Re: lmbench ctxsw regression with CFS

Ingo Molnar
2007-08-04T06:51:04Z

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

> Oh good. Thanks for getting to the bottom of it. We have normally
> disliked too much runtime tunables in the scheduler, so I assume these
> are mostly going away or under a CONFIG option for 2.6.23? Or...?

yeah, they are all already under CONFIG_SCHED_DEBUG. (it's just that the
add-on optimization is not upstream yet - the tunings are still being
tested) Btw., with SCHED_DEBUG we now also have your domain-tree sysctl
patch upstream, which has been in -mm for a near eternity.

> What CPU did you get these numbers on? Do the indirect calls hurt much
> on those without an indirect predictor? (I'll try running some tests).

it was on an older Athlon64 X2. I never saw indirect calls really
hurting on modern x86 CPUs - dont both CPU makers optimize them pretty
efficiently? (as long as the target function is always the same - which
it is here.)

> I must say that I don't really like the indirect calls a great deal,
> and they could be eliminated just with a couple of branches and direct
> calls.

yeah - i'll try that too. We can make the indirect call the uncommon
case and a NULL pointer be the common case, combined with a 'default',
direct function call. But i doubt it makes a big (or even measurable)
difference.

Ingo