Messages in this thread | | | Date | Fri, 19 Jun 2015 14:58:20 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 4/6] mm, compaction: always skip compound pages by order in migrate scanner |
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On Wed, Jun 10, 2015 at 11:32:32AM +0200, Vlastimil Babka wrote: > The compaction migrate scanner tries to skip compound pages by their order, to > reduce number of iterations for pages it cannot isolate. The check is only done > if PageLRU() is true, which means it applies to THP pages, but not e.g. > hugetlbfs pages or any other non-LRU compound pages, which we have to iterate > by base pages. > > This limitation comes from the assumption that it's only safe to read > compound_order() when we have the zone's lru_lock and THP cannot be split under > us. But the only danger (after filtering out order values that are not below > MAX_ORDER, to prevent overflows) is that we skip too much or too little after > reading a bogus compound_order() due to a rare race. This is the same reasoning > as patch 99c0fd5e51c4 ("mm, compaction: skip buddy pages by their order in the > migrate scanner") introduced for unsafely reading PageBuddy() order. > > After this patch, all pages are tested for PageCompound() and we skip them by > compound_order(). The test is done after the test for balloon_page_movable() > as we don't want to assume if balloon pages (or other pages with own isolation > and migration implementation if a generic API gets implemented) are compound > or not. > > When tested with stress-highalloc from mmtests on 4GB system with 1GB hugetlbfs > pages, the vmstat compact_migrate_scanned count decreased by 15%. > > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
-- Mel Gorman SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in Please read the FAQ at http://www.tux.org/lkml/
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