On 02/15/2017 12:56 PM, Xishi Qiu wrote: > On 2017/2/15 18:47, Vlastimil Babka wrote: > >> On 02/14/2017 11:07 AM, Xishi Qiu wrote: >>> On 2017/2/11 1:23, Vlastimil Babka wrote: >>> >>>> When stealing pages from pageblock of a different migratetype, we count how >>>> many free pages were stolen, and change the pageblock's migratetype if more >>>> than half of the pageblock was free. This might be too conservative, as there >>>> might be other pages that are not free, but were allocated with the same >>>> migratetype as our allocation requested. >>>> >>>> While we cannot determine the migratetype of allocated pages precisely (at >>>> least without the page_owner functionality enabled), we can count pages that >>>> compaction would try to isolate for migration - those are either on LRU or >>>> __PageMovable(). The rest can be assumed to be MIGRATE_RECLAIMABLE or >>>> MIGRATE_UNMOVABLE, which we cannot easily distinguish. This counting can be >>>> done as part of free page stealing with little additional overhead. >>>> >>>> The page stealing code is changed so that it considers free pages plus pages >>>> of the "good" migratetype for the decision whether to change pageblock's >>>> migratetype. >>>> >>>> The result should be more accurate migratetype of pageblocks wrt the actual >>>> pages in the pageblocks, when stealing from semi-occupied pageblocks. This >>>> should help the efficiency of page grouping by mobility. >>>> >>>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> >>> >>> Hi Vlastimil, >>> >>> How about these two changes? >>> >>> 1. If we steal some free pages, we will add these page at the head of start_migratetype >>> list, it will cause more fixed, because these pages will be allocated more easily. >> >> What do you mean by "more fixed" here? >> >>> So how about use list_move_tail instead of list_move? >> >> Hmm, not sure if it can make any difference. We steal because the lists >> are currently empty (at least for the order we want), so it shouldn't >> matter if we add to head or tail. >> > > Hi Vlastimil, > > Please see the following case, I am not sure if it is right. > > MIGRATE_MOVABLE > order: 0 1 2 3 4 5 6 7 8 9 10 > free num: 1 1 1 1 1 1 1 1 1 1 0 // one page(e.g. page A) was allocated before > > MIGRATE_UNMOVABLE > order: 0 1 2 3 4 5 6 7 8 9 10 > free num: x x x x 0 0 0 0 0 0 0 // we want order=4, so steal from MIGRATE_MOVABLE > > We alloc order=4 in MIGRATE_UNMOVABLE, then it will fallback to steal pages from > MIGRATE_MOVABLE, and we will move free pages form MIGRATE_MOVABLE list to > MIGRATE_UNMOVABLE list. > > List of order 4-9 in MIGRATE_UNMOVABLE is empty, so add head or tail is the same. > But order 0-3 is not empty, so if we add to the head, we will allocate pages which > stolen from MIGRATE_MOVABLE first later. So we will have less chance to make a large > block(order=10) when the one page(page A) free again. I see. But do we know that page A, and the order-4 page we just allocated, are both going to be freed soon? It's not a clear win to me, so maybe you can try implementing it and see if it makes any difference? > Also we will split order=9 which from MIGRATE_MOVABLE to alloc order=4 in expand(), Yes, for pageblock order == 9. > so if we add to the head, we will allocate pages which split from order=9 first later. > So we will have less chance to make a large block(order=9) when the order=4 page > free again. Again that assumes our order-4 allocation is temporary. Is there a significant chance of this? >>> __rmqueue_fallback >>> steal_suitable_fallback >>> move_freepages_block >>> move_freepages >>> list_move >>> >>> 2. When doing expand() - list_add(), usually the list is empty, but in the >>> following case, the list is not empty, because we did move_freepages_block() >>> before. >>> >>> __rmqueue_fallback >>> steal_suitable_fallback >>> move_freepages_block // move to the list of start_migratetype >>> expand // split the largest order >>> list_add // add to the list of start_migratetype >>> >>> So how about use list_add_tail instead of list_add? Then we can merge the large >>> block again as soon as the page freed. >> >> Same here. The lists are not empty, but contain probably just the pages >> from our stolen pageblock. It shouldn't matter how we order them within >> the same block. >> >> So maybe it could make some difference for higher-order allocations, but >> it's unclear to me. Making e.g. expand() more complex with a flag to >> tell it the head vs tail add could mean extra overhead in allocator fast >> path that would offset any gains. >> >>> 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>
