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Date:	Mon, 30 Jun 2014 14:14:04 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 4/4] mm: page_alloc: Reduce cost of the fair zone
 allocation policy

On Mon, 30 Jun 2014 17:48:03 +0100 Mel Gorman <mgorman@...e.de> 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.
> 
> This patch makes a number of changes that should reduce the overall cost
> 
> o Abort the fair zone allocation policy once remote zones are encountered
> o Use a simplier scan when resetting NR_ALLOC_BATCH
> o Use a simple flag to identify depleted zones instead of accessing a
>   potentially write-intensive cache line for counters
> 
> On UMA machines, the effect on overall performance is marginal. The main
> impact is on system CPU usage which is small enough on UMA to begin with.
> This comparison shows the system CPu usage between vanilla, the previous
> patch and this patch.
> 
>           3.16.0-rc2  3.16.0-rc2  3.16.0-rc2
>              vanilla checklow-v4 fairzone-v4
> User          390.13      400.85      396.13
> System        404.41      393.60      389.61
> Elapsed      5412.45     5166.12     5163.49
> 
> There is a small reduction and it appears consistent.
> 
> On NUMA machines, the scanning overhead is higher as zones are scanned
> that are ineligible for use by zone allocation policy. This patch fixes
> the zone-order zonelist policy and reduces the numbers of zones scanned
> by the allocator leading to an overall reduction of CPU usage.
> 
>           3.16.0-rc2  3.16.0-rc2  3.16.0-rc2
>              vanilla checklow-v4 fairzone-v4
> User          744.05      763.26      778.53
> System      70148.60    49331.48    44905.73
> Elapsed     28094.08    27476.72    27378.98

That's a large change in system time.  Does this all include kswapd
activity?

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