Re: [PATCH 0/2] locking/mutex: Enable optimistic spinning of lock waiter

[Jason Low]

On Tue, 2016-02-09 at 14:47 -0500, Waiman Long wrote:
> This patchset is a variant of PeterZ's "locking/mutex: Avoid spinner
> vs waiter starvation" patch. The major difference is that the
> waiter-spinner won't enter into the OSQ used by the spinners. Instead,
> it will spin directly on the lock in parallel with the queue head
> of the OSQ. So there will be a bit more cacheline contention on the
> lock cacheline, but that shouldn't cause noticeable impact on system
> performance.
> 
> This patchset tries to address 2 issues with Peter's patch:
> 
>  1) Ding Tianhong still find that hanging task could happen in some cases.
>  2) Jason Low found that there was performance regression for some AIM7
>     workloads.

This might help address the hang that Ding reported.

Performance wise, this patchset reduced AIM7 fserver throughput on the 8
socket machine by -70%+ at 1000+ users.

                | fserver JPM
-----------------------------
baseline	| ~450000
Peter's patch	| ~410000
This patchset	| ~100000

My guess is that waiters spinning/acquiring the lock is less efficient,
and this patchset further increases the chance for waiters to
spin/acquire the lock over the fastpath optimistic spinners.

Jason