linux-kernel - Re: [PATCH] mm: do not drain pagevecs for mlock

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Date:	Fri, 30 Dec 2011 16:48:35 +0800
From:	Tao Ma <tm@....ma>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <jweiner@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: do not drain pagevecs for mlock

On 12/30/2011 04:11 PM, KOSAKI Motohiro wrote:
> 2011/12/30 Tao Ma <tm@....ma>:
>> In our test of mlock, we have found some severe performance regression
>> in it. Some more investigations show that mlocked is blocked heavily
>> by lur_add_drain_all which calls schedule_on_each_cpu and flush the work
>> queue which is very slower if we have several cpus.
>>
>> So we have tried 2 ways to solve it:
>> 1. Add a per cpu counter for all the pagevecs so that we don't schedule
>>   and flush the lru_drain work if the cpu doesn't have any pagevecs(I
>>   have finished the codes already).
>> 2. Remove the lru_add_drain_all.
>>
>> The first one has some problems since in our product system, all the cpus
>> are busy, so I guess there is very little chance for a cpu to have 0 pagevecs
>> except that you run several consecutive mlocks.
>>
>> From the commit log which added this function(8891d6da), it seems that we
>> don't have to call it. So the 2nd one seems to be both easy and workable and
>> comes this patch.
> 
> Could you please show us your system environment and benchmark programs?
> Usually lru_drain_** is very fast than mlock() body because it makes
> plenty memset(page).
The system environment is: 16 core Xeon E5620. 24G memory.

I have attached the program. It is very simple and just uses mlock/munlock.

Thanks
Tao

View attachment "test_mlock.c" of type "text/x-csrc" (982 bytes)