linux-kernel - Re: [LKP] [futex] 76835b0ebf8: -8.1% will-it-scale.per_thread

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Date:	Thu, 26 Feb 2015 09:58:13 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, LKP ML <lkp@...org>
Subject: Re: [LKP] [futex] 76835b0ebf8: -8.1% will-it-scale.per_thread_ops

On Thu, Feb 26, 2015 at 05:16:53AM +0000, Huang Ying wrote:
> FYI, we noticed the below changes on
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> commit 76835b0ebf8a7fe85beb03c75121419a7dec52f0 ("futex: Ensure get_futex_key_refs() always implies a barrier")
> 
> 
> testbox/testcase/testparams: lkp-wsx01/will-it-scale/performance-futex4
> 
> 0429fbc0bdc297d6  76835b0ebf8a7fe85beb03c751
> ----------------  --------------------------
>          %stddev     %change         %stddev
>              \          |                \
>    6314259 ±  0%      -8.1%    5800079 ±  0%  will-it-scale.per_thread_ops
>    6274871 ±  0%      -8.1%    5768747 ±  0%  will-it-scale.per_process_ops

This was raised before but I'm not sure we can do much about it:

https://lkml.org/lkml/2014/10/26/189

It would be interesting to know the difference in performance with the
code prior to commits b0c29f79ecea and 11d4616bd07f which optimised the
futex code but missed a barrier.

-- 
Catalin
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