Messages in this thread | | | Date | Wed, 07 May 2014 11:22:59 +0200 | From | Vlastimil Babka <> | Subject | Re: [PATCH 2/2] mm/compaction: avoid rescanning pageblocks in isolate_freepages |
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On 05/07/2014 12:19 AM, Naoya Horiguchi wrote: > On Fri, May 02, 2014 at 05:27:55PM +0200, Vlastimil Babka wrote: >> The compaction free scanner in isolate_freepages() currently remembers PFN of >> the highest pageblock where it successfully isolates, to be used as the >> starting pageblock for the next invocation. The rationale behind this is that >> page migration might return free pages to the allocator when migration fails >> and we don't want to skip them if the compaction continues. >> >> Since migration now returns free pages back to compaction code where they can >> be reused, this is no longer a concern. This patch changes isolate_freepages() >> so that the PFN for restarting is updated with each pageblock where isolation >> is attempted. Using stress-highalloc from mmtests, this resulted in 10% >> reduction of the pages scanned by the free scanner. >> >> Note that the somewhat similar functionality that records highest successful >> pageblock in zone->compact_cached_free_pfn, remains unchanged. This cache is >> used when the whole compaction is restarted, not for multiple invocations of >> the free scanner during single compaction. >> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz> >> Cc: Minchan Kim <minchan@kernel.org> >> Cc: Mel Gorman <mgorman@suse.de> >> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> >> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> >> Cc: Christoph Lameter <cl@linux.com> >> Cc: Rik van Riel <riel@redhat.com> >> --- >> mm/compaction.c | 18 ++++++------------ >> 1 file changed, 6 insertions(+), 12 deletions(-) >> >> diff --git a/mm/compaction.c b/mm/compaction.c >> index 873d7de..1967850 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -686,12 +686,6 @@ static void isolate_freepages(struct zone *zone, >> low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); >> >> /* >> - * If no pages are isolated, the block_start_pfn < low_pfn check >> - * will kick in. >> - */ >> - next_free_pfn = 0; >> - >> - /* >> * Isolate free pages until enough are available to migrate the >> * pages on cc->migratepages. We stop searching if the migrate >> * and free page scanners meet or enough free pages are isolated. >> @@ -731,19 +725,19 @@ static void isolate_freepages(struct zone *zone, >> continue; >> >> /* Found a block suitable for isolating free pages from */ >> + next_free_pfn = block_start_pfn; >> isolated = isolate_freepages_block(cc, block_start_pfn, >> block_end_pfn, freelist, false); >> nr_freepages += isolated; >> >> /* >> - * Record the highest PFN we isolated pages from. When next >> - * looking for free pages, the search will restart here as >> - * page migration may have returned some pages to the allocator >> + * Set a flag that we successfully isolated in this pageblock. >> + * In the next loop iteration, zone->compact_cached_free_pfn >> + * will not be updated and thus it will effectively contain the >> + * highest pageblock we isolated pages from. >> */ >> - if (isolated && next_free_pfn == 0) { >> + if (isolated) >> cc->finished_update_free = true; >> - next_free_pfn = block_start_pfn; >> - } > > Why don't you completely remove next_free_pfn and update cc->free_pfn directly?
Hi,
well you could ask the same about the nr_freepages variable. For me personally local variable (now with a comment for next_free_pfn) looks more readable. The function currently received cleanup from several people (including me), so I wouldn't want to change it again, unless others also think it would be better. From compiler standpoint both variants should be the same I guess.
Vlastimil
> Thanks, > Naoya Horiguchi >
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