linux-kernel - Re: [tip:sched/core] sched: Turn off child_runs

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Date:	Wed, 9 Sep 2009 21:48:52 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Theodore Tso <tytso@....edu>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	efault@....de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Turn off child_runs_first

Hi!

> > > It hurts 'optimal' make -j<NR_CPUS> workloads as make jobs
> > > get preempted by child tasks, reducing parallelism.
> > 
> > Wasn't one of the reasons why we historically did child_runs_first 
> > was so that for fork/exit workloads, the child has a chance to 
> > exec the new process?  If the parent runs first, then more pages 
> > will probably need to be COW'ed.
> 
> That kind of workload should be using vfork() anyway, and be even 
> faster because it can avoid the fork overhead, right?

Well.. one should not have to update userspace to keep
performance.... and vfork is extremely ugly interface.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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