Re: [PATCH 6/8] mm: vmscan: Throttle reclaim if encountering toomany dirty pages under writeback

[Johannes Weiner]

On Thu, Jul 21, 2011 at 05:28:48PM +0100, 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. If a percentage of them are dirty, the process will be
> throttled if a blocking device is congested or the zone being scanned
> is marked congested. The percentage that must be dirty depends on
> the priority. At default priority, all of them must be dirty. At
> DEF_PRIORITY-1, 50% of them must be dirty, 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@xxxxxxx>
> ---
>  mm/vmscan.c |   21 ++++++++++++++++++---
>  1 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cf7b501..b0060f8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -720,7 +720,8 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
>  static unsigned long shrink_page_list(struct list_head *page_list,
>  				      struct zone *zone,
>  				      struct scan_control *sc,
> -				      int priority)
> +				      int priority,
> +				      unsigned long *ret_nr_dirty)
>  {
>  	LIST_HEAD(ret_pages);
>  	LIST_HEAD(free_pages);
> @@ -971,6 +972,7 @@ keep_lumpy:
>  
>  	list_splice(&ret_pages, page_list);
>  	count_vm_events(PGACTIVATE, pgactivate);
> +	*ret_nr_dirty += nr_dirty;

Note that this includes anon pages, which means that swapping is
throttled as well.

I don't think it is a downside to throttle swapping during IO
congestion - waiting for pages under writeback to become reclaimable
is better than kicking off even more IO in this case as well - but the
changelog and the comments should include it, I guess.

Otherwise,
Acked-by: Johannes Weiner <jweiner@xxxxxxxxxx>
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