linux-kernel - Re: [PATCH v8 0/9] rwsem performance optimizations

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Date:	Thu, 10 Oct 2013 09:54:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Waiman Long <Waiman.Long@...com>, linux-kernel@...r.kernel.org,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v8 0/9] rwsem performance optimizations


* Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> The throughput of pure mmap with mutex is below vs pure mmap is below:
> 
> % change in performance of the mmap with pthread-mutex vs pure mmap
> #threads        vanilla 	all rwsem    	without optspin
> 				patches
> 1               3.0%    	-1.0%   	-1.7%
> 5               7.2%    	-26.8%  	5.5%
> 10              5.2%    	-10.6%  	22.1%
> 20              6.8%    	16.4%   	12.5%
> 40              -0.2%   	32.7%   	0.0%
> 
> So with mutex, the vanilla kernel and the one without optspin both run 
> faster.  This is consistent with what Peter reported.  With optspin, the 
> picture is more mixed, with lower throughput at low to moderate number 
> of threads and higher throughput with high number of threads.

So, going back to your orignal table:

> % change in performance of the mmap with pthread-mutex vs pure mmap
> #threads        vanilla all     without optspin
> 1               3.0%    -1.0%   -1.7%
> 5               7.2%    -26.8%  5.5%
> 10              5.2%    -10.6%  22.1%
> 20              6.8%    16.4%   12.5%
> 40              -0.2%   32.7%   0.0%
>
> In general, vanilla and no-optspin case perform better with 
> pthread-mutex.  For the case with optspin, mmap with pthread-mutex is 
> worse at low to moderate contention and better at high contention.

it appears that 'without optspin' appears to be a pretty good choice - if 
it wasn't for that '1 thread' number, which, if I correctly assume is the 
uncontended case, is one of the most common usecases ...

How can the single-threaded case get slower? None of the patches should 
really cause noticeable overhead in the non-contended case. That looks 
weird.

It would also be nice to see the 2, 3, 4 thread numbers - those are the 
most common contention scenarios in practice - where do we see the first 
improvement in performance?

Also, it would be nice to include a noise/sttdev figure, it's really hard 
to tell whether -1.7% is statistically significant.

Thanks,

	Ingo
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