From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Allocation/free pattern is usually sequantial. If they are freed to
the buddy list, they can be coalesced. However, we first keep these freed
pages at the pcp list and try to reuse them until threshold is reached
so we don't have enough chance to get a high order freepage. This reusing
would provide us some performance advantages since we don't need to
get the zone lock and we don't pay the cost to check buddy merging.
But, less fragmentation and more high order freepage would compensate
this overhead in other ways. First, we would trigger less direct
compaction which has high overhead. And, there are usecases that uses
high order page to boost their performance.
Instantly resuing freed page seems to provide us computational benefit
but the other affects more precious things like as I/O performance and
memory consumption so I think that it's a good idea to weight
later advantage more.