Re: getting processor numbers

Ulrich Drepper
2007-04-03T19:32:52Z

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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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