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

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Date:	Fri, 27 Aug 2010 10:19:52 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [RFC PATCH 00/11] sched: CFS low-latency features

On Fri, 2010-08-27 at 09:42 +0200, Peter Zijlstra wrote:
> On Thu, 2010-08-26 at 19:49 -0400, Mathieu Desnoyers wrote:
> > AFAIK, I don't think we would end up starving the system in any possible way.
> 
> Correct, it does maintain fairness.
> 
> > So far I cannot see a situation where selecting the next buddy would _not_ make
> > sense in any kind of input-driven wakeups (interactive, timer, disk, network,
> > etc). But maybe it's just a lack of imagination on my part. 
> 
> The risk is that you end up with always using next-buddy, and we tried
> that a while back and that didn't work well for some, Mike might
> remember.

I turned it off because it was ripping spread apart badly, and last
buddy did a better job of improving scalability without it.

> Also, when you use timers things like time-outs you really couldn't care
> less if its handled sooner rather than later.
> 
> Disk is usually so slow you really don't want to consider it
> interactive, but then sometimes you might,.. its a really hard problem.

(very hard)

> The only clear situation is the direct input, that's a direct link
> between the user and our wakeup chain and the user is always important.

Yeah, directly linked wakeups using next could be a good thing, but the
trouble with using any linkage to the user is that you have to pass it
on to reap benefit.. so when do you disconnect?

	-Mike

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