Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it left

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 Jun 2012 13:55:20 -0400
Rik van Riel <riel@redhat.com> wrote:

> Order > 0 compaction stops when enough free pages of the correct
> page order have been coalesced. When doing subsequent higher order
> allocations, it is possible for compaction to be invoked many times.
> 
> However, the compaction code always starts out looking for things to
> compact at the start of the zone, and for free pages to compact things
> to at the end of the zone.
> 
> This can cause quadratic behaviour, with isolate_freepages starting
> at the end of the zone each time, even though previous invocations
> of the compaction code already filled up all free memory on that end
> of the zone.
> 
> This can cause isolate_freepages to take enormous amounts of CPU
> with certain workloads on larger memory systems.
> 
> The obvious solution is to have isolate_freepages remember where
> it left off last time, and continue at that point the next time
> it gets invoked for an order > 0 compaction. This could cause
> compaction to fail if cc->free_pfn and cc->migrate_pfn are close
> together initially, in that case we restart from the end of the
> zone and try once more.
> 
> Forced full (order == -1) compactions are left alone.

Is there a quality of service impact here?  Newly-compactable pages
at lower pfns than compact_cached_free_pfn will now get missed, leading
to a form of fragmentation?

> @@ -463,6 +474,8 @@ static void isolate_freepages(struct zone *zone,
>  		 */
>  		if (isolated)
>  			high_pfn = max(high_pfn, pfn);
> +		if (cc->order > 0)
> +			zone->compact_cached_free_pfn = high_pfn;

Is high_pfn guaranteed to be aligned to pageblock_nr_pages here?  I
assume so, if lots of code in other places is correct but it's
unobvious from reading this function.

>  	}
>  
>  	/* split_free_page does not map the pages */
>
> ...
>
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -118,8 +118,10 @@ struct compact_control {
>  	unsigned long nr_freepages;	/* Number of isolated free pages */
>  	unsigned long nr_migratepages;	/* Number of pages to migrate */
>  	unsigned long free_pfn;		/* isolate_freepages search base */
> +	unsigned long start_free_pfn;	/* where we started the search */
>  	unsigned long migrate_pfn;	/* isolate_migratepages search base */
>  	bool sync;			/* Synchronous migration */
> +	bool wrapped;			/* Last round for order>0 compaction */

This comment is incomprehensible :(

>  
>  	int order;			/* order a direct compactor needs */
>  	int migratetype;		/* MOVABLE, RECLAIMABLE etc */

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