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

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Date:	Fri, 30 Dec 2011 05:07:57 -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

>> 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.
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