Re: [PATCH V4 02/15] mm, compaction: defer each zone individually instead of preferred zone

[Mel Gorman]

On Wed, Jul 16, 2014 at 03:48:10PM +0200, Vlastimil Babka wrote:
> When direct sync compaction is often unsuccessful, it may become deferred for
> some time to avoid further useless attempts, both sync and async. Successful
> high-order allocations un-defer compaction, while further unsuccessful
> compaction attempts prolong the copmaction deferred period.
> 
> Currently the checking and setting deferred status is performed only on the
> preferred zone of the allocation that invoked direct compaction. But compaction
> itself is attempted on all eligible zones in the zonelist, so the behavior is
> suboptimal and may lead both to scenarios where 1) compaction is attempted
> uselessly, or 2) where it's not attempted despite good chances of succeeding,
> as shown on the examples below:
> 
> 1) A direct compaction with Normal preferred zone failed and set deferred
>    compaction for the Normal zone. Another unrelated direct compaction with
>    DMA32 as preferred zone will attempt to compact DMA32 zone even though
>    the first compaction attempt also included DMA32 zone.
> 
>    In another scenario, compaction with Normal preferred zone failed to compact
>    Normal zone, but succeeded in the DMA32 zone, so it will not defer
>    compaction. In the next attempt, it will try Normal zone which will fail
>    again, instead of skipping Normal zone and trying DMA32 directly.
> 
> 2) Kswapd will balance DMA32 zone and reset defer status based on watermarks
>    looking good. A direct compaction with preferred Normal zone will skip
>    compaction of all zones including DMA32 because Normal was still deferred.
>    The allocation might have succeeded in DMA32, but won't.
> 
> This patch makes compaction deferring work on individual zone basis instead of
> preferred zone. For each zone, it checks compaction_deferred() to decide if the
> zone should be skipped. If watermarks fail after compacting the zone,
> defer_compaction() is called. The zone where watermarks passed can still be
> deferred when the allocation attempt is unsuccessful. When allocation is
> successful, compaction_defer_reset() is called for the zone containing the
> allocated page. This approach should approximate calling defer_compaction()
> only on zones where compaction was attempted and did not yield allocated page.
> There might be corner cases but that is inevitable as long as the decision
> to stop compacting dues not guarantee that a page will be allocated.
> 
> During testing on a two-node machine with a single very small Normal zone on
> node 1, this patch has improved success rates in stress-highalloc mmtests
> benchmark. The success here were previously made worse by commit 3a025760fc
> ("mm: page_alloc: spill to remote nodes before waking kswapd") as kswapd was
> no longer resetting often enough the deferred compaction for the Normal zone,
> and DMA32 zones on both nodes were thus not considered for compaction.
> On different machine, success rates were improved with __GFP_NO_KSWAPD
> allocations.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
> Reviewed-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxx>

-- 
Mel Gorman
SUSE Labs
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