Re: [PATCH] [3/6] scheduler: Do devirtualization for sched_fair

Ingo Molnar
2007-10-08T12:39:50Z

* Andi Kleen <ak@suse.de> wrote:

> > hm, i'm not convinced about this one. It increases the code size a
> > bit
>
> Tiny bit (<200 bytes) and the wait_for/sleep_on refactor patch in the
> series saves over 1K so I should have some room for code size
> increase. Overall it will be still considerable smaller.

there's no forced dependency between those two patches :-) So for now
i've applied the one that saves text and skipped the one that bloats it.

> > and it's a sched.c local hack. If then this should be done on a
> > generic infrastructure level - lots of other code (VFS, networking,
> > etc.) could benefit from it i suspect - and then should be
> > .configurable as well.
>
> Unfortunately not -- for this to work (especially for inlining)
> requires to
> #include files implementing the sub calls. Except for the scheduler
> #that
> is pretty uncommon unfortunately. Also the situation regarding which
> call target is the common one is typically much less clear than with
> sched_fair / other scheduling classes.

some workloads would call sched_fair uncommon too. To me this seems like
a workaround for the lack of a particular hardware feature.

> > Then the benefit might become measurable too.
>
> It might have been measurable if the context switch was measurable at
> all. Unfortunately the lmbench3 lat_ctx test I tired fluctuated by
> itself over 50%. Ok I suppose it would be possible to instrument the
> kernel itself to measure cycles. Would that convince you?

dunno, it would depend on the numbers. But really, in most workloads we
do a lot more VFS indirect calls than scheduler indirect calls. So if
this was an issue i'd really suggest to attack it in a generic way.

Ingo