Messages in this thread | | | Date | Wed, 10 Apr 2019 21:38:44 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH-tip v3 04/14] locking/rwsem: Implement lock handoff to prevent lock starvation |
| |
Hurph, I was still looking at v2.. I suppose I'll go stare at this verison, I don't think you said there were many changes, right?
This version seems to still suffer that HANDOFF issue I found on v2.
On Wed, Apr 10, 2019 at 02:42:21PM -0400, Waiman Long wrote: > Because of writer lock stealing, it is possible that a constant > stream of incoming writers will cause a waiting writer or reader to > wait indefinitely leading to lock starvation. > > The mutex code has a lock handoff mechanism to prevent lock starvation. > This patch implements a similar lock handoff mechanism to disable > lock stealing and force lock handoff to the first waiter in the queue > after at least a 4ms waiting period unless it is a RT writer task which > doesn't need to wait. The waiting period is used to avoid discouraging > lock stealing too much to affect performance. > > A rwsem microbenchmark was run for 5 seconds on a 2-socket 40-core > 80-thread Skylake system with a v5.1 based kernel and 240 write_lock > threads with 5us sleep critical section. > > Before the patch, the min/mean/max numbers of locking operations for > the locking threads were 1/7,792/173,696. After the patch, the figures > became 5,842/6,542/7,458. It can be seen that the rwsem became much > more fair, though there was a drop of about 16% in the mean locking > operations done which was a tradeoff of having better fairness. > > Making the waiter set the handoff bit right after the first wakeup can
What does 'right after the first wakeup' mean? If that the top-waiter setting it if it fails to acquire the lock due to steals?
> impact performance especially with a mixed reader/writer workload. With > the same microbenchmark with short critical section and equal number of > reader and writer threads (40/40), the reader/writer locking operation > counts with the current patch were: > > 40 readers, Iterations Min/Mean/Max = 1,793/1,794/1,796 > 40 writers, Iterations Min/Mean/Max = 1,793/34,956/86,081 > > By making waiter set handoff bit immediately after wakeup: > > 40 readers, Iterations Min/Mean/Max = 43/44/46 > 40 writers, Iterations Min/Mean/Max = 43/1,263/3,191
| |