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

Ingo Molnar
2007-04-17T09:52:09Z

* Nick Piggin <npiggin@suse.de> wrote:

> > > Maybe the progress is that more key people are becoming open to
> > > the idea of changing the scheduler.
> >
> > Could be. All was quiet for quite a while, but when RSDL showed up,
> > it aroused enough interest to show that scheduling woes is on folks
> > radar.
>
> Well I know people have had woes with the scheduler for ever (I guess
> that isn't going to change :P). [...]

yes, that part isnt going to change, because the CPU is a _scarce
resource_ that is perhaps the most frequently overcommitted physical
computer resource in existence, and because the kernel does not (yet)
track eye movements of humans to figure out which tasks are more
important them. So critical human constraints are unknown to the
scheduler and thus complaints will always come.

The upstream scheduler thought it had enough information: the sleep
average. So now the attempt is to go back and _simplify_ the scheduler
and remove that information, and concentrate on getting fairness
precisely right. The magic thing about 'fairness' is that it's a pretty
good default policy if we decide that we simply have not enough
information to do an intelligent choice.

( Lets be cautious though: the jury is still out whether people actually
like this more than the current approach. While CFS feedback looks
promising after a whopping 3 days of it being released [ ;-) ], the
test coverage of all 'fairness centric' schedulers, even considering
years of availability is less than 1% i'm afraid, and that < 1% was
mostly self-selecting. )

Ingo