On 2017/2/15 18:47, Vlastimil Babka wrote:

On 02/14/2017 11:07 AM, Xishi Qiu wrote:

quoted

On 2017/2/11 1:23, Vlastimil Babka wrote:

quoted

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 <redacted>

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?

quoted

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.

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
free again.

quoted

__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.

quoted

Thanks,
Xishi Qiu

.

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