On 2016/10/26 12:37, Joonsoo Kim wrote: > On Mon, Oct 17, 2016 at 05:21:54PM +0800, Xishi Qiu wrote: >> 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? > > Yeah... I don't think deeply but, at a glance, it would be helpful. > >> >> 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. > > I guess that it has no effect. When we do __rmqueue_fallback() and > expand(), we don't have any freepage on this or more order. So, > list_add or list_add_tail will show the same result. > Hi Joonsoo, Usually this list is empty, but in the following case, the list is not empty. __rmqueue_fallback steal_suitable_fallback move_freepages_block // move to the list of start_migratetype expand // split the largest order first list_add // add to the list of start_migratetype 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>