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 <mgorman@xxxxxxx> > Reviewed-by: Rik van Riel <riel@xxxxxxxxxx> Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>