Re: [PATCH 0/3] OOM detection rework v4

[Joonsoo Kim]

2016-03-02 21:37 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
> On Wed 02-03-16 11:55:07, Joonsoo Kim wrote:
>> On Tue, Mar 01, 2016 at 07:14:08PM +0100, Vlastimil Babka wrote:
> [...]
>> > Yes, compaction is historically quite careful to avoid making low
>> > memory conditions worse, and to prevent work if it doesn't look like
>> > it can ultimately succeed the allocation (so having not enough base
>> > pages means that compacting them is considered pointless). This
>> > aspect of preventing non-zero-order OOMs is somewhat unexpected :)
>>
>> It's better not to assume that compaction would succeed all the times.
>> Compaction has some limitations so it sometimes fails.
>> For example, in lowmem situation, it only scans small parts of memory
>> and if that part is fragmented by non-movable page, compaction would fail.
>> And, compaction would defer requests 64 times at maximum if successive
>> compaction failure happens before.
>>
>> Depending on compaction heavily is right direction to go but I think
>> that it's not ready for now. More reclaim would relieve problem.
>
> I really fail to see why. The reclaimable memory can be migrated as
> well, no? Relying on the order-0 reclaim makes only sense to get over
> wmarks.

Attached link on previous reply mentioned limitation of current compaction
implementation. Briefly speaking, It would not scan all range of memory
due to algorithm limitation so even if there is reclaimable memory that
can be also migrated, compaction could fail.

There is no such limitation on reclaim and that's why I think that compaction
is not ready for now.

Thanks.