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

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Date:	Fri, 19 Nov 2010 15:24:18 +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 12:57:24 +0100, a écrit :
> On Fri, 2010-11-19 at 01:07 +0100, Samuel Thibault wrote:
> > Also note that having a hierarchical process structure should permit to
> > make things globally more efficient: avoid putting e.g. your cpp, cc1,
> > and asm processes at three corners of your 4-socket NUMA machine :) 
> 
> And no, using that to load-balance between CPUs doesn't necessarily help
> with the NUMA case,

It doesn't _necessarily_ help, but it should help in quite a few cases.

> load-balancing is an impossible job (equivalent to
> page-replacement -- you simply don't know the future), applications
> simply do wildly weird stuff. 

Sure. Not a reason not to get the low-hanging fruits :)

> From a process hierarchy there's absolutely no difference between a
> cc1/cpp/asm and some MPI jobs, both can be parent-child relations with
> pipes between, some just run short and have data affinity, others run
> long and don't have any.

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.

Samuel
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