linux-kernel - Re: Scheduler(?) regression from 2.6.22 to 2.6.24 for short-lived threads

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Date:	Mon, 11 Feb 2008 20:58:46 +0100
From:	Mike Galbraith <efault@....de>
To:	Olof Johansson <olof@...om.net>
Cc:	Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Scheduler(?) regression from 2.6.22 to 2.6.24 for short-lived
	threads

On Mon, 2008-02-11 at 11:26 -0600, Olof Johansson wrote: 
> On Mon, Feb 11, 2008 at 09:15:55AM +0100, Mike Galbraith wrote:
> > Piddling around with your testcase, it still looks to me like things
> > improved considerably in latest greatest git.  Hopefully that means
> > happiness is in the pipe for the real workload... synthetic load is
> > definitely happier here as burst is shortened.
> 
> The real workload doesn't see much of an improvement. The changes I did
> when tinkering yesterday seem like they're better at modelling just
> what's going on with that one.

So the real application is trying to yield?  If so, you could try
prodding /proc/sys/kernel/sched_compat_yield.

It shouldn't matter if you yield or not really, that should reduce the
number of non-work spin cycles wasted awaiting preemption as threads
execute in series (the problem), and should improve your performance
numbers, but not beyond single threaded.

If I plugged a yield into the busy wait, I would expect to see a large
behavioral difference due to yield implementation changes, but that
would only be a symptom in this case, no?  Yield should be a noop.

	-Mike

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