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    SubjectRe: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback
    On Wed, 10 Aug 2011 11:47:19 +0100
    Mel Gorman <mgorman@suse.de> wrote:

    > Workloads that are allocating frequently and writing files place a
    > large number of dirty pages on the LRU. With use-once logic, it is
    > possible for them to reach the end of the LRU quickly requiring the
    > reclaimer to scan more to find clean pages. Ordinarily, processes that
    > are dirtying memory will get throttled by dirty balancing but this
    > is a global heuristic and does not take into account that LRUs are
    > maintained on a per-zone basis. This can lead to a situation whereby
    > reclaim is scanning heavily, skipping over a large number of pages
    > under writeback and recycling them around the LRU consuming CPU.
    >
    > This patch checks how many of the number of pages isolated from the
    > LRU were dirty and under writeback. If a percentage of them under
    > writeback, the process will be throttled if a backing device or the
    > zone is congested. Note that this applies whether it is anonymous or
    > file-backed pages that are under writeback meaning that swapping is
    > potentially throttled. This is intentional due to the fact if the
    > swap device is congested, scanning more pages and dispatching more
    > IO is not going to help matters.
    >
    > The percentage that must be in writeback depends on the priority. At
    > default priority, all of them must be dirty. At DEF_PRIORITY-1, 50%
    > of them must be, DEF_PRIORITY-2, 25% etc. i.e. as pressure increases
    > the greater the likelihood the process will get throttled to allow
    > the flusher threads to make some progress.
    >
    > Signed-off-by: Mel Gorman <mgorman@suse.de>
    > Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
    > Acked-by: Johannes Weiner <jweiner@redhat.com>

    Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    Maybe I need to add memcg_is_congested() ...



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