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?
> @@ -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 */
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>