Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback

[Rik van Riel <riel@redhat.com>]

On 08/10/2011 06:47 AM, Mel Gorman 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>

Acked-by: Rik van Riel <riel@redhat.com>

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