From 90afa5de6f3fa89a733861e843377302479fcf7e Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Tue, 16 Jun 2009 15:33:20 -0700 Subject: vmscan: properly account for the number of page cache pages zone_reclaim() can reclaim A bug was brought to my attention against a distro kernel but it affects mainline and I believe problems like this have been reported in various guises on the mailing lists although I don't have specific examples at the moment. The reported problem was that malloc() stalled for a long time (minutes in some cases) if a large tmpfs mount was occupying a large percentage of memory overall. The pages did not get cleaned or reclaimed by zone_reclaim() because the zone_reclaim_mode was unsuitable, but the lists are uselessly scanned frequencly making the CPU spin at near 100%. This patchset intends to address that bug and bring the behaviour of zone_reclaim() more in line with expectations which were noticed during investigation. It is based on top of mmotm and takes advantage of Kosaki's work with respect to zone_reclaim(). Patch 1 fixes the heuristics that zone_reclaim() uses to determine if the scan should go ahead. The broken heuristic is what was causing the malloc() stall as it uselessly scanned the LRU constantly. Currently, zone_reclaim is assuming zone_reclaim_mode is 1 and historically it could not deal with tmpfs pages at all. This fixes up the heuristic so that an unnecessary scan is more likely to be correctly avoided. Patch 2 notes that zone_reclaim() returning a failure automatically means the zone is marked full. This is not always true. It could have failed because the GFP mask or zone_reclaim_mode were unsuitable. Patch 3 introduces a counter zreclaim_failed that will increment each time the zone_reclaim scan-avoidance heuristics fail. If that counter is rapidly increasing, then zone_reclaim_mode should be set to 0 as a temporarily resolution and a bug reported because the scan-avoidance heuristic is still broken. This patch: On NUMA machines, the administrator can configure zone_reclaim_mode that is a more targetted form of direct reclaim. On machines with large NUMA distances for example, a zone_reclaim_mode defaults to 1 meaning that clean unmapped pages will be reclaimed if the zone watermarks are not being met. There is a heuristic that determines if the scan is worthwhile but the problem is that the heuristic is not being properly applied and is basically assuming zone_reclaim_mode is 1 if it is enabled. The lack of proper detection can manfiest as high CPU usage as the LRU list is scanned uselessly. Historically, once enabled it was depending on NR_FILE_PAGES which may include swapcache pages that the reclaim_mode cannot deal with. Patch vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included pages that were not file-backed such as swapcache and made a calculation based on the inactive, active and mapped files. This is far superior when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a reasonable starting figure. This patch alters how zone_reclaim() works out how many pages it might be able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set in the reclaim_mode it will either consider NR_FILE_PAGES as potential candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount swapcache and other non-file-backed pages. If RECLAIM_WRITE is not set, then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is not set, then NR_FILE_MAPPED are not. [kosaki.motohiro@jp.fujitsu.com: Estimate unmapped pages minus tmpfs pages] [fengguang.wu@intel.com: Fix underflow problem in Kosaki's estimate] Signed-off-by: Mel Gorman Reviewed-by: Rik van Riel Acked-by: Christoph Lameter Cc: KOSAKI Motohiro Cc: Wu Fengguang Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/sysctl/vm.txt | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) (limited to 'Documentation') diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt index 0ea5adbc5b16..c4de6359d440 100644 --- a/Documentation/sysctl/vm.txt +++ b/Documentation/sysctl/vm.txt @@ -315,10 +315,14 @@ min_unmapped_ratio: This is available only on NUMA kernels. -A percentage of the total pages in each zone. Zone reclaim will only -occur if more than this percentage of pages are file backed and unmapped. -This is to insure that a minimal amount of local pages is still available for -file I/O even if the node is overallocated. +This is a percentage of the total pages in each zone. Zone reclaim will +only occur if more than this percentage of pages are in a state that +zone_reclaim_mode allows to be reclaimed. + +If zone_reclaim_mode has the value 4 OR'd, then the percentage is compared +against all file-backed unmapped pages including swapcache pages and tmpfs +files. Otherwise, only unmapped pages backed by normal files but not tmpfs +files and similar are considered. The default is 1 percent. -- cgit v1.2.2