Re: [PATCH v4 10/16] locking/rwsem: Wake up almost all readers in wait queue

[Peter Zijlstra]

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