Commit 48ee5f36 authored May 19, 2016 by Mel Gorman Committed by Linus Torvalds May 19, 2016

mm, page_alloc: shortcut watermark checks for order-0 pages

Watermarks have to be checked on every allocation including the number
of pages being allocated and whether reserves can be accessed. The
reserves only matter if memory is limited and the free_pages adjustment
only applies to high-order pages. This patch adds a shortcut for
order-0 pages that avoids numerous calculations if there is plenty of
free memory yielding the following performance difference in a page
allocator microbenchmark;

4.6.0-rc2 4.6.0-rc2
optfair-v1r20 fastmark-v1r20
Min alloc-odr0-1 380.00 ( 0.00%) 364.00 ( 4.21%)
Min alloc-odr0-2 273.00 ( 0.00%) 262.00 ( 4.03%)
Min alloc-odr0-4 227.00 ( 0.00%) 214.00 ( 5.73%)
Min alloc-odr0-8 196.00 ( 0.00%) 186.00 ( 5.10%)
Min alloc-odr0-16 183.00 ( 0.00%) 173.00 ( 5.46%)
Min alloc-odr0-32 173.00 ( 0.00%) 165.00 ( 4.62%)
Min alloc-odr0-64 169.00 ( 0.00%) 161.00 ( 4.73%)
Min alloc-odr0-128 169.00 ( 0.00%) 159.00 ( 5.92%)
Min alloc-odr0-256 180.00 ( 0.00%) 168.00 ( 6.67%)
Min alloc-odr0-512 190.00 ( 0.00%) 180.00 ( 5.26%)
Min alloc-odr0-1024 198.00 ( 0.00%) 190.00 ( 4.04%)
Min alloc-odr0-2048 204.00 ( 0.00%) 196.00 ( 3.92%)
Min alloc-odr0-4096 209.00 ( 0.00%) 202.00 ( 3.35%)
Min alloc-odr0-8192 213.00 ( 0.00%) 206.00 ( 3.29%)
Min alloc-odr0-16384 214.00 ( 0.00%) 206.00 ( 3.74%)

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 30534755

mm/page_alloc.c

+27 −1

Original line number	Diff line number	Diff line
		@@ -2623,6 +2623,32 @@ bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
		zone_page_state(z, NR_FREE_PAGES));
		}

		static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
		unsigned long mark, int classzone_idx, unsigned int alloc_flags)
		{
		long free_pages = zone_page_state(z, NR_FREE_PAGES);
		long cma_pages = 0;

		#ifdef CONFIG_CMA
		/* If allocation can't use CMA areas don't use free CMA pages */
		if (!(alloc_flags & ALLOC_CMA))
		cma_pages = zone_page_state(z, NR_FREE_CMA_PAGES);
		#endif

		/*
		* Fast check for order-0 only. If this fails then the reserves
		* need to be calculated. There is a corner case where the check
		* passes but only the high-order atomic reserve are free. If
		* the caller is !atomic then it'll uselessly search the free
		* list. That corner case is then slower but it is harmless.
		*/
		if (!order && (free_pages - cma_pages) > mark + z->lowmem_reserve[classzone_idx])
		return true;

		return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
		free_pages);
		}

		bool zone_watermark_ok_safe(struct zone *z, unsigned int order,
		unsigned long mark, int classzone_idx)
		{
		@@ -2744,7 +2770,7 @@ zonelist_scan:
		continue;

		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
		if (!zone_watermark_ok(zone, order, mark,
		if (!zone_watermark_fast(zone, order, mark,
		ac->classzone_idx, alloc_flags)) {
		int ret;

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