On 2016/10/26 12:37, Joonsoo Kim wrote:
> On Mon, Oct 17, 2016 at 05:21:54PM +0800, Xishi Qiu wrote:
>> 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
>>> 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?
>
> Yeah... I don't think deeply but, at a glance, it would be helpful.
>
>>
>> 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.
>
> I guess that it has no effect. When we do __rmqueue_fallback() and
> expand(), we don't have any freepage on this or more order. So,
> list_add or list_add_tail will show the same result.
>
Hi Joonsoo,
Usually this list is empty, but in the following case, the list is not empty.
__rmqueue_fallback
steal_suitable_fallback
move_freepages_block // move to the list of start_migratetype
expand // split the largest order first
list_add // add to the list of start_migratetype
Thanks,
Xishi Qiu
