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

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Date:	Fri, 29 Jun 2012 11:58:29 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.org,
	akpm@...ux-foundation.org, minchan@...nel.org,
	linux-kernel@...r.kernel.org, jaschut@...dia.gov
Subject: Re: [PATCH -mm] mm: have order>0 compaction start off where it left

(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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