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

[Mel Gorman]

On Thu, Jan 17, 2019 at 06:17:28PM +0100, Vlastimil Babka wrote:
> 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 :)
> 

No, unfortunately it wouldn't. I did find though that sync_light often
ran very quickly after async when compaction was having trouble
succeeding. The time window was short enough that states like
Dirty/Writeback were highly unlikely to be cleared. It might have played
nice when fragmentation was very low but any benefit then would be very
difficult to detect.

-- 
Mel Gorman
SUSE Labs