Re: [PATCH 04/10] mm: vmscan: Decide whether to compact the pgdat based on reclaim progress

[Hillf Danton <dhillf@gmail.com>]

On Sun, Mar 17, 2013 at 9:04 PM, Mel Gorman <mgorman@suse.de> wrote:
> In the past, kswapd makes a decision on whether to compact memory after the
> pgdat was considered balanced. This more or less worked but it is late to
> make such a decision and does not fit well now that kswapd makes a decision
> whether to exit the zone scanning loop depending on reclaim progress.
>
> This patch will compact a pgdat if at least  the requested number of pages
> were reclaimed from unbalanced zones for a given priority. If any zone is
> currently balanced, kswapd will not call compaction as it is expected the
> necessary pages are already available.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>  mm/vmscan.c | 52 +++++++++++++++++++++-------------------------------
>  1 file changed, 21 insertions(+), 31 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 279d0c2..7513bd1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2694,8 +2694,11 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>
>         do {
>                 unsigned long lru_pages = 0;
> +               unsigned long nr_to_reclaim = 0;
>                 unsigned long nr_reclaimed = sc.nr_reclaimed;
> +               unsigned long this_reclaimed;
>                 bool raise_priority = true;
> +               bool pgdat_needs_compaction = true;

To show that compaction is needed iff non order-o reclaim,
                bool do_compaction = !!order;

>
>                 /*
>                  * Scan in the highmem->dma direction for the highest
> @@ -2743,7 +2746,17 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>                 for (i = 0; i <= end_zone; i++) {
>                         struct zone *zone = pgdat->node_zones + i;
>
> +                       if (!populated_zone(zone))
> +                               continue;
> +
>                         lru_pages += zone_reclaimable_pages(zone);
> +
> +                       /* Check if the memory needs to be defragmented */
Enrich the comment with, say,
/*If any zone is
currently balanced, kswapd will not call compaction as it is expected the
necessary pages are already available.*/
please since a big one is deleted below.

> +                       if (order && pgdat_needs_compaction &&
> +                                       zone_watermark_ok(zone, order,
> +                                               low_wmark_pages(zone),
> +                                               *classzone_idx, 0))
> +                               pgdat_needs_compaction = false;
>                 }
>
>                 /*
> @@ -2814,6 +2827,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>                                  */
>                                 if (kswapd_shrink_zone(zone, &sc, lru_pages))
>                                         raise_priority = false;
> +
> +                               nr_to_reclaim += sc.nr_to_reclaim;
>                         }
>
>                         /*
> @@ -2864,46 +2879,21 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>                 if (try_to_freeze() || kthread_should_stop())
>                         break;
>
> -               /* If no reclaim progress then increase scanning priority */
> -               if (sc.nr_reclaimed - nr_reclaimed == 0)
> -                       raise_priority = true;
> +               /* Compact if necessary and kswapd is reclaiming efficiently */
> +               this_reclaimed = sc.nr_reclaimed - nr_reclaimed;
> +               if (order && pgdat_needs_compaction &&
> +                               this_reclaimed > nr_to_reclaim)
> +                       compact_pgdat(pgdat, order);
>
>                 /*
>                  * Raise priority if scanning rate is too low or there was no
>                  * progress in reclaiming pages
>                  */
> -               if (raise_priority || sc.nr_reclaimed - nr_reclaimed == 0)
> +               if (raise_priority || !this_reclaimed)
>                         sc.priority--;
>         } while (sc.priority >= 0 &&
>                  !pgdat_balanced(pgdat, order, *classzone_idx));
>
> -       /*
> -        * If kswapd was reclaiming at a higher order, it has the option of
> -        * sleeping without all zones being balanced. Before it does, it must
> -        * ensure that the watermarks for order-0 on *all* zones are met and
> -        * that the congestion flags are cleared. The congestion flag must
> -        * be cleared as kswapd is the only mechanism that clears the flag
> -        * and it is potentially going to sleep here.
> -        */
> -       if (order) {
> -               int zones_need_compaction = 1;
> -
> -               for (i = 0; i <= end_zone; i++) {
> -                       struct zone *zone = pgdat->node_zones + i;
> -
> -                       if (!populated_zone(zone))
> -                               continue;
> -
> -                       /* Check if the memory needs to be defragmented. */
> -                       if (zone_watermark_ok(zone, order,
> -                                   low_wmark_pages(zone), *classzone_idx, 0))
> -                               zones_need_compaction = 0;
> -               }
> -
> -               if (zones_need_compaction)
> -                       compact_pgdat(pgdat, order);
> -       }
> -
>  out:
>         /*
>          * Return the order we were reclaiming at so prepare_kswapd_sleep()
> --
> 1.8.1.4

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