Re: [PATCH v2 2/2] locking/qrwlock: Don't contend with readers when setting _QW_WAITING

[Ingo Molnar <mingo@kernel.org>]

* 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