* Tim Chen <[email protected]> wrote:
> On Wed, 2013-06-19 at 09:53 -0700, Tim Chen wrote:
> > On Wed, 2013-06-19 at 15:16 +0200, Ingo Molnar wrote:
> >
> > > > vmstat for mutex implementation:
> > > > procs -----------memory---------- ---swap-- -----io---- --system--
> > > > -----cpu-----
> > > > r b swpd free buff cache si so bi bo in cs us sy
> > > > id wa st
> > > > 38 0 0 130957920 47860 199956 0 0 0 56 236342
> > > > 476975 14 72 14 0 0
> > > > 41 0 0 130938560 47860 219900 0 0 0 0 236816
> > > > 479676 14 72 14 0 0
> > > >
> > > > vmstat for rw-sem implementation (3.10-rc4)
> > > > procs -----------memory---------- ---swap-- -----io---- --system--
> > > > -----cpu-----
> > > > r b swpd free buff cache si so bi bo in cs us sy
> > > > id wa st
> > > > 40 0 0 130933984 43232 202584 0 0 0 0 321817
> > > > 690741 13 71 16 0 0
> > > > 39 0 0 130913904 43232 224812 0 0 0 0 322193
> > > > 692949 13 71 16 0 0
> > >
> > > It appears the main difference is that the rwsem variant context-switches
> > > about 36% more than the mutex version, right?
> > >
> > > I'm wondering how that's possible - the lock is mostly write-locked,
> > > correct? So the lock-stealing from Davidlohr Bueso and Michel Lespinasse
> > > ought to have brought roughly the same lock-stealing behavior as mutexes
> > > do, right?
> > >
> > > So the next analytical step would be to figure out why rwsem
> > > lock-stealing
> > > is not behaving in an equivalent fashion on this workload. Do readers
> > > come
> > > in frequently enough to disrupt write-lock-stealing perhaps?
>
> Ingo,
>
> I did some instrumentation on the write lock failure path. I found that
> for the exim workload, there are no readers blocking for the rwsem when
> write locking failed. The lock stealing is successful for 9.1% of the
> time and the rest of the write lock failure caused the writer to go to
> sleep. About 1.4% of the writers sleep more than once. Majority of the
> writers sleep once.
>
> It is weird that lock stealing is not successful more often.
For this to be comparable to the mutex scalability numbers you'd have to
compare wlock-stealing _and_ adaptive spinning for failed-wlock rwsems.
Are both techniques applied in the kernel you are running your tests on?
Thanks,
Ingo
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