On 2016/10/13 16:08, js1304@gmail.com wrote:

From: Joonsoo Kim <redacted>

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?

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.

Thanks,
Xishi Qiu

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