On Wed, Oct 26, 2016 at 01:50:37PM +0800, Xishi Qiu wrote: > 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 In this case, stealed freepage on steal_suitable_fallback() and splitted freepage would come from the same pageblock. So, it doen't matter to use whatever list_add* function. Thanks. -- 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>