Re: [PATCH] microoptimize by trying to avoid locking a locked mutex

On Thursday, November 05, 2015 01:45:19 PM Adrian Chadd wrote:
> On 5 November 2015 at 11:26, Mateusz Guzik <mjguzik_at_gmail.com> wrote:
> > On Thu, Nov 05, 2015 at 11:04:13AM -0800, John Baldwin wrote:
> >> On Thursday, November 05, 2015 04:26:28 PM Konstantin Belousov wrote:
> >> > On Thu, Nov 05, 2015 at 12:32:18AM +0100, Mateusz Guzik wrote:
> >> > > mtx_lock will unconditionally try to grab the lock and if that fails,
> >> > > will call __mtx_lock_sleep which will immediately try to do the same
> >> > > atomic op again.
> >> > >
> >> > > So, the obvious microoptimization is to check the state in
> >> > > __mtx_lock_sleep and avoid the operation if the lock is not free.
> >> > >
> >> > > This gives me ~40% speedup in a microbenchmark of 40 find processes
> >> > > traversing tmpfs and contending on mount mtx (only used as an easy
> >> > > benchmark, I have WIP patches to get rid of it).
> >> > >
> >> > > Second part of the patch is optional and just checks the state of the
> >> > > lock prior to doing any atomic operations, but it gives a very modest
> >> > > speed up when applied on top of the __mtx_lock_sleep change. As such,
> >> > > I'm not going to defend this part.
> >> > Shouldn't the same consideration applied to all spinning loops, i.e.
> >> > also to the spin/thread mutexes, and to the spinning parts of sx and
> >> > lockmgr ?
> >>
> >> I agree.  I think both changes are good and worth doing in our other
> >> primitives.
> >>
> >
> > I glanced over e.g. rw_rlock and it did not have the issue, now that I
> > see _sx_xlock_hard it wuld indeed use fixing.
> >
> > Expect a patch in few h for all primitives I'll find. I'll stress test
> > the kernel, but it is unlikely I'll do microbenchmarks for remaining
> > primitives.
> 
> Is this stuff you're proposing still valid for non-x86 platforms?

Yes.  It just does a read before trying the atomic compare and swap and
falls through to the hard case as if the atomic op failed if the result
of the read would result in a compare failure.

-- 
John Baldwin