From d527caf22e48480b102c7c6ee5b9ba12170148f7 Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Tue, 22 Mar 2011 16:30:38 -0700 Subject: mm: compaction: prevent kswapd compacting memory to reduce CPU usage This patch reverts 5a03b051 ("thp: use compaction in kswapd for GFP_ATOMIC order > 0") due to reports stating that kswapd CPU usage was higher and IRQs were being disabled more frequently. This was reported at http://www.spinics.net/linux/fedora/alsa-user/msg09885.html. Without this patch applied, CPU usage by kswapd hovers around the 20% mark according to the tester (Arthur Marsh: http://www.spinics.net/linux/fedora/alsa-user/msg09899.html). With this patch applied, it's around 2%. The problem is not related to THP which specifies __GFP_NO_KSWAPD but is triggered by high-order allocations hitting the low watermark for their order and waking kswapd on kernels with CONFIG_COMPACTION set. The most common trigger for this is network cards configured for jumbo frames but it's also possible it'll be triggered by fork-heavy workloads (order-1) and some wireless cards which depend on order-1 allocations. The symptoms for the user will be high CPU usage by kswapd in low-memory situations which could be confused with another writeback problem. While a patch like 5a03b051 may be reintroduced in the future, this patch plays it safe for now and reverts it. [mel@csn.ul.ie: Beefed up the changelog] Signed-off-by: Andrea Arcangeli Signed-off-by: Mel Gorman Reported-by: Arthur Marsh Tested-by: Arthur Marsh Cc: [2.6.38.1] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmscan.c | 18 +----------------- 1 file changed, 1 insertion(+), 17 deletions(-) (limited to 'mm/vmscan.c') diff --git a/mm/vmscan.c b/mm/vmscan.c index 6771ea70bfe7..3b4a41d72489 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2397,7 +2397,6 @@ loop_again: * cause too much scanning of the lower zones. */ for (i = 0; i <= end_zone; i++) { - int compaction; struct zone *zone = pgdat->node_zones + i; int nr_slab; @@ -2428,24 +2427,9 @@ loop_again: sc.nr_reclaimed += reclaim_state->reclaimed_slab; total_scanned += sc.nr_scanned; - compaction = 0; - if (order && - zone_watermark_ok(zone, 0, - high_wmark_pages(zone), - end_zone, 0) && - !zone_watermark_ok(zone, order, - high_wmark_pages(zone), - end_zone, 0)) { - compact_zone_order(zone, - order, - sc.gfp_mask, false, - COMPACT_MODE_KSWAPD); - compaction = 1; - } - if (zone->all_unreclaimable) continue; - if (!compaction && nr_slab == 0 && + if (nr_slab == 0 && !zone_reclaimable(zone)) zone->all_unreclaimable = 1; /* -- cgit v1.2.3-59-g8ed1b