linux-kernel - Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

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Date:	Fri, 19 Nov 2010 15:55:05 +0100
From:	Samuel Thibault <samuel.thibault@...-lyon.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Hans-Peter Jansen <hpj@...la.net>,
	linux-kernel@...r.kernel.org,
	Lennart Poettering <mzxreary@...inter.de>, david@...g.hm,
	Dhaval Giani <dhaval.giani@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

Peter Zijlstra, le Fri 19 Nov 2010 15:43:13 +0100, a écrit :
> > MPI jobs typically communicate with each other. Keeping them on the same
> > socket permits to keep shared-memory MPI drivers to mostly remain in
> > e.g. the L3 cache. That typically gives benefits.
> 
> Pushing them away permits them to use a larger part of that same L3
> cache allowing them to work on larger data sets.

But then you are not benefitting from all CPU cores.

> Most of the MPI apps
> have a large compute to communication ratio because that is what allows
> them to run in parallel so well (traditionally the interconnects were
> terribly slow to boot), that suggests that working on larger data sets
> is a good thing and running on the same node really doesn't matter since
> communication is assumes slow anyway.

Err, if the compute to communication ratio is big, then you should use
all CPU cores, up to the point where communication becomes a matter
again, and making sure that related MPI processes end up on the same
socket will permit to got a it further.

> There really is no simple solution to his.

I never said there was even a solution, actually (in particular any kind
of generic solution), but that there are a a few simple ways exist to
make things better.

Samuel
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