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

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Date:	Wed, 9 Sep 2009 20:08:56 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	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


* Theodore Tso <tytso@....edu> wrote:

> On Wed, Sep 09, 2009 at 03:37:07PM +0000, tip-bot for Mike Galbraith wrote:
> > Commit-ID:  2bba22c50b06abe9fd0d23933b1e64d35b419262
> > Gitweb:     http://git.kernel.org/tip/2bba22c50b06abe9fd0d23933b1e64d35b419262
> > Author:     Mike Galbraith <efault@....de>
> > AuthorDate: Wed, 9 Sep 2009 15:41:37 +0200
> > Committer:  Ingo Molnar <mingo@...e.hu>
> > CommitDate: Wed, 9 Sep 2009 17:30:05 +0200
> > 
> > sched: Turn off child_runs_first
> > 
> > Set child_runs_first default to off.
> > 
> > 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?

Also, on SMP we do that anyway - there's good likelyhood on an idle 
system that we wake the child on the other core straight away.

	Ingo
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