On 2016/2/1 21:53, Mel Gorman wrote: > On Fri, Jan 29, 2016 at 10:03:52PM +0100, Vlastimil Babka wrote: >>> Since the root cause is that fallbacks might frequently split order-2 >>> and order-3 pages of the other migration types. This patch tweaks >>> fallback mechanism to avoid splitting order-2 and order-3 pages. while >>> fallbacks happen, if the largest feasible pages are less than or queal to >>> COSTLY_ORDER, i.e. 3, then try to select the smallest feasible pages. The >>> reason why fallbacks prefer the largest feasiable pages is to increase >>> fallback efficiency since fallbacks are likely to happen again. By >>> stealing the largest feasible pages, it could reduce the oppourtunities >>> of antoher fallback. Besides, it could make consecutive allocations more >>> approximate to each other and make system less fragment. However, if the >>> largest feasible pages are less than or equal to order-3, fallbacks might >>> split it and make the upcoming order-3 page allocations fail. >> >> In theory I don't see immediately why preferring smaller pages for >> fallback should be a clear win. If it's Unmovable allocations stealing >> from Movable pageblocks, the allocations will spread over larger areas >> instead of being grouped together. Maybe, for Movable allocations >> stealing from Unmovable allocations, preferring smallest might make >> sense and be safe, as any extra fragmentation is fixable bycompaction. > > I strongly agree that spreading the fallback allocations over a larger > area is likely to have a negative impact. Given the age of the kernel > being tested, it would make sense to either rebase or at the very last > backport the patches that affect watermark calculations and the > treatment of high-order pages. > Is it the feature of MIGRATE_HIGHATOMIC? Thanks, Xishi Qiu -- 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>