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

Ingo Molnar
2007-10-08T11:42:46Z

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

> Some CPUs like K8 cannot predict indirect calls. A common optimization
> in object oriented languages is to check for the most common call
> target and then call it directly; otherwise do an indirect call. This
> patch does this manually for sched_fair calls in sched.c
[...]

> I unfortunately wasn't able to measure a consistent difference in
> lmbench3 lat_ctx -- the change seems to be below its (large)
> instability.

> + * Simple devirtualization:
> + * Most tasks are fair tasks. Avoid the overhead of
> + * doing indirect calls for them by checking for the fair case and doing
> + * a direct call. Use only in real fast paths since it generates larger code.
> + */
> +#define __CLASS_CALL(class, name, args) \
> + ((class) == &fair_sched_class ? \
> + name ## _fair args : (class)->name args)
> +#define CLASS_CALL(p, name, args) \
> + __CLASS_CALL((p)->sched_class, name, args)

hm, i'm not convinced about this one. It increases the code size a bit
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.
Then the benefit might become measurable too.

Ingo