On Tue 09-04-13 12:06:56, Mel Gorman wrote:
> The number of pages kswapd can reclaim is bound by the number of pages it
> scans which is related to the size of the zone and the scanning priority. In
> many cases the priority remains low because it's reset every SWAP_CLUSTER_MAX
> reclaimed pages but in the event kswapd scans a large number of pages it
> cannot reclaim, it will raise the priority and potentially discard a large
> percentage of the zone as sc->nr_to_reclaim is ULONG_MAX. The user-visible
> effect is a reclaim "spike" where a large percentage of memory is suddenly
> freed. It would be bad enough if this was just unused memory but because
> of how anon/file pages are balanced it is possible that applications get
> pushed to swap unnecessarily.
>
> This patch limits the number of pages kswapd will reclaim to the high
> watermark. Reclaim will still overshoot due to it not being a hard limit as
> shrink_lruvec() will ignore the sc.nr_to_reclaim at DEF_PRIORITY but it
> prevents kswapd reclaiming the world at higher priorities. The number of
> pages it reclaims is not adjusted for high-order allocations as kswapd will
> reclaim excessively if it is to balance zones for high-order allocations.
>
> Signed-off-by: Mel Gorman <mgor...@suse.de>
> Reviewed-by: Rik van Riel <r...@redhat.com>
> Acked-by: Johannes Weiner <han...@cmpxchg.org>
Reviewed-by: Michal Hocko <mho...@suse.cz>
> ---
> mm/vmscan.c | 53 +++++++++++++++++++++++++++++------------------------
> 1 file changed, 29 insertions(+), 24 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 88c5fed..4835a7a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2593,6 +2593,32 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int
> order, long remaining,
> }
>
> /*
> + * kswapd shrinks the zone by the number of pages required to reach
> + * the high watermark.
> + */
> +static void kswapd_shrink_zone(struct zone *zone,
> + struct scan_control *sc,
> + unsigned long lru_pages)
> +{
> + unsigned long nr_slab;
> + struct reclaim_state *reclaim_state = current->reclaim_state;
> + struct shrink_control shrink = {
> + .gfp_mask = sc->gfp_mask,
> + };
> +
> + /* Reclaim above the high watermark. */
> + sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
> + shrink_zone(zone, sc);
> +
> + reclaim_state->reclaimed_slab = 0;
> + nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
> + sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> +
> + if (nr_slab == 0 && !zone_reclaimable(zone))
> + zone->all_unreclaimable = 1;
> +}
> +
> +/*
> * For kswapd, balance_pgdat() will work across all this node's zones until
> * they are all at high_wmark_pages(zone).
> *
> @@ -2619,27 +2645,16 @@ static unsigned long balance_pgdat(pg_data_t *pgdat,
> int order,
> bool pgdat_is_balanced = false;
> int i;
> int end_zone = 0; /* Inclusive. 0 = ZONE_DMA */
> - unsigned long total_scanned;
> - struct reclaim_state *reclaim_state = current->reclaim_state;
> unsigned long nr_soft_reclaimed;
> unsigned long nr_soft_scanned;
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .may_unmap = 1,
> .may_swap = 1,
> - /*
> - * kswapd doesn't want to be bailed out while reclaim. because
> - * we want to put equal scanning pressure on each zone.
> - */
> - .nr_to_reclaim = ULONG_MAX,
> .order = order,
> .target_mem_cgroup = NULL,
> };
> - struct shrink_control shrink = {
> - .gfp_mask = sc.gfp_mask,
> - };
> loop_again:
> - total_scanned = 0;
> sc.priority = DEF_PRIORITY;
> sc.nr_reclaimed = 0;
> sc.may_writepage = !laptop_mode;
> @@ -2710,7 +2725,7 @@ loop_again:
> */
> for (i = 0; i <= end_zone; i++) {
> struct zone *zone = pgdat->node_zones + i;
> - int nr_slab, testorder;
> + int testorder;
> unsigned long balance_gap;
>
> if (!populated_zone(zone))
> @@ -2730,7 +2745,6 @@ loop_again:
> order, sc.gfp_mask,
> &nr_soft_scanned);
> sc.nr_reclaimed += nr_soft_reclaimed;
> - total_scanned += nr_soft_scanned;
>
> /*
> * We put equal pressure on every zone, unless
> @@ -2759,17 +2773,8 @@ loop_again:
>
> if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
> !zone_balanced(zone, testorder,
> - balance_gap, end_zone)) {
> - shrink_zone(zone, &sc);
> -
> - reclaim_state->reclaimed_slab = 0;
> - nr_slab = shrink_slab(&shrink, sc.nr_scanned,
> lru_pages);
> - sc.nr_reclaimed +=
> reclaim_state->reclaimed_slab;
> - total_scanned += sc.nr_scanned;
> -
> - if (nr_slab == 0 && !zone_reclaimable(zone))
> - zone->all_unreclaimable = 1;
> - }
> + balance_gap, end_zone))
> + kswapd_shrink_zone(zone, &sc, lru_pages);
>
> /*
> * If we're getting trouble reclaiming, start doing
> --
> 1.8.1.4
>
--
Michal Hocko
SUSE Labs
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