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SubjectRe: [PATCH 2/2] vmscan: kill shrink_all_zones()
Date
On Friday 09 October 2009, KOSAKI Motohiro wrote:
> shrink_all_zone() was introduced by commit d6277db4ab (swsusp: rework
> memory shrinker) for hibernate performance improvement. and sc.swap_cluster_max
> was introduced by commit a06fe4d307 (Speed freeing memory for suspend).
>
> commit a06fe4d307 said
>
> Without the patch:
> Freed 14600 pages in 1749 jiffies = 32.61 MB/s (Anomolous!)
> Freed 88563 pages in 14719 jiffies = 23.50 MB/s
> Freed 205734 pages in 32389 jiffies = 24.81 MB/s
>
> With the patch:
> Freed 68252 pages in 496 jiffies = 537.52 MB/s
> Freed 116464 pages in 569 jiffies = 798.54 MB/s
> Freed 209699 pages in 705 jiffies = 1161.89 MB/s
>
> At that time, their patch was pretty worth. However, Modern Hardware
> trend and recent VM improvement broke its worth. From several reason,
> I think we should remove shrink_all_zones() at all.
>
> detail:
>
> 1) Old days, shrink_zone()'s slowness was mainly caused by stupid congestion_wait()
> at no i/o congestion.
> but current shrink_zone() is sane, not slow.
>
> 2) shrink_all_zone() try to shrink all pages at a time. but it doesn't works
> fine on numa system.
> example)
> System has 4GB memory and each node have 2GB. and hibernate need 1GB.
>
> optimal)
> steal 500MB from each node.
> shrink_all_zones)
> steal 1GB from node-0.
>
> Oh, Cache balancing was broke ;)
> Unfortunately, Desktop system moved ahead NUMA.
> (Side note, if hibernate require 2GB, shrink_all_zones() never success)
>
> 3) if the node has several I/O flighting pages, shrink_all_zones() makes
> pretty bad result.
>
> schenario) hibernate need 1GB
>
> 1) shrink_all_zones() try to reclaim 1GB from Node-0
> 2) but it only reclaimed 990MB
> 3) stupidly, shrink_all_zones() try to reclaim 1GB from Node-1
> 4) it reclaimed 990MB
>
> Oh, well. it reclaimed twice much than required.
> In the other hand, current shrink_zone() has sane baling out logic.
> then, it doesn't make overkill reclaim. then, we lost shrink_zones()'s risk.
>
> 4) SplitLRU VM always keep active/inactive ratio very carefully. inactive list only
> shrinking break its assumption. it makes unnecessary OOM risk. it obviously suboptimal.
>
> Throuput comparision
> ==============================================
> old 2192.10 MB/s
> new 2222.22 MB/s
>
> ok, it's almost same throuput.
>
> Cc: Rafael J. Wysocki <rjw@sisk.pl>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

I have no objections to any of the two patches, but I think we may want to drop
shrink_all_memory() altogether. Everything should work without it and the
reason I didn't remove it was because I saw a performance regression on one
system without it. It may not be worth keeping it, though.

Have you done any tests with shrink_all_memory() removed?

Rafael


> ---
> mm/vmscan.c | 75 +++++++++++++---------------------------------------------
> 1 files changed, 17 insertions(+), 58 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 80e727d..9f28166 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2130,51 +2130,6 @@ unsigned long global_lru_pages(void)
>
> #ifdef CONFIG_HIBERNATION
> /*
> - * Helper function for shrink_all_memory(). Tries to reclaim 'nr_pages' pages
> - * from LRU lists system-wide, for given pass and priority.
> - *
> - * For pass > 3 we also try to shrink the LRU lists that contain a few pages
> - */
> -static void shrink_all_zones(unsigned long nr_pages, int prio,
> - int pass, struct scan_control *sc)
> -{
> - struct zone *zone;
> - unsigned long nr_reclaimed = 0;
> -
> - for_each_populated_zone(zone) {
> - enum lru_list l;
> -
> - if (zone_is_all_unreclaimable(zone) && prio != DEF_PRIORITY)
> - continue;
> -
> - for_each_evictable_lru(l) {
> - enum zone_stat_item ls = NR_LRU_BASE + l;
> - unsigned long lru_pages = zone_page_state(zone, ls);
> -
> - /* For pass = 0, we don't shrink the active list */
> - if (pass == 0 && (l == LRU_ACTIVE_ANON ||
> - l == LRU_ACTIVE_FILE))
> - continue;
> -
> - zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1;
> - if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) {
> - unsigned long nr_to_scan;
> -
> - zone->lru[l].nr_saved_scan = 0;
> - nr_to_scan = min(nr_pages, lru_pages);
> - nr_reclaimed += shrink_list(l, nr_to_scan, zone,
> - sc, prio);
> - if (nr_reclaimed >= nr_pages) {
> - sc->nr_reclaimed += nr_reclaimed;
> - return;
> - }
> - }
> - }
> - }
> - sc->nr_reclaimed += nr_reclaimed;
> -}
> -
> -/*
> * Try to free `nr_pages' of memory, system-wide, and return the number of
> * freed pages.
> *
> @@ -2188,12 +2143,18 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
> int pass;
> struct reclaim_state reclaim_state;
> struct scan_control sc = {
> - .gfp_mask = GFP_KERNEL,
> + .gfp_mask = GFP_HIGHUSER_MOVABLE,
> + .may_swap = 0,
> .may_unmap = 0,
> .may_writepage = 1,
> + .swap_cluster_max = SWAP_CLUSTER_MAX,
> + .nr_to_reclaim = nr_pages,
> + .swappiness = vm_swappiness,
> + .order = 0,
> .isolate_pages = isolate_pages_global,
> - .nr_reclaimed = 0,
> };
> + struct zonelist * zonelist = node_zonelist(first_online_node,
> + GFP_HIGHUSER_MOVABLE);
>
> current->reclaim_state = &reclaim_state;
>
> @@ -2215,9 +2176,9 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
>
> /*
> * We try to shrink LRUs in 5 passes:
> - * 0 = Reclaim from inactive_list only
> - * 1 = Reclaim from active list but don't reclaim mapped
> - * 2 = 2nd pass of type 1
> + * 0 = Reclaim unmapped pages
> + * 1 = 2nd pass of type 0
> + * 2 = 3rd pass of type 0
> * 3 = Reclaim mapped (normal reclaim)
> * 4 = 2nd pass of type 3
> */
> @@ -2225,15 +2186,15 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
> int prio;
>
> /* Force reclaiming mapped pages in the passes #3 and #4 */
> - if (pass > 2)
> + if (pass > 2) {
> sc.may_unmap = 1;
> + sc.may_swap = 1;
> + }
>
> for (prio = DEF_PRIORITY; prio >= 0; prio--) {
> - unsigned long nr_to_scan = nr_pages - sc.nr_reclaimed;
> -
> sc.nr_scanned = 0;
> - sc.swap_cluster_max = nr_to_scan;
> - shrink_all_zones(nr_to_scan, prio, pass, &sc);
> +
> + shrink_zones(prio, zonelist, &sc);
> if (sc.nr_reclaimed >= nr_pages)
> goto out;
>
> @@ -2243,10 +2204,8 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
> sc.nr_reclaimed += reclaim_state.reclaimed_slab;
> if (sc.nr_reclaimed >= nr_pages)
> goto out;
> -
> - if (sc.nr_scanned && prio < DEF_PRIORITY - 2)
> - congestion_wait(BLK_RW_ASYNC, HZ / 10);
> }
> + congestion_wait(BLK_RW_ASYNC, HZ / 10);
> }
>
> /*
>



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