From 1a501907bbea8e6ebb0b16cf6db9e9cbf1d2c813 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Wed, 4 Jun 2014 16:10:49 -0700 Subject: [PATCH] mm: vmscan: use proportional scanning during direct reclaim and full scan at DEF_PRIORITY Commit "mm: vmscan: obey proportional scanning requirements for kswapd" ensured that file/anon lists were scanned proportionally for reclaim from kswapd but ignored it for direct reclaim. The intent was to minimse direct reclaim latency but Yuanhan Liu pointer out that it substitutes one long stall for many small stalls and distorts aging for normal workloads like streaming readers/writers. Hugh Dickins pointed out that a side-effect of the same commit was that when one LRU list dropped to zero that the entirety of the other list was shrunk leading to excessive reclaim in memcgs. This patch scans the file/anon lists proportionally for direct reclaim to similarly age page whether reclaimed by kswapd or direct reclaim but takes care to abort reclaim if one LRU drops to zero after reclaiming the requested number of pages. Based on ext4 and using the Intel VM scalability test 3.15.0-rc5 3.15.0-rc5 shrinker proportion Unit lru-file-readonce elapsed 5.3500 ( 0.00%) 5.4200 ( -1.31%) Unit lru-file-readonce time_range 0.2700 ( 0.00%) 0.1400 ( 48.15%) Unit lru-file-readonce time_stddv 0.1148 ( 0.00%) 0.0536 ( 53.33%) Unit lru-file-readtwice elapsed 8.1700 ( 0.00%) 8.1700 ( 0.00%) Unit lru-file-readtwice time_range 0.4300 ( 0.00%) 0.2300 ( 46.51%) Unit lru-file-readtwice time_stddv 0.1650 ( 0.00%) 0.0971 ( 41.16%) The test cases are running multiple dd instances reading sparse files. The results are within the noise for the small test machine. The impact of the patch is more noticable from the vmstats 3.15.0-rc5 3.15.0-rc5 shrinker proportion Minor Faults 35154 36784 Major Faults 611 1305 Swap Ins 394 1651 Swap Outs 4394 5891 Allocation stalls 118616 44781 Direct pages scanned 4935171 4602313 Kswapd pages scanned 15921292 16258483 Kswapd pages reclaimed 15913301 16248305 Direct pages reclaimed 4933368 4601133 Kswapd efficiency 99% 99% Kswapd velocity 670088.047 682555.961 Direct efficiency 99% 99% Direct velocity 207709.217 193212.133 Percentage direct scans 23% 22% Page writes by reclaim 4858.000 6232.000 Page writes file 464 341 Page writes anon 4394 5891 Note that there are fewer allocation stalls even though the amount of direct reclaim scanning is very approximately the same. Signed-off-by: Mel Gorman Cc: Johannes Weiner Cc: Hugh Dickins Cc: Tim Chen Cc: Dave Chinner Tested-by: Yuanhan Liu Cc: Bob Liu Cc: Jan Kara Cc: Rik van Riel Cc: Al Viro Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmscan.c | 36 +++++++++++++++++++++++++----------- 1 file changed, 25 insertions(+), 11 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index cc29fca8d989..9149444f947d 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2064,13 +2064,27 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) unsigned long nr_reclaimed = 0; unsigned long nr_to_reclaim = sc->nr_to_reclaim; struct blk_plug plug; - bool scan_adjusted = false; + bool scan_adjusted; get_scan_count(lruvec, sc, nr); /* Record the original scan target for proportional adjustments later */ memcpy(targets, nr, sizeof(nr)); + /* + * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal + * event that can occur when there is little memory pressure e.g. + * multiple streaming readers/writers. Hence, we do not abort scanning + * when the requested number of pages are reclaimed when scanning at + * DEF_PRIORITY on the assumption that the fact we are direct + * reclaiming implies that kswapd is not keeping up and it is best to + * do a batch of work at once. For memcg reclaim one check is made to + * abort proportional reclaim if either the file or anon lru has already + * dropped to zero at the first pass. + */ + scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() && + sc->priority == DEF_PRIORITY); + blk_start_plug(&plug); while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] || nr[LRU_INACTIVE_FILE]) { @@ -2090,18 +2104,9 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) if (nr_reclaimed < nr_to_reclaim || scan_adjusted) continue; - /* - * For global direct reclaim, reclaim only the number of pages - * requested. Less care is taken to scan proportionally as it - * is more important to minimise direct reclaim stall latency - * than it is to properly age the LRU lists. - */ - if (global_reclaim(sc) && !current_is_kswapd()) - break; - /* * For kswapd and memcg, reclaim at least the number of pages - * requested. Ensure that the anon and file LRUs shrink + * requested. Ensure that the anon and file LRUs are scanned * proportionally what was requested by get_scan_count(). We * stop reclaiming one LRU and reduce the amount scanning * proportional to the original scan target. @@ -2109,6 +2114,15 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE]; nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON]; + /* + * It's just vindictive to attack the larger once the smaller + * has gone to zero. And given the way we stop scanning the + * smaller below, this makes sure that we only make one nudge + * towards proportionality once we've got nr_to_reclaim. + */ + if (!nr_file || !nr_anon) + break; + if (nr_file > nr_anon) { unsigned long scan_target = targets[LRU_INACTIVE_ANON] + targets[LRU_ACTIVE_ANON] + 1;