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

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Date:	Fri, 6 Jan 2012 01:18:02 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Tao Ma <tm@....ma>
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

2012/1/6 Tao Ma <tm@....ma>:
> Hi Kosaki,
> On 12/30/2011 06:07 PM, KOSAKI Motohiro wrote:
>>>> Because your test program is too artificial. 20sec/100000times =
>>>> 200usec. And your
>>>> program repeat mlock and munlock the exact same address. so, yes, if
>>>> lru_add_drain_all() is removed, it become near no-op. but it's
>>>> worthless comparision.
>>>> none of any practical program does such strange mlock usage.
>>> yes, I should say it is artificial. But mlock did cause the problem in
>>> our product system and perf shows that the mlock uses the system time
>>> much more than others. That's the reason we created this program to test
>>> whether mlock really sucks. And we compared the result with
>>> rhel5(2.6.18) which runs much much faster.
>>>
>>> And from the commit log you described, we can remove lru_add_drain_all
>>> safely here, so why add it? At least removing it makes mlock much faster
>>> compared to the vanilla kernel.
>>
>> If we remove it, we lose to a test way of mlock. "Memlocked" field of
>> /proc/meminfo
>> show inaccurate number very easily. So, if 200usec is no avoidable,
>> I'll ack you.
>> But I'm not convinced yet.
> Do you find something new for this?

No.

Or more exactly, 200usec is my calculation mistake. your program call mlock
3 times per each iteration. so, correct cost is 66usec.
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