Re: [PATCH 0/7] locking/rwsem: enable reader opt-spinning & writer respin

[Waiman Long <waiman.long@hp.com>]

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