* Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> 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 <tim.c.chen@xxxxxxxxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>