* Tim Chen <[email protected]> wrote:
> On Wed, 2013-06-26 at 14:36 -0700, Tim Chen wrote:
> > On Wed, 2013-06-26 at 11:51 +0200, Ingo Molnar wrote:
> > > * 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?
> > >
> >
> > Ingo,
> >
> > The previous experiment was done on a kernel without spinning.
> > I've redone the testing on two kernel for a 15 sec stretch of the
> > workload run. One with the adaptive (or optimistic)
> > spinning and the other without. Both have the patches from Alex to avoid
> > cmpxchg induced cache bouncing.
> >
> > With the spinning, I sleep much less for lock acquisition (18.6% vs 91.58%).
> > However, I've got doubling of write lock acquisition getting
> > blocked. So that offset the gain from spinning which may be why
> > I didn't see gain for this particular workload.
> >
> > No Opt Spin Opt Spin
> > Writer acquisition blocked count 3448946 7359040
> > Blocked by reader 0.00% 0.55%
> > Lock acquired first attempt (lock stealing) 8.42% 16.92%
> > Lock acquired second attempt (1 sleep) 90.26% 17.60%
> > Lock acquired after more than 1 sleep 1.32% 1.00%
> > Lock acquired with optimistic spin N/A 64.48%
> >
>
> Adding also the mutex statistics for the 3.10-rc4 kernel with mutex
> implemenation of lock for anon_vma tree. Wonder if Ingo has any
> insight on why mutex performs better from these stats.
>
> Mutex acquisition blocked count 14380340
> Lock acquired in slowpath (no sleep) 0.06%
> Lock acquired in slowpath (1 sleep) 0.24%
> Lock acquired in slowpath more than 1 sleep 0.98%
> Lock acquired with optimistic spin 99.6%
This is how I interpret the stats:
It does appear that in the mutex case we manage to acquire via spinning
with a very high percentage - i.e. it essentialy behaves as a spinlock.
That is actually good news in a way, because it makes it rather simple how
rwsems should behave in this case: since they have no substantial
read-locking aspect in this workload, the down_write()/up_write()s should
essentially behave like spinlocks as well, right?
Yet in the rwsem-spinning case the stats show that we only acquire the
lock via spinning in 65% of the cases, plus we lock-steal in 16.9% of the
cases:
Because lock stealing is essentially a single-spin spinning as well:
> > Lock acquired first attempt (lock stealing) ...... 16.92%
So rwsems in this case behave like spinlocks in 65%+16.9% == 81.9% of the
time.
What remains is the sleeping component:
> > Lock acquired second attempt (1 sleep) ...... 17.60%
Yet the 17.6% sleep percentage is still much higher than the 1% in the
mutex case. Why doesn't spinning work - do we time out of spinning
differently?
Is there some other aspect that defeats optimistic spinning and forces the
slowpath and creates sleeping, scheduling and thus extra overhead?
For example after a failed lock-stealing, do we still try optimistic
spinning to write-acquire the rwsem, or go into the slowpath and thus
trigger excessive context-switches?
Thanks,
Ingo
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