Date: Wed, 9 Sep 2009 21:48:52 +0200
From: Pavel Machek <pavel@ucw.cz>
To: Ingo Molnar <mingo@elte.hu>
Cc: Theodore Tso <tytso@mit.edu>, mingo@redhat.com, hpa@zytor.com,
       linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, efault@gmx.de,
       tglx@linutronix.de, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:sched/core] sched: Turn off child_runs_first
Message-ID: <20090909194849.GA499@atrey.karlin.mff.cuni.cz>
References: <1252486344.28645.18.camel@marge.simson.net> <tip-2bba22c50b06abe9fd0d23933b1e64d35b419262@git.kernel.org> <20090909175724.GY22901@mit.edu> <20090909180856.GA7323@elte.hu>
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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.

-- 
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