Re: lmbench ctxsw regression with CFS

Jens Axboe
2007-08-13T14:59:38Z

On Mon, Aug 06 2007, Nick Piggin wrote:
> > > 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 think a lot of CPUs do. I think ia64 does not. It predicts
> based on the contents of a branch target register which has to
> be loaded I presume before instructoin fetch reaches the branch.
> I don't know if this would hurt or not.

Testing on ia64 showed that the indirect calls in the io scheduler hurt
quite a bit, so I'd be surprised if the impact here wasn't an issue
there.

--
Jens Axboe