On 2016/10/13 16:08, js1...@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo....@lge.com>
> 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
> 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.
I find another two places to reduce fragmentation.
If we steal some free pages, we will add these page at the head of
this will cause more fixed migratetype, because this pages will be allocated
So how about use list_move_tail instead of list_move?
How about use list_add_tail instead of list_add? If add the tail, then the rest
will be hard to be allocated and we can merge them again as soon as the page