(2012/06/29 1:30), Rik van Riel wrote:

On 06/28/2012 06:29 AM, Mel Gorman wrote:

quoted

Lets say there are two parallel compactions running. Process A meets
the migration PFN and moves to the end of the zone to restart. Process B
finishes scanning mid-way through the zone and updates last_free_pfn. This
will cause Process A to "jump" to where Process B left off which is not
necessarily desirable.

Another side effect is that a workload that allocations/frees
aggressively will not compact as well as the "free" scanner is not
scanning the end of the zone each time. It would be better if
last_free_pfn was updated when a full pageblock was encountered

So;

1. Initialise last_free_pfn to the end of the zone
2. On compaction, scan from last_free_pfn and record where it started
3. If a pageblock is full, update last_free_pfn
4. If the migration and free scanner meet, reset last_free_pfn and
    the free scanner. Abort if the free scanner wraps to where it started

Does that make sense?

Yes, that makes sense.  We still have to keep track
of whether we have wrapped around, but I guess that
allows for a better name for the bool :)

Maybe cc->wrapped?

Does anyone have a better name?

cc->second_scan ? (I have no sense of naming ;)

As for point (4), should we abort when we wrap
around to where we started, or should we abort
when free_pfn and migrate_pfn meet after we
wrapped around?

I'd like to vote for aborting earlier.

Regards,
-Kame

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