Re: [PATCH 21/25] mm, compaction: Round-robin the order while searching the free lists for a target

From: Vlastimil Babka
Date: Fri Jan 18 2019 - 04:18:02 EST

On 1/4/19 1:50 PM, Mel Gorman wrote:
> As compaction proceeds and creates high-order blocks, the free list
> search gets less efficient as the larger blocks are used as compaction
> targets. Eventually, the larger blocks will be behind the migration
> scanner for partially migrated pageblocks and the search fails. This
> patch round-robins what orders are searched so that larger blocks can be
> ignored and find smaller blocks that can be used as migration targets.
> The overall impact was small on 1-socket but it avoids corner cases where
> the migration/free scanners meet prematurely or situations where many of
> the pageblocks encountered by the free scanner are almost full instead of
> being properly packed. Previous testing had indicated that without this
> patch there were occasional large spikes in the free scanner without this
> patch. By co-incidence, the 2-socket results showed a 54% reduction in
> the free scanner but will not be universally true.
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>