On Sat, Apr 13, 2019 at 01:22:53PM -0400, Waiman Long wrote: > When the front of the wait queue is a reader, other readers > immediately following the first reader will also be woken up at the > same time. However, if there is a writer in between. Those readers > behind the writer will not be woken up. > > Because of optimistic spinning, the lock acquisition order is not FIFO > anyway. The lock handoff mechanism will ensure that lock starvation > will not happen. > > Assuming that the lock hold times of the other readers still in the > queue will be about the same as the readers that are being woken up, > there is really not much additional cost other than the additional > latency due to the wakeup of additional tasks by the waker. Therefore > all the readers up to a maximum of 256 in the queue are woken up when > the first waiter is a reader to improve reader throughput. > > With a locking microbenchmark running on 5.1 based kernel, the total > locking rates (in kops/s) on a 8-socket IvyBridge-EX system with > equal numbers of readers and writers before and after this patch were > as follows: > > # of Threads Pre-Patch Post-patch > ------------ --------- ---------- > 4 1,641 1,674 > 8 731 1,062 > 16 564 924 > 32 78 300 > 64 38 195 > 240 50 149 > > There is no performance gain at low contention level. At high contention > level, however, this patch gives a pretty decent performance boost.
Right, so this basically completes the convertion from task-fair (FIFO) to phase-fair. https://cs.unc.edu/~anderson/papers/rtsj10-for-web.pdf
