Re: [PATCH v5 3/3] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

Peter Zijlstra
2015-06-25T18:35:31Z

On Fri, Jun 19, 2015 at 11:50:02AM -0400, Waiman Long wrote:
> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
> in queue_write_lock_slowpath() will contend with incoming readers
> causing possibly extra cmpxchg() operations that are wasteful. This
> patch changes the code to do a byte cmpxchg() to eliminate contention
> with new readers.
>
> A multithreaded microbenchmark running 5M read_lock/write_lock loop
> on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel
> with the qspinlock patch have the following execution times (in ms)
> with and without the patch:
>
> With R:W ratio = 5:1
>
> Threads w/o patch with patch % change
> ------- --------- ---------- --------
> 2 990 895 -9.6%
> 3 2136 1912 -10.5%
> 4 3166 2830 -10.6%
> 5 3953 3629 -8.2%
> 6 4628 4405 -4.8%
> 7 5344 5197 -2.8%
> 8 6065 6004 -1.0%
> 9 6826 6811 -0.2%
> 10 7599 7599 0.0%
> 15 9757 9766 +0.1%
> 20 13767 13817 +0.4%
>
> With small number of contending threads, this patch can improve
> locking performance by up to 10%. With more contending threads,
> however, the gain diminishes.
>
> With the extended qrwlock structure defined in asm-generic/qrwlock,
> the queue_write_unlock() function is also simplified to a
> smp_store_release() call.
>
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>

This one does not in fact apply, seeing how I applied a previous
version.

Please send an incremental patch if you still want to change things to
this form.