Re: [PATCH v2 04/10] mm, page_alloc: count movable pages when stealing from pageblock
From: Xishi Qiu
Date: Wed Feb 15 2017 - 06:56:58 EST
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
>>> 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.
Please see the following case, I am not sure if it is right.
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
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
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.
Also we will split order=9 which from MIGRATE_MOVABLE to alloc order=4 in expand(),
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
>> 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()
>> 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.
>> Xishi Qiu