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SubjectRe: [PATCH 5/5] mm: have order > 0 compaction start near a pageblock with free pages
On Wed, Aug 08, 2012 at 08:08:44PM +0100, Mel Gorman wrote:
> commit [7db8889a: mm: have order > 0 compaction start off where it left]
> introduced a caching mechanism to reduce the amount work the free page
> scanner does in compaction. However, it has a problem. Consider two process
> simultaneously scanning free pages
> C
> Process A M S F
> |---------------------------------------|
> Process B M FS
> C is zone->compact_cached_free_pfn
> S is cc->start_pfree_pfn
> M is cc->migrate_pfn
> F is cc->free_pfn
> In this diagram, Process A has just reached its migrate scanner, wrapped
> around and updated compact_cached_free_pfn accordingly.
> Simultaneously, Process B finishes isolating in a block and updates
> compact_cached_free_pfn again to the location of its free scanner.
> Process A moves to "end_of_zone - one_pageblock" and runs this check
> if (cc->order > 0 && (!cc->wrapped ||
> zone->compact_cached_free_pfn >
> cc->start_free_pfn))
> pfn = min(pfn, zone->compact_cached_free_pfn);
> compact_cached_free_pfn is above where it started so the free scanner skips
> almost the entire space it should have scanned. When there are multiple
> processes compacting it can end in a situation where the entire zone is
> not being scanned at all. Further, it is possible for two processes to
> ping-pong update to compact_cached_free_pfn which is just random.
> Overall, the end result wrecks allocation success rates.
> There is not an obvious way around this problem without introducing new
> locking and state so this patch takes a different approach.
> First, it gets rid of the skip logic because it's not clear that it matters
> if two free scanners happen to be in the same block but with racing updates
> it's too easy for it to skip over blocks it should not.
> Second, it updates compact_cached_free_pfn in a more limited set of
> circumstances.
> If a scanner has wrapped, it updates compact_cached_free_pfn to the end
> of the zone. When a wrapped scanner isolates a page, it updates
> compact_cached_free_pfn to point to the highest pageblock it
> can isolate pages from.
> If a scanner has not wrapped when it has finished isolated pages it
> checks if compact_cached_free_pfn is pointing to the end of the
> zone. If so, the value is updated to point to the highest
> pageblock that pages were isolated from. This value will not
> be updated again until a free page scanner wraps and resets
> compact_cached_free_pfn.
> This is not optimal and it can still race but the compact_cached_free_pfn
> will be pointing to or very near a pageblock with free pages.
> Signed-off-by: Mel Gorman <>
> Reviewed-by: Rik van Riel <>
Reviewed-by: Minchan Kim <>

Kind regards,
Minchan Kim

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