Re: [RFC PATCH 00/10] redesign compaction algorithm

[Vlastimil Babka <vbabka@suse.cz>]

On 25.6.2015 20:14, Joonsoo Kim wrote:
>> The long-term success rate of fragmentation avoidance depends on
>> > minimsing the number of UNMOVABLE allocation requests that use a
>> > pageblock belonging to another migratetype. Once such a fallback occurs,
>> > that pageblock potentially can never be used for a THP allocation again.
>> >
>> > Lets say there is an unmovable pageblock with 500 free pages in it. If
>> > the freepage scanner uses that pageblock and allocates all 500 free
>> > pages then the next unmovable allocation request needs a new pageblock.
>> > If one is not completely free then it will fallback to using a
>> > RECLAIMABLE or MOVABLE pageblock forever contaminating it.
> Yes, I can imagine that situation. But, as I said above, we already use
> non-movable pageblock for migration scanner. While unmovable
> pageblock with 500 free pages fills, some other unmovable pageblock
> with some movable pages will be emptied. Number of freepage
> on non-movable would be maintained so fallback doesn't happen.

There's nothing that guarantees that the migration scanner will be emptying
unmovable pageblock, or am I missing something? Worse, those pageblocks would be
marked to skip by the free scanner if it isolated free pages from them, so
migration scanner would skip them.