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

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Date:	Tue, 17 Apr 2007 16:39:54 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	William Lee Irwin III <wli@...omorphy.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Mike Galbraith <efault@....de>,
	Con Kolivas <kernel@...ivas.org>, Ingo Molnar <mingo@...e.hu>,
	ck list <ck@....kolivas.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Tue, Apr 17, 2007 at 09:01:55AM +0200, Nick Piggin wrote:
> On Mon, Apr 16, 2007 at 11:26:21PM -0700, William Lee Irwin III wrote:
> > On Mon, Apr 16, 2007 at 11:09:55PM -0700, William Lee Irwin III wrote:
> > >> All things are not equal; they all have different properties. I like
> > 
> > On Tue, Apr 17, 2007 at 08:15:03AM +0200, Nick Piggin wrote:
> > > Exactly. So we have to explore those properties and evaluate performance
> > > (in all meanings of the word). That's only logical.
> > 
> > Any chance you'd be willing to put down a few thoughts on what sorts
> > of standards you'd like to set for both correctness (i.e. the bare
> > minimum a scheduler implementation must do to be considered valid
> > beyond not oopsing) and performance metrics (i.e. things that produce
> > numbers for each scheduler you can compare to say "this scheduler is
> > better than this other scheduler at this.").
> 
> Yeah I guess that's the hard part :)
> 
> For correctness, I guess fairness is an easy one. I think that unfairness
> is basically a bug and that it would be very unfortunate to merge something
> unfair. But this is just within the context of a single runqueue... for
> better or worse, we allow some unfairness in multiprocessors for performance
> reasons of course.

I'm a big fan of fairness, but I think it's a bit early to declare it
a mandatory feature. Bounded unfairness is probably something we can
agree on, ie "if we decide to be unfair, no process suffers more than
a factor of x".
 
> Latency. Given N tasks in the system, an arbitrary task should get
> onto the CPU in a bounded amount of time (excluding events like freak
> IRQ holdoffs and such, obviously -- ie. just considering the context
> of the scheduler's state machine).

This is a slightly stronger statement than starvation-free (which is
obviously mandatory). I think you're looking for something like
"worst-case scheduling latency is proportional to the number of
runnable tasks". Which I think is quite a reasonable requirement.

I'm pretty sure the stock scheduler falls short of both of these
guarantees though.

-- 
Mathematics is the supreme nostalgia of our time.
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