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SubjectRe: [PATCH v2 2/2] locking/qrwlock: Don't contend with readers when setting _QW_WAITING
On 06/10/2015 03:35 AM, Ingo Molnar wrote:
> * Waiman Long<Waiman.Long@hp.com> 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.
> Mind posting the microbenchmark?
>
> Thanks,
>
> Ingo

I have attached the tool that I used for testing.

Cheers,
Longman
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