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

Nick Piggin
2007-04-19T03:18:57Z

On Wed, Apr 18, 2007 at 07:48:21AM -0700, Linus Torvalds wrote:
>
>
> On Wed, 18 Apr 2007, Matt Mackall wrote:
> >
> > Why is X special? Because it does work on behalf of other processes?
> > Lots of things do this. Perhaps a scheduler should focus entirely on
> > the implicit and directed wakeup matrix and optimizing that
> > instead[1].
>
> I 100% agree - the perfect scheduler would indeed take into account where
> the wakeups come from, and try to "weigh" processes that help other
> processes make progress more. That would naturally give server processes
> more CPU power, because they help others
>
> I don't believe for a second that "fairness" means "give everybody the
> same amount of CPU". That's a totally illogical measure of fairness. All
> processes are _not_ created equal.

I believe that unless the kernel is told of these inequalities, then it
must schedule fairly.

And yes, by fairly, I mean fairly among all threads as a base resource
class, because that's what Linux has always done (and if you aggregate
into higher classes, you still need that per-thread scheduling).

So I'm not excluding extra scheduling classes like per-process, per-user,
but among any class of equal schedulable entities, fair scheduling is the
only option because the alternative of unfairness is just insane.

> That said, even trying to do "fairness by effective user ID" would
> probably already do a lot. In a desktop environment, X would get as much
> CPU time as the user processes, simply because it's in a different
> protection domain (and that's really what "effective user ID" means: it's
> not about "users", it's really about "protection domains").
>
> And "fairness by euid" is probably a hell of a lot easier to do than
> trying to figure out the wakeup matrix.

Well my X server has an euid of root, which would mean my X clients can
cause X to do work and eat into root's resources. Or as Ingo said, X
may not be running as root. Seems like just another hack to try to
implicitly solve the X problem and probably create a lot of others
along the way.

All fairness issues aside, in the context of keeping a very heavily
loaded desktop interactive, X is special. That you are trying to think
up funny rules that would implicitly give X better priority is kind of
indicative of that.