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Date:	Mon, 20 Oct 2014 11:44:18 -0400
From:	Rik van Riel <>
To:	Vlastimil Babka <>,
	Andrew Morton <>
	Minchan Kim <>, Mel Gorman <>,
	Joonsoo Kim <>,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	David Rientjes <>
Subject: Re: [PATCH 5/5] mm, compaction: more focused lru and pcplists draining

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 <>

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