Re: [PATCH -mm] mm: have order>0 compaction start off where it left

[Kamezawa Hiroyuki]

(2012/06/29 1:30), Rik van Riel wrote:
> On 06/28/2012 06:29 AM, Mel Gorman wrote:
>
> > 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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