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SubjectRe: CFS review

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, 28 Aug 2007, Al Boldi wrote:
> >
> > I like your analysis, but how do you explain that these stalls
> > vanish when __update_curr is disabled?
>
> It's entirely possible that what happens is that the X scheduling is
> just a slightly unstable system - which effectively would turn a small
> scheduling difference into a *huge* visible difference.

i think it's because disabling __update_curr() in essence removes the
ability of scheduler to preempt tasks - that hack in essence results in
a non-scheduler. Hence the gears + X pair of tasks becomes a synchronous
pair of tasks in essence - and thus gears cannot "overload" X.

Normally gears + X is an asynchronous pair of tasks, with gears (or
xperf, or devel versions of firefox, etc.) not being throttled at all
and thus being able to overload/spam the X server with requests. (And we
generally want to _reward_ asynchronity and want to allow tasks to
overlap each other and we want each task to go as fast and as parallel
as it can.)

Eventually X's built-in "bad, abusive client" throttling code kicks in,
which, AFAIK is pretty crude and might yield to such artifacts. But ...
it would be nice for an X person to confirm - and in any case i'll try
Al's workload - i thought i had a reproducer but i barked up the wrong
tree :-) My laptop doesnt run with the vesa driver, so i have no easy
reproducer for now.

( also, it would be nice if Al could try rc4 plus my latest scheduler
tree as well - just on the odd chance that something got fixed
meanwhile. In particular Mike's sleeper-bonus-limit fix could be
related. )

Ingo
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