Messages in this thread | | | Date | Mon, 03 Nov 2014 09:12:33 +0100 | From | Vlastimil Babka <> | Subject | Re: [PATCH 5/5] mm, compaction: more focused lru and pcplists draining |
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On 10/27/2014 08:41 AM, Joonsoo Kim wrote: > On Tue, Oct 07, 2014 at 05:33:39PM +0200, Vlastimil Babka wrote: >> The goal of memory compaction is to create high-order freepages through page >> migration. Page migration however puts pages on the per-cpu lru_add cache, >> which is later flushed to per-cpu pcplists, and only after pcplists are >> drained the pages can actually merge. This can happen due to the per-cpu >> caches becoming full through further freeing, or explicitly. >> >> During direct compaction, it is useful to do the draining explicitly so that >> pages merge as soon as possible and compaction can detect success immediately >> and keep the latency impact at minimum. However the current implementation is >> far from ideal. Draining is done only in __alloc_pages_direct_compact(), >> after all zones were already compacted, and the decisions to continue or stop >> compaction in individual zones was done without the last batch of migrations >> being merged. It is also missing the draining of lru_add cache before the >> pcplists. >> >> This patch moves the draining for direct compaction into compact_zone(). It >> adds the missing lru_cache draining and uses the newly introduced single zone >> pcplists draining to reduce overhead and avoid impact on unrelated zones. >> Draining is only performed when it can actually lead to merging of a page of >> desired order (passed by cc->order). This means it is only done when migration >> occurred in the previously scanned cc->order aligned block(s) and the >> migration scanner is now pointing to the next cc->order aligned block. >> >> The patch has been tested with stress-highalloc benchmark from mmtests. >> Although overal allocation success rates of the benchmark were not affected, >> the number of detected compaction successes has doubled. This suggests that >> allocations were previously successful due to implicit merging caused by >> background activity, making a later allocation attempt succeed immediately, >> but not attributing the success to compaction. Since stress-highalloc always >> tries to allocate almost the whole memory, it cannot show the improvement in >> its reported success rate metric. However after this patch, compaction should >> detect success and terminate earlier, reducing the direct compaction latencies >> in a real scenario. >> >> 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: Michal Nazarewicz <mina86@mina86.com> >> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> >> Cc: Christoph Lameter <cl@linux.com> >> Cc: Rik van Riel <riel@redhat.com> >> Cc: David Rientjes <rientjes@google.com> >> --- >> mm/compaction.c | 41 ++++++++++++++++++++++++++++++++++++++++- >> mm/page_alloc.c | 4 ---- >> 2 files changed, 40 insertions(+), 5 deletions(-) >> >> diff --git a/mm/compaction.c b/mm/compaction.c >> index 8fa888d..41b49d7 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -1179,6 +1179,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) >> while ((ret = compact_finished(zone, cc, migratetype)) == >> COMPACT_CONTINUE) { >> int err; >> + unsigned long last_migrated_pfn = 0; > > I think that this definition looks odd. > In every iteration, last_migrated_pfn is re-defined as 0. > Maybe, it is on outside of the loop.
Oops you're right, that's a mistake and it makes the code miss some of the drain points (a minority I think but anyway).
>> >> switch (isolate_migratepages(zone, cc)) { >> case ISOLATE_ABORT: >> @@ -1187,7 +1188,12 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) >> cc->nr_migratepages = 0; >> goto out; >> case ISOLATE_NONE: >> - continue; >> + /* >> + * We haven't isolated and migrated anything, but >> + * there might still be unflushed migrations from >> + * previous cc->order aligned block. >> + */ >> + goto check_drain; >> case ISOLATE_SUCCESS: >> ; >> } >> @@ -1212,6 +1218,39 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) >> goto out; >> } >> } >> + >> + /* >> + * Record where we have freed pages by migration and not yet >> + * flushed them to buddy allocator. Subtract 1, because often >> + * we finish a pageblock and migrate_pfn points to the first >> + * page* of the next one. In that case we want the drain below >> + * to happen immediately. >> + */ >> + if (!last_migrated_pfn) >> + last_migrated_pfn = cc->migrate_pfn - 1; > > And, I wonder why last_migrated_pfn is set after isolate_migratepages().
Not sure I understand your question. With the mistake above, it cannot currently be set at the point isolate_migratepages() is called, so you might question the goto check_drain in the ISOLATE_NONE case, if that's what you are wondering about.
When I correct that, it might be set when COMPACT_CLUSTER_MAX pages are isolated and migrated the middle of a pageblock, and then the rest of the pageblock contains no pages that could be isolated, so the last isolate_migratepages() attempt in the pageblock returns with ISOLATE_NONE. Still there were some migrations that produced free pages that should be drained at that point.
> > Thanks. >
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