linux-kernel - Re: [RFC PATCH 00/11] sched: CFS low-latency features

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Message-ID: <20100827155013.GF14926@Krystal>
Date:	Fri, 27 Aug 2010 11:50:13 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC PATCH 00/11] sched: CFS low-latency features

* Thomas Gleixner (tglx@...utronix.de) wrote:
> On Thu, 26 Aug 2010, Mathieu Desnoyers wrote:
[...] 
> > Hrm, thinking about it a little more, one of the "plus" sides of these
> > SIGEV_THREAD timers is that a single timer can fork threads that will run on
> > many cores on a multi-core system. If we go for preallocation of a single
> > thread, we lose that. Maybe we could think of a way to preallocate a thread pool
> > instead ?
> 
> Why should a single timer fork many threads? Just because a previous
> thread did not complete before the timer fires again? That's
> braindamage as all threads call the same function which then needs to
> be serialized anyway. We really do not need a function which creates
> tons of threads which get all stuck on the same resource.

It could make sense if the workload is mostly CPU-bound and there is only a very
short critical section shared between the threads. But I agree that in many
cases this will generate an utter contention mess.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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