linux-kernel - Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2

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Date:	Tue, 23 Feb 2016 12:04:16 -0800
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/27] Move LRU page reclaim from zones to nodes v2

On Tue, Feb 23, 2016 at 03:04:23PM +0000, Mel Gorman wrote:
> In many benchmarks, there is an obvious difference in the number of
> allocations from each zone as the fair zone allocation policy is removed
> towards the end of the series. For example, this is the allocation stats
> when running blogbench that showed no difference in headling performance
> 
>                           mmotm-20160209   nodelru-v2
> DMA allocs                           0           0
> DMA32 allocs                   7218763      608067
> Normal allocs                 12701806    18821286
> Movable allocs                       0           0

According to the mmotm numbers, your DMA32 zone is over a third of
available memory, yet in the nodelru-v2 kernel sees only 3% of the
allocations. That's an insanely high level of aging inversion, where
the lifetime of a cache entry is again highly dependent on placement.

The fact that this doesn't make a performance difference in the
specific benchmarks you ran only proves just that: these specific
benchmarks don't care. IMO, benchmarking is not enough here. If this
is truly supposed to be unproblematic, then I think we need a reasoned
explanation. I can't imagine how it possibly could be, though.

If reclaim can't guarantee a balanced zone utilization then the
allocator has to keep doing it. :( As far as I'm concerned, the
original reason for the fair zone allocator still applies.