Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

[Pan Xinhui]

å 2017/1/4 17:41, Peter Zijlstra åé:
> On Tue, Jan 03, 2017 at 05:07:54PM -0500, Waiman Long wrote:
> > On 01/03/2017 11:18 AM, Peter Zijlstra wrote:
> > > On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
> > > > A number of cmpxchg calls in qspinlock_paravirt.h were replaced by more
> > > > relaxed versions to improve performance on architectures that use LL/SC.
> > > Claim without numbers ;-)
> >
> > Well it is hard to produce actual numbers here as I don't have the setup
> > to gather data.
>
> Surely RHT has big PPC machines around? I know that getting to them is a
> wee bit of a bother, but they should be available somewhere.
hi,

I do some tests about cmpxchg and cmpxchg_acquire before on ppc.

loops in 15s of each cmpxchg is below.

cmpxchg_relaxed: 336663
cmpxchg_release: 369054
cmpxchg_acquire: 363364
cmpxchg:	 179435

so cmpxchg is really expensive than others.
but I also have doubt about the cmpxchg_relaxed, it should be the cheapest, but from the tests, release/acquire are faster than it.

thanks
xinhui