From bb13ffeb9f6bfeb301443994dfbf29f91117dfb3 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Mon, 8 Oct 2012 16:32:41 -0700 Subject: mm: compaction: cache if a pageblock was scanned and no pages were isolated When compaction was implemented it was known that scanning could potentially be excessive. The ideal was that a counter be maintained for each pageblock but maintaining this information would incur a severe penalty due to a shared writable cache line. It has reached the point where the scanning costs are a serious problem, particularly on long-lived systems where a large process starts and allocates a large number of THPs at the same time. Instead of using a shared counter, this patch adds another bit to the pageblock flags called PG_migrate_skip. If a pageblock is scanned by either migrate or free scanner and 0 pages were isolated, the pageblock is marked to be skipped in the future. When scanning, this bit is checked before any scanning takes place and the block skipped if set. The main difficulty with a patch like this is "when to ignore the cached information?" If it's ignored too often, the scanning rates will still be excessive. If the information is too stale then allocations will fail that might have otherwise succeeded. In this patch o CMA always ignores the information o If the migrate and free scanner meet then the cached information will be discarded if it's at least 5 seconds since the last time the cache was discarded o If there are a large number of allocation failures, discard the cache. The time-based heuristic is very clumsy but there are few choices for a better event. Depending solely on multiple allocation failures still allows excessive scanning when THP allocations are failing in quick succession due to memory pressure. Waiting until memory pressure is relieved would cause compaction to continually fail instead of using reclaim/compaction to try allocate the page. The time-based mechanism is clumsy but a better option is not obvious. Signed-off-by: Mel Gorman Acked-by: Rik van Riel Cc: Richard Davies Cc: Shaohua Li Cc: Avi Kivity Acked-by: Rafael Aquini Cc: Fengguang Wu Cc: Michal Nazarewicz Cc: Bartlomiej Zolnierkiewicz Cc: Kyungmin Park Cc: Mark Brown Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mmzone.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include/linux/mmzone.h') diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 16a4cc2950a0..f85ecc9cfa1b 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -369,6 +369,9 @@ struct zone { */ spinlock_t lock; int all_unreclaimable; /* All pages pinned */ +#if defined CONFIG_COMPACTION || defined CONFIG_CMA + unsigned long compact_blockskip_expire; +#endif #ifdef CONFIG_MEMORY_HOTPLUG /* see spanned/present_pages for more description */ seqlock_t span_seqlock; -- cgit v1.2.2