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    SubjectRe: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim
    On Wed, Jul 07, 2010 at 01:03:38PM +0800, Wu Fengguang wrote:
    > Hi Mel,
    > > Second, using systemtap, I was able to see that file-backed dirty
    > > pages have a tendency to be near the end of the LRU even though they
    > > are a small percentage of the overall pages in the LRU. I'm hoping
    > > to figure out why this is as it would make avoiding writeback a lot
    > > less controversial.
    > Your intuitions are correct -- the current background writeback logic
    > fails to write elder inodes first. Under heavy loads the background
    > writeback job may run for ever, totally ignoring the time order of
    > inode->dirtied_when. This is probably why you see lots of dirty pages
    > near the end of LRU.

    Possible. In a mail to Christoph, I asserted that writeback of elder inodes
    was happening first but I obviously could be mistaken.

    > Here is an old patch for fixing this. Sorry for being late. I'll
    > pick up and refresh the patch series ASAP. (I made a mistake last
    > year to post too many patches at one time. I'll break them up into
    > more manageable pieces.)
    > [PATCH 31/45] writeback: sync old inodes first in background writeback
    > <>

    I'll check it out as an alternative to forward-flushing based on the
    amount of dirty pages encountered during scanning. Thanks.

    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab

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