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

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Date:	Fri, 06 Jan 2012 14:30:54 +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 01/06/2012 02:18 PM, KOSAKI Motohiro wrote:
> 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.
yes, so mlock can do 15000/s, it is even slower than the whole i/o time
for some not very fast ssd disk and I don't think it is endurable. I
guess we should remove it, right? Or you have another other suggestion
that I can try for it?

Thanks
Tao
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