linux-kernel - Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

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Message-ID: <20070417082417.GT8915@holomorphy.com>
Date:	Tue, 17 Apr 2007 01:24:17 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"Michael K. Edwards" <medwards.linux@...il.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Ingo Molnar <mingo@...e.hu>, Matt Mackall <mpm@...enic.com>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

On Mon, Apr 16, 2007 at 04:10:59PM -0700, Michael K. Edwards wrote:
>> This observation of Peter's is the best thing to come out of this
>> whole foofaraw.  Looking at what's happening in CPU-land, I think it's
>> going to be necessary, within a couple of years, to replace the whole
>> idea of "CPU scheduling" with "run queue scheduling" across a complex,
>> possibly dynamic mix of CPU-ish resources.  Ergo, there's not much
>> point in churning the mainline scheduler through a design that isn't
>> significantly more flexible than any of those now under discussion.

On Tue, Apr 17, 2007 at 05:55:28AM +0200, Nick Piggin wrote:
> Why? If you do that, then your load balancer just becomes less flexible
> because it is harder to have tasks run on one or the other.

On Tue, Apr 17, 2007 at 05:55:28AM +0200, Nick Piggin wrote:
> You can have single-runqueue-per-domain behaviour (or close to) just by
> relaxing all restrictions on idle load balancing within that domain. It
> is harder to go the other way and place any per-cpu affinity or
> restirctions with multiple cpus on a single runqueue.

The big sticking point here is order-sensitivity. One can point to
stringent sched_yield() ordering but that's not so important in and of
itself. The more significant case is RT applications which are order-
sensitive. Per-cpu runqueues rather significantly disturb the ordering
requirements of applications that care about it.

In terms of a plugging framework, the per-cpu arrangement precludes or
makes extremely awkward scheduling policies that don't have per-cpu
runqueues, for instance, the 2.4.x policy. There is also the alternate
SMP scalability strategy of a lockless scheduler with a single global
queue, which is more performance-oriented.


-- wli
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