Re: [PATCH] mm: vmscan: Stop reclaim/compaction earlier due toinsufficient progress if !__GFP_REPEAT

[Andrew Morton]

On Wed, 16 Feb 2011 09:50:49 +0000
Mel Gorman <mel@xxxxxxxxx> wrote:

> should_continue_reclaim() for reclaim/compaction allows scanning to continue
> even if pages are not being reclaimed until the full list is scanned. In
> terms of allocation success, this makes sense but potentially it introduces
> unwanted latency for high-order allocations such as transparent hugepages
> and network jumbo frames that would prefer to fail the allocation attempt
> and fallback to order-0 pages.  Worse, there is a potential that the full
> LRU scan will clear all the young bits, distort page aging information and
> potentially push pages into swap that would have otherwise remained resident.

afaict the patch affects order-0 allocations as well.  What are the
implications of this?

Also, what might be the downsides of this change, and did you test for
them?

> This patch will stop reclaim/compaction if no pages were reclaimed in the
> last SWAP_CLUSTER_MAX pages that were considered.

a) Why SWAP_CLUSTER_MAX?  Is (SWAP_CLUSTER_MAX+7) better or worse?

b) The sentence doesn't seem even vaguely accurate.  shrink_zone()
   will scan vastly more than SWAP_CLUSTER_MAX pages before calling
   should_continue_reclaim().  Confused.

c) The patch doesn't "stop reclaim/compaction" fully.  It stops it
   against one zone.  reclaim will then advance on to any other
   eligible zones.

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