Re: [PATCH] mm: page_alloc: Avoid marking zones full prematurelyafter zone_reclaim()

From: Simon Jeons
Date: Thu Mar 21 2013 - 04:32:19 EST

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.
dd will use page cache instead of direct IO? Where can I got dd source codes?
One offline question, when should use page cache and when should use direct IO?

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