On 2016/10/13 16:08, js1304@xxxxxxxxx wrote: > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > Currently, freeing page can stay longer in the buddy list if next higher > order page is in the buddy list in order to help coalescence. However, > it doesn't work for the simplest sequential free case. For example, think > about the situation that 8 consecutive pages are freed in sequential > order. > > page 0: attached at the head of order 0 list > page 1: merged with page 0, attached at the head of order 1 list > page 2: attached at the tail of order 0 list > page 3: merged with page 2 and then merged with page 0, attached at > the head of order 2 list > page 4: attached at the head of order 0 list > page 5: merged with page 4, attached at the tail of order 1 list > page 6: attached at the tail of order 0 list > page 7: merged with page 6 and then merged with page 4. Lastly, merged > with page 0 and we get order 3 freepage. > > With excluding page 0 case, there are three cases that freeing page is > attached at the head of buddy list in this example and if just one > corresponding ordered allocation request comes at that moment, this page > in being a high order page will be allocated and we would fail to make > order-3 freepage. > > Allocation usually happens in sequential order and free also does. So, it > would be important to detect such a situation and to give some chance > to be coalesced. > > I think that simple and effective heuristic about this case is just > attaching freeing page at the tail of the buddy list unconditionally. > If freeing isn't merged during one rotation, it would be actual > fragmentation and we don't need to care about it for coalescence. > Hi Joonsoo, I find another two places to reduce fragmentation. 1) __rmqueue_fallback steal_suitable_fallback move_freepages_block move_freepages list_move If we steal some free pages, we will add these page at the head of start_migratetype list, this will cause more fixed migratetype, because this pages will be allocated more easily. So how about use list_move_tail instead of list_move? 2) __rmqueue_fallback expand list_add How about use list_add_tail instead of list_add? If add the tail, then the rest of pages will be hard to be allocated and we can merge them again as soon as the page freed. 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>