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mm, compaction: restrict async compaction to pageblocks of same migratetype

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The migrate scanner in async compaction is currently limited to
MIGRATE_MOVABLE pageblocks.  This is a heuristic intended to reduce
latency, based on the assumption that non-MOVABLE pageblocks are
unlikely to contain movable pages.

However, with the exception of THP's, most high-order allocations are
not movable.  Should the async compaction succeed, this increases the
chance that the non-MOVABLE allocations will fallback to a MOVABLE
pageblock, making the long-term fragmentation worse.

This patch attempts to help the situation by changing async direct
compaction so that the migrate scanner only scans the pageblocks of the
requested migratetype.  If it's a non-MOVABLE type and there are such
pageblocks that do contain movable pages, chances are that the
allocation can succeed within one of such pageblocks, removing the need
for a fallback.  If that fails, the subsequent sync attempt will ignore
this restriction.

In testing based on 4.9 kernel with stress-highalloc from mmtests
configured for order-4 GFP_KERNEL allocations, this patch has reduced
the number of unmovable allocations falling back to movable pageblocks
by 30%.  The number of movable allocations falling back is reduced by
12%.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Vlastimil Babka 2017-05-08 15:54:49 -07:00 • committed by Linus Torvalds
commit 282722b0d2
2 changed files with 22 additions and 9 deletions
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11
mm/compaction.c
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@ -986,10 +986,17 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
static bool suitable_migration_source(struct compact_control *cc,
struct page *page)
{
if (cc->mode != MIGRATE_ASYNC)
int block_mt;
if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
return true;
return is_migrate_movable(get_pageblock_migratetype(page));
block_mt = get_pageblock_migratetype(page);
if (cc->migratetype == MIGRATE_MOVABLE)
return is_migrate_movable(block_mt);
else
return block_mt == cc->migratetype;
}
/* Returns true if the page is within a block suitable for migration to */