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    SubjectRe: [PATCH] mm: vmscan: fix setting reclaim mode
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    On Wed, Jan 11, 2012 at 12:44 AM, Mel Gorman <mgorman@suse.de> wrote:
    > On Tue, Jan 10, 2012 at 11:58:03PM +0800, Hillf Danton wrote:
    >> From: Hillf Danton <dhillf@gmail.com>
    >> [PATCH] mm: vmscan: fix setting reclaim mode
    >>
    >> The comment says, initially assume we are entering either lumpy reclaim or
    >> reclaim/compaction, and depending on the reclaim order, we will either set the
    >> sync mode or just reclaim order-0 pages later.
    >>
    >> On other hand, order-0 reclaim, instead of sync reclaim, is expected when
    >> under memory pressure, but the check for memory pressure is incorrect,
    >> leading to sync reclaim at low reclaim priorities.
    >>
    >> And the result is sync reclaim is set for high priorities.
    >>
    >
    > RECLAIM_MODE_SYNC is only set for RECLAIM_MODE_LUMPYRECLAIM. Even when
    > using RECLAIM_MODE_LUMPYRECLAIM, it should only be set when reclaim
    > is under memory pressure and failing to reclaim the necessry pages
    > (priority < DEF_PRIORITY - 2). Once in symc reclaim, reclaim will call
    > wait_on_page_writeback() on dirty pages which potentially leads to
    > significant stalls (one of the reasons why RECLAIM_MODE_LUMPYRECLAIM
    > sucks and why compaction is preferred). Your patch means sync reclaim
    > is used even when priority == DEF_PRIORITY. This is unexpected.
    >
    > Your changelog really needs to explain what the problem is that you
    > have encountered and why this patch fixes it. It's not like some of
    > your other patches which were minor performance optimisations that
    > were self-evident.
    >
    Hi Mel

    To avoid another step in wrong direction, I'd goto bed now,
    it is about 1:09 AM.
    When sane again, I will try to understand your comment.

    Best
    Hillf


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