linux-kernel - Re: [PATCH 1/3] mm: kswapd: Stop high-order balancing when any suitable zone is balanced

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Date:	Wed, 01 Dec 2010 10:13:56 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Simon Kirby <sim@...tway.ca>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm: kswapd: Stop high-order balancing when any
 suitable zone is balanced

On Wed, 2010-12-01 at 01:15 +0800, Mel Gorman wrote:
> When the allocator enters its slow path, kswapd is woken up to balance the
> node. It continues working until all zones within the node are balanced. For
> order-0 allocations, this makes perfect sense but for higher orders it can
> have unintended side-effects. If the zone sizes are imbalanced, kswapd
> may reclaim heavily on a smaller zone discarding an excessive number of
> pages. The user-visible behaviour is that kswapd is awake and reclaiming
> even though plenty of pages are free from a suitable zone.
> 
> This patch alters the "balance" logic to stop kswapd if any suitable zone
> becomes balanced to reduce the number of pages it reclaims from other zones.
from my understanding, the patch will break reclaim high zone if a low
zone meets the high order allocation, even the high zone doesn't meet
the high order allocation. This, for example, will make a high order
allocation from a high zone fallback to low zone and quickly exhaust low
zone, for example DMA. This will break some drivers.

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