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    SubjectRe: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]
    On Tue, Apr 17, 2007 at 11:38:31PM -0500, Matt Mackall wrote:
    > On Wed, Apr 18, 2007 at 05:15:11AM +0200, Nick Piggin wrote:
    > >
    > > I don't know why this would be a useful feature (of course I'm talking
    > > about processes at the same nice level). One of the big problems with
    > > the current scheduler is that it is unfair in some corner cases. It
    > > works OK for most people, but when it breaks down it really hurts. At
    > > least if you start with a fair scheduler, you can alter priorities
    > > until it satisfies your need... with an unfair one your guess is as
    > > good as mine.
    > >
    > > So on what basis would you allow unfairness? On the basis that it doesn't
    > > seem to harm anyone? It doesn't seem to harm testers?
    > On the basis that there's only anecdotal evidence thus far that
    > fairness is the right approach.
    > It's not yet clear that a fair scheduler can do the right thing with X,
    > with various kernel threads, etc. without fiddling with nice levels.
    > Which makes it no longer "completely fair".

    Of course I mean SCHED_OTHER tasks at the same nice level. Otherwise
    I would be arguing to make nice basically a noop.

    > It's also not yet clear that a scheduler can't be taught to do the
    > right thing with X without fiddling with nice levels.

    Being fair doesn't prevent that. Implicit unfairness is wrong though,
    because it will bite people.

    What's wrong with allowing X to get more than it's fair share of CPU
    time by "fiddling with nice levels"? That's what they're there for.

    > So I'm just not yet willing to completely rule out systems that aren't
    > "completely fair".
    > But I think we should rule out schedulers that don't have rigid bounds on
    > that unfairness. That's where the really ugly behavior lies.

    Been a while since I really looked at the mainline scheduler, but I
    don't think it can permanently starve something, so I don't know what
    your bounded unfairness would help with.
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