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

Waiman Long
2015-06-25T20:33:48Z

On 06/25/2015 02:35 PM, Peter Zijlstra wrote:
> 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.

I saw that Ingo has merged a previous version of the patch. I am fine
with that version. As Will is working on a qrwlock patch to enable ARM
to use it, I will let him make the structure move to qrwlock.h if he
choose to do so.

Cheers,
Longman