linux-kernel - Re: [PATCH 0/5] Prevent kswapd dumping excessive amounts of memory in response to high-order allocations V2

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Message-ID: <20101209015530.GD3796@hostway.ca>
Date:	Wed, 8 Dec 2010 17:55:30 -0800
From:	Simon Kirby <sim@...tway.ca>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] Prevent kswapd dumping excessive amounts of memory
	in response to high-order allocations V2

Hmm...

Wouldn't it make more sense for the fast path page allocator to allocate
weighted-round-robin (by zone size) from each zone, rather than just
starting from the highest and working down?

This would mean that each zone would get a proportional amount of
allocations and reclaiming a bit from each would likely throw out the
oldest allocations, rather than some of that and and some more recent
stuff that was allocated at the beginning of the lower zone.

For example, with the current approach, a time progression of allocations
looks like this (N=Normal, D=DMA32): 1N 2N 3N 4D 5D 6D 7D 8D 9D

...once the watermark is hit, kswapd reclaims 1 and 4, since they're
oldest in each zone, but 2 and 3 were allocated earlier.

Versus a weighted-round-robin approach: 1N 2D 3D 4N 5D 6D 7N 8D 9D

...kswapd reclaims 1 and 2, and they're oldest in time and maybe LRU.

Things probably eventually mix up enough once the system has reclaimed
and allocated more for a while with the current approach, but the
allocations are still chunky depending on how many extra things kswapd
reclaims to reach higher-order watermarks, and doesn't this always mess
with the LRU when the there are multiple usable zones?

Anyway, this approach might be horrible for some other reasons (page
allocations hoping to be sequential?  bigger cache footprint?), but it
might reduce the requirements for other other workarounds, and it would
make the LRU node-wide instead of zone-wide.

Simon-
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