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

[Andrew Morton]

On Wed, 25 Jun 2014 08:58:46 +0100 Mel Gorman <mgorman@xxxxxxx> 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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