message-Id: <20070403131623.c6831607.akpm@linux-foundation.org>
Date:	Tue, 3 Apr 2007 13:16:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: getting processor numbers

On Tue, 03 Apr 2007 09:54:46 -0700
Ulrich Drepper <drepper@...hat.com> wrote:

> More and more code depends on knowing the number of processors in the
> system to efficiently scale the code.  E.g., in OpenMP it is used by
> default to determine how many threads to create.  Creating more threads
> than there are processors/cores doesn't make sense.

but...  It would be a mistake for an application to assume that it is
allowed to _use_ all the present CPUs.  People can and do run applications
within cpusets, and under sched_setaffinity().

So I'd have thought that in general an application should be querying its
present affinity mask - something like sched_getaffinity()?  That fixes the
CPU hotplug issues too, of course.

But we discussed this all a couple years back and it was decided that
sched_getaffinity() was unsuitable.  I remember at the time not really
understanding why?

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