linux-kernel - Re: [PATCH v6 00/11] locking/rwsem: Rework rwsem-xadd & enable new rwsem features

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Date:   Wed, 11 Oct 2017 16:57:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 00/11] locking/rwsem: Rework rwsem-xadd & enable new
 rwsem features

On 10/11/2017 04:50 PM, Dave Chinner wrote:
> On Wed, Oct 11, 2017 at 02:01:51PM -0400, Waiman Long wrote:
>> In term of rwsem performance, a rwsem microbenchmark and fio randrw
>> test with a xfs filesystem on a ramdisk were used to verify the
>> performance changes due to these patches. Both tests were run on a
>> 2-socket, 40-core Gold 6148 system. The rwsem microbenchmark (1:1
>> reader/writer ratio) has short critical section while the fio randrw
>> test has long critical section (4k read/write).
>>
>> The following table shows the performance of the rwsem microbenchmark
>> and fio radrw test with different number of patches applied on 4.14
>> based kernels:
>>
>>   # of Patches	Locking Rate	FIO Bandwidth	FIO Bandwidth
>>     Applied	 40 threads	 32 threads	 16 threads
>>   ------------	------------	-------------	-------------
>> 	0	  38.7 kop/s	  706 MB/s	  704 MB/s
>> 	7	  38.6 kop/s	  668 MB/s	  663 MB/s
>> 	8	  38.9 kop/s	  704 MB/s	  701 MB/s
>> 	9	  39.1 kop/s	  702 MB/s	  707 MB/s
>>        11	3218.0 kop/s	 2594 MB/s	 2614 MB/s
>>
>> So this patchset improves mixed read/write rwsem microbench by 83X
>> and randrw fio bandwidth by about 3.7X.
> Overall improvement in bandwidth is not necessarily a good thing -
> this could simply demonstrate total write bandwidth starvation and
> so it's only reporting read bandwith. It's much more important to
> look at the change in read bandwidth vs write bandwidth in the fio
> test. i.e. exactly how did the IO balance change as a result of
> changing the locking bias?

Thanks for the input. I can take out the reader lock stealing part. That
will give it a more fair reader/writer bias. It can also be an option
that be set when the rwsem is inited.

Cheers,
Longman