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

Matt Mackall
2007-04-17T21:53:51Z

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.