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

[Waiman Long]

On 04/17/2019 09:39 AM, Peter Zijlstra wrote:
> 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

Right, the changes that I am making is similar in concept to the
phase-fair rwlock mentioned in the article. That is an interesting
article even though I was not aware of it before you brought it up.

Cheers,
Longman