linux-kernel - Re: [PATCH] mm: page_alloc: Avoid marking zones full prematurely after zone

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Date:	Thu, 21 Mar 2013 09:44:41 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hedi Berriche <hedi@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: Avoid marking zones full prematurely
 after zone_reclaim()

On Thu 21-03-13 16:32:03, Simon Jeons wrote:
> Hi Michal,
> On 03/21/2013 04:19 PM, Michal Hocko wrote:
> >On Thu 21-03-13 10:33:07, Simon Jeons wrote:
> >>Hi Mel,
> >>On 03/21/2013 02:19 AM, Mel Gorman wrote:
> >>>The following problem was reported against a distribution kernel when
> >>>zone_reclaim was enabled but the same problem applies to the mainline
> >>>kernel. The reproduction case was as follows
> >>>
> >>>1. Run numactl -m +0 dd if=largefile of=/dev/null
> >>>    This allocates a large number of clean pages in node 0
> >>I confuse why this need allocate a large number of clean pages?
> >It reads from file and puts pages into the page cache. The pages are not
> >modified so they are clean. Output file is /dev/null so no pages are
> >written. dd doesn't call fadvise(POSIX_FADV_DONTNEED) on the input file
> >by default so pages from the file stay in the page cache
> 
> Thanks for your clarify Michal.

This is getting off-topic.

> dd will use page cache instead of direct IO?

no by default. You can use direct option. Refer to man dd for more
information.

> Where can I got dd source codes?

dd is part of coreutils: http://www.gnu.org/software/coreutils/
Please do not be afraid to use google. Most of these answers are there
already...

> One offline question, when should use page cache and when should use
> direct IO?

And this is really off-topic. The simplest answer would be. Use direct
IO when you want to prevent from caching because you are doing it
yourselvef. Please try to search the web it is full of more specific
examples.
-- 
Michal Hocko
SUSE Labs
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