On Tue, Feb 17, 2015 at 10:46:04AM +0100, Vlastimil Babka wrote:
> On 02/12/2015 08:15 AM, 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.
>
> Hm that's weird. I'll try to investigate this later. Meanwhile it
> can stay as it is.
>
> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Okay. Thanks.
>
> But you'll need to fix:
>
> >---
> > mm/compaction.c | 14 ++++++++++++--
> > mm/internal.h | 2 ++
> > mm/page_alloc.c | 19 ++++++++++++++-----
> > 3 files changed, 28 insertions(+), 7 deletions(-)
> >
> >diff --git a/mm/compaction.c b/mm/compaction.c
> >index 782772d..d40c426 100644
> >--- 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]))
> > return COMPACT_PARTIAL;
> >
> >- /* 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]))
>
> This won't compile with !CONFIG_CMA, right? I recall pointing it on
> v3 already (or something similar elsewhere).
Will fix.
>
> >+ return COMPACT_PARTIAL;
> >+
> >+ /*
> >+ * Job done if allocation would steal freepages from
> >+ * other migratetype buddy lists.
> >+ */
> >+ if (find_suitable_fallback(area, order, migratetype,
> >+ true, &can_steal) != -1)
> > return COMPACT_PARTIAL;
> > }
> >
> >diff --git a/mm/internal.h b/mm/internal.h
> >index c4d6c9b..9640650 100644
> >--- a/mm/internal.h
> >+++ b/mm/internal.h
> >@@ -200,6 +200,8 @@ isolate_freepages_range(struct compact_control *cc,
> > unsigned long
> > isolate_migratepages_range(struct compact_control *cc,
> > unsigned long low_pfn, unsigned long end_pfn);
> >+int find_suitable_fallback(struct free_area *area, unsigned int order,
> >+ int migratetype, bool only_stealable, bool *can_steal);
> >
> > #endif
> >
> >diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >index 64a4974..95654f9 100644
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -1191,9 +1191,14 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
> > set_pageblock_migratetype(page, start_type);
> > }
> >
> >-/* Check whether there is a suitable fallback freepage with requested order. */
> >-static int find_suitable_fallback(struct free_area *area, unsigned int order,
> >- int migratetype, bool *can_steal)
> >+/*
> >+ * Check whether there is a suitable fallback freepage with requested order.
> >+ * If only_stealable is true, this function returns fallback_mt only if
> >+ * we can steal other freepages all together. This would help to reduce
> >+ * fragmentation due to mixed migratetype pages in one pageblock.
> >+ */
> >+int find_suitable_fallback(struct free_area *area, unsigned int order,
> >+ int migratetype, bool only_stealable, bool *can_steal)
> > {
> > int i;
> > int fallback_mt;
> >@@ -1213,7 +1218,11 @@ static int find_suitable_fallback(struct free_area *area, unsigned int order,
> > if (can_steal_fallback(order, migratetype))
> > *can_steal = true;
> >
> >- return fallback_mt;
> >+ if (!only_stealable)
> >+ return fallback_mt;
> >+
> >+ if (*can_steal)
> >+ return fallback_mt;
>
> Why not just single if (!only_stealable || *can_steal)
Will fix, too.
Thanks.
--
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>