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Re: GNU Parallel Bug Reports Auto-detection of available CPU cores

From: Johannes Dröge
Subject: Re: GNU Parallel Bug Reports Auto-detection of available CPU cores
Date: Tue, 24 Feb 2015 00:35:11 +0100
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Am 23.02.2015 um 21:48 schrieb Ole Tange:
I just tested this on a 32-core and a 64-core machine:

$ ssh cpu32 cat  /proc/self/status |grep -m 1 '^Cpus_allowed:'
Cpus_allowed:   ffffffff,ffffffff
$ ssh cpu64 cat  /proc/self/status |grep -m 1 '^Cpus_allowed:'
Cpus_allowed:   ffffffff,ffffffff

It does the right thing if you prepend with 'taskset ffff...', so it
should be possible to do min(existing_code,your_code).

I don't quite get what you mean but the calculation of the mask should be somewhere in the kernel docs and it should be valid for all architectures. You could probably port this code to Perl easily.

Is this a real problem or is it a theoretical problem?

When you are using CPU pinning with taskset, which seems to be the new standard, this will be a problem. For example, I believe containers like Docker see the full set of cores but will be restricted to a subset using the cpuset functionality. So if you run parallel in such containers, auto-detection will fail.

Gruß Johannes

Johannes Dröge, M.Sc.
Algorithmic Bioinformatics, Heinrich Heine University Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany
PGP: http://keys.fungs.de/6ea5e4.asc (55F2720303A7F236A94666F20E2360727A6EA5E4)
Web: algbio.cs.uni-duesseldorf.de | Tel/Fax: +49 211 81-12644/13464

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