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SubjectRe: [PATCH 0/7] locking/rwsem: enable reader opt-spinning & writer respin
On 08/04/2014 12:25 AM, Davidlohr Bueso wrote:
> On Sun, 2014-08-03 at 22:36 -0400, Waiman Long wrote:
>> This patch set improves upon the rwsem optimistic spinning patch set
>> from Davidlohr to enable better performing rwsem and more aggressive
>> use of optimistic spinning.
>>
>> By using a microbenchmark running 1 million lock-unlock operations per
>> thread on a 4-socket 40-core Westmere-EX x86-64 test machine running
>> 3.16-rc7 based kernels, the following table shows the execution times
>> with 2/10 threads running on different CPUs on the same socket where
>> load is the number of pause instructions in the critical section:
>>
>> lock/r:w ratio # of threads Load:Execution Time (ms)
>> -------------- ------------ ------------------------
>> mutex 2 1:530.7, 5:406.0, 10:472.7
>> mutex 10 1:1848 , 5:2046 , 10:4394
>>
>> Before patch:
>> rwsem/0:1 2 1:339.4, 5:368.9, 10:394.0
>> rwsem/1:1 2 1:2915 , 5:2621 , 10:2764
>> rwsem/10:1 2 1:891.2, 5:779.2, 10:827.2
>> rwsem/0:1 10 1:5618 , 5:5722 , 10:5683
>> rwsem/1:1 10 1:14562, 5:14561, 10:14770
>> rwsem/10:1 10 1:5914 , 5:5971 , 10:5912
>>
>> After patch:
>> rwsem/0:1 2 1:161.1, 5:244.4, 10:271.4
>> rwsem/1:1 2 1:188.8, 5:212.4, 10:312.9
>> rwsem/10:1 2 1:168.8, 5:179.5, 10:209.8
>> rwsem/0:1 10 1:1306 , 5:1733 , 10:1998
>> rwsem/1:1 10 1:1512 , 5:1602 , 10:2093
>> rwsem/10:1 10 1:1267 , 5:1458 , 10:2233
>>
>> % Change:
>> rwsem/0:1 2 1:-52.5%, 5:-33.7%, 10:-31.1%
>> rwsem/1:1 2 1:-93.5%, 5:-91.9%, 10:-88.7%
>> rwsem/10:1 2 1:-81.1%, 5:-77.0%, 10:-74.6%
>> rwsem/0:1 10 1:-76.8%, 5:-69.7%, 10:-64.8%
>> rwsem/1:1 10 1:-89.6%, 5:-89.0%, 10:-85.8%
>> rwsem/10:1 10 1:-78.6%, 5:-75.6%, 10:-62.2%
> So at a very low level you see nicer results, which aren't really
> translating to much of a significant impact at a higher level (aim7).

I was using a 4-socket system for testing. I believe the performance
gain will be higher on larger machine. I will run some tests on those
larger machine as well.
>> It can be seen that there is dramatic reduction in the execution
>> times. The new rwsem is now even faster than mutex whether it is all
>> writers or a mixture of writers and readers.
>>
>> Running the AIM7 benchmarks on the same 40-core system (HT off),
>> the performance improvements on some of the workloads were as follows:
>>
>> Workload Before Patch After Patch % Change
>> -------- ------------ ----------- --------
>> custom (200-1000) 446135 477404 +7.0%
>> custom (1100-2000) 449665 484734 +7.8%
>> high_systime 152437 154217 +1.2%
>> (200-1000)
>> high_systime 269695 278942 +3.4%
>> (1100-2000)
> I worry about complicating rwsems even _more_ than they are, specially
> for such a marginal gain. You might want to try other workloads -- ie:
> postgresql (pgbench), I normally get pretty useful data when dealing
> with rwsems.
>

Thank for the info. I will try running pgbench as well.

-Longman


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