Messages in this thread | | | Date | Thu, 5 Jan 2017 23:40:34 +0800 | From | Boqun Feng <> | Subject | Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs |
| |
On Thu, Jan 05, 2017 at 10:17:46AM -0500, Waiman Long wrote: > On 01/05/2017 03:16 AM, Pan Xinhui wrote: > > > > > > 在 2017/1/4 17:41, Peter Zijlstra 写道: > >> 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 > > > Thanks for doing the test. It looks like we should just focus on using > either cmpxchg_release or cmpxchg_acquire and forget about cmpxchg_relaxed. >
_relaxed is more lightweight in two aspects:
1. _relaxed is not a compiler barrier, so it allows compilers do more optimizations than _acquire and _release.
2. _relaxed doesn't introduce HW barriers and doesn't performs worse than _acquire and _release in _most_ _usual_ cases.
And on some ll/sc archs without a lightweigt barrier like "lwsync" for _acquire and _release, #2 could be more significant.
So don't hesitate to use _relaxed whenever it's possible and reasonable ;-)
Regards, Boqun
> Cheers, > Longman > [unhandled content-type:application/pgp-signature] | |