linux-kernel - Re: [PATCH 3/6] mm: vmscan: Do not reclaim from lower zones if they are balanced

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Date:	Wed, 25 Jun 2014 16:32:50 -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>,
	Jens Axboe <axboe@...nel.dk>, Jeff Moyer <jmoyer@...hat.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 3/6] mm: vmscan: Do not reclaim from lower zones if they
 are balanced

On Wed, 25 Jun 2014 08:58:46 +0100 Mel Gorman <mgorman@...e.de> wrote:

> Historically kswapd scanned from DMA->Movable in the opposite direction
> to the page allocator to avoid allocating behind kswapd direction of
> progress. The fair zone allocation policy altered this in a non-obvious
> manner.
> 
> Traditionally, the page allocator prefers to use the highest eligible zone
> until the watermark is depleted, woke kswapd and moved onto the next zone.
> kswapd scans zones in the opposite direction so the scanning lists on
> 64-bit look like this;
> 
> ...
>
> Note that this patch makes a large performance difference for lower
> numbers of threads and brings performance closer to 3.0 figures. It was
> also tested against xfs and there are similar gains although I don't have
> 3.0 figures to compare against. There are still regressions for higher
> number of threads but this is related to changes in the CFQ IO scheduler.
> 

Why did this patch make a difference to sequential read performance? 
IOW, by what means was/is reclaim interfering with sequential reads?

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