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

[Simon Jeons]

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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