Re: [PATCH 17/25] mm, compaction: Keep cached migration PFNs synced for unusable pageblocks

[Vlastimil Babka]

On 1/4/19 1:50 PM, Mel Gorman wrote:
> Migrate has separate cached PFNs for ASYNC and SYNC* migration on the
> basis that some migrations will fail in ASYNC mode. However, if the cached
> PFNs match at the start of scanning and pageblocks are skipped due to
> having no isolation candidates, then the sync state does not matter.
> This patch keeps matching cached PFNs in sync until a pageblock with
> isolation candidates is found.
> 
> The actual benefit is marginal given that the sync scanner following the
> async scanner will often skip a number of pageblocks but it's useless
> work. Any benefit depends heavily on whether the scanners restarted
> recently so overall the reduction in scan rates is a mere 2.8% which
> is borderline noise.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

My easlier suggestion to check more thoroughly if pages can be migrated (which
depends on the mode) before isolating them wouldn't play nice with this :)