Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy

[Johannes Weiner]

On Wed, Jul 09, 2014 at 09:13:08AM +0100, Mel Gorman wrote:
> The fair zone allocation policy round-robins allocations between zones
> within a node to avoid age inversion problems during reclaim. If the
> first allocation fails, the batch counts is reset and a second attempt
> made before entering the slow path.
> 
> One assumption made with this scheme is that batches expire at roughly the
> same time and the resets each time are justified. This assumption does not
> hold when zones reach their low watermark as the batches will be consumed
> at uneven rates.  Allocation failure due to watermark depletion result in
> additional zonelist scans for the reset and another watermark check before
> hitting the slowpath.
> 
> On UMA, the benefit is negligible -- around 0.25%. On 4-socket NUMA
> machine it's variable due to the variability of measuring overhead with
> the vmstat changes. The system CPU overhead comparison looks like
> 
>           3.16.0-rc3  3.16.0-rc3  3.16.0-rc3
>              vanilla   vmstat-v5 lowercost-v5
> User          746.94      774.56      802.00
> System      65336.22    32847.27    40852.33
> Elapsed     27553.52    27415.04    27368.46
> 
> However it is worth noting that the overall benchmark still completed
> faster and intuitively it makes sense to take as few passes as possible
> through the zonelists.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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