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

[Rik van Riel]

On 06/28/2012 04:59 PM, Andrew Morton wrote:
> On Thu, 28 Jun 2012 13:55:20 -0400
> Rik van Riel<riel@xxxxxxxxxx>  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?

The compaction side of the zone always starts at the
very beginning of the zone.  I believe we can get
away with this, because skipping a whole transparent
hugepage or non-movable block is 512 times faster than
scanning an entire block for target pages in
isolate_freepages.

> > @@ -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.

Reading the code a few more times, I believe that it is
indeed aligned to pageblock size.

> > --- 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 :(

Agreed.  I'm not sure how to properly describe that variable
in 30 or so characters :)

It denotes whether the current invocation of compaction,
called with order > 0, has had free_pfn and migrate_pfn
meet, resulting in free_pfn being reset to the top of
the zone.

Now, how to describe that briefly?

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