Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

Thomas Gleixner
2013-11-25T19:52:29Z

On Mon, 25 Nov 2013, Davidlohr Bueso wrote:
> On Mon, 2013-11-25 at 18:32 +0100, Thomas Gleixner wrote:
> > If the smp_mb() is heavy weight, then it will hurt massivly in the
> > case where the hash bucket is not empty, because we add the price for
> > the smp_mb() just for no gain.
> >
> > In that context it would also be helpful to measure the overhead on
> > x86 for the !empty case.
>
> Absolutely, I will add these comparisons. If we do notice that we end up
> hurting the !empty case, would the current patch using atomic ops still
> be considered? We have made sure that none of the changes in this set
> affects performance on other workloads/smaller systems.

Please read my last reply to the atomic ops approach.

Aside of that we need numbers for a significant range of !x86.

Thanks,

tglx