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    SubjectRe: [patch 1/5] mm: exclude reserved pages from dirtyable memory
    On Fri, Sep 30, 2011 at 09:17:20AM +0200, Johannes Weiner wrote:
    > The amount of dirtyable pages should not include the full number of
    > free pages: there is a number of reserved pages that the page
    > allocator and kswapd always try to keep free.
    >
    > The closer (reclaimable pages - dirty pages) is to the number of
    > reserved pages, the more likely it becomes for reclaim to run into
    > dirty pages:
    >
    > +----------+ ---
    > | anon | |
    > +----------+ |
    > | | |
    > | | -- dirty limit new -- flusher new
    > | file | | |
    > | | | |
    > | | -- dirty limit old -- flusher old
    > | | |
    > +----------+ --- reclaim
    > | reserved |
    > +----------+
    > | kernel |
    > +----------+
    >
    > This patch introduces a per-zone dirty reserve that takes both the
    > lowmem reserve as well as the high watermark of the zone into account,
    > and a global sum of those per-zone values that is subtracted from the
    > global amount of dirtyable pages. The lowmem reserve is unavailable
    > to page cache allocations and kswapd tries to keep the high watermark
    > free. We don't want to end up in a situation where reclaim has to
    > clean pages in order to balance zones.
    >
    > Not treating reserved pages as dirtyable on a global level is only a
    > conceptual fix. In reality, dirty pages are not distributed equally
    > across zones and reclaim runs into dirty pages on a regular basis.
    >
    > But it is important to get this right before tackling the problem on a
    > per-zone level, where the distance between reclaim and the dirty pages
    > is mostly much smaller in absolute numbers.
    >
    > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
    > Reviewed-by: Rik van Riel <riel@redhat.com>

    Acked-by: Mel Gorman <mgorman@suse.de>

    --
    Mel Gorman
    SUSE Labs


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