Re: [PATCH 5/5] mm, compaction: more focused lru and pcplists draining

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On 10/07/2014 11:33 AM, 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.


Acked-by: Rik van Riel <riel@xxxxxxxxxx>

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