Re: [PATCH v4 3/3] mm/compaction: enhance compaction finish condition

From: Andrew Morton
Date: Wed Feb 18 2015 - 19:04:13 EST

On Thu, 12 Feb 2015 16:15:05 +0900 Joonsoo Kim <> wrote:

> Compaction has anti fragmentation algorithm. It is that freepage
> should be more than pageblock order to finish the compaction if we don't
> find any freepage in requested migratetype buddy list. This is for
> mitigating fragmentation, but, there is a lack of migratetype
> consideration and it is too excessive compared to page allocator's anti
> fragmentation algorithm.
> Not considering migratetype would cause premature finish of compaction.
> For example, if allocation request is for unmovable migratetype,
> freepage with CMA migratetype doesn't help that allocation and
> compaction should not be stopped. But, current logic regards this
> situation as compaction is no longer needed, so finish the compaction.
> Secondly, condition is too excessive compared to page allocator's logic.
> We can steal freepage from other migratetype and change pageblock
> migratetype on more relaxed conditions in page allocator. This is designed
> to prevent fragmentation and we can use it here. Imposing hard constraint
> only to the compaction doesn't help much in this case since page allocator
> would cause fragmentation again.
> To solve these problems, this patch borrows anti fragmentation logic from
> page allocator. It will reduce premature compaction finish in some cases
> and reduce excessive compaction work.
> stress-highalloc test in mmtests with non movable order 7 allocation shows
> considerable increase of compaction success rate.
> Compaction success rate (Compaction success * 100 / Compaction stalls, %)
> 31.82 : 42.20
> I tested it on non-reboot 5 runs stress-highalloc benchmark and found that
> there is no more degradation on allocation success rate than before. That
> roughly means that this patch doesn't result in more fragmentations.
> Vlastimil suggests additional idea that we only test for fallbacks
> when migration scanner has scanned a whole pageblock. It looked good for
> fragmentation because chance of stealing increase due to making more
> free pages in certain pageblock. So, I tested it, but, it results in
> decreased compaction success rate, roughly 38.00. I guess the reason that
> if system is low memory condition, watermark check could be failed due to
> not enough order 0 free page and so, sometimes, we can't reach a fallback
> check although migrate_pfn is aligned to pageblock_nr_pages. I can insert
> code to cope with this situation but it makes code more complicated so
> I don't include his idea at this patch.
> ...
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1170,13 +1170,23 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
> /* Direct compactor: Is a suitable page free? */
> for (order = cc->order; order < MAX_ORDER; order++) {
> struct free_area *area = &zone->free_area[order];
> + bool can_steal;
> /* Job done if page is free of the right migratetype */
> if (!list_empty(&area->free_list[migratetype]))
> - /* Job done if allocation would set block type */
> - if (order >= pageblock_order && area->nr_free)
> + /* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
> + if (migratetype == MIGRATE_MOVABLE &&
> + !list_empty(&area->free_list[MIGRATE_CMA]))

MIGRATE_CMA isn't defined if CONFIG_CMA=n.

--- a/mm/compaction.c~mm-compaction-enhance-compaction-finish-condition-fix
+++ a/mm/compaction.c
@@ -1180,11 +1180,12 @@ static int __compact_finished(struct zon
if (!list_empty(&area->free_list[migratetype]))

+#ifdef CONFIG_CMA
/* MIGRATE_MOVABLE can fallback on MIGRATE_CMA */
if (migratetype == MIGRATE_MOVABLE &&
* Job done if allocation would steal freepages from
* other migratetype buddy lists.

Please review the rest of the patchset for the CONFIG_CMA=n case (is it
all necessary?), runtime test it and let me know?

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