Subject: Re: [PATCH 1/3] mm: kswapd: Stop high-order balancing when any
 suitable zone is balanced
From: Shaohua Li <shaohua.li@intel.com>
To: Mel Gorman <mel@csn.ul.ie>
Cc: Simon Kirby <sim@hostway.ca>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Dave Hansen <dave@linux.vnet.ibm.com>, linux-mm <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <1291137339-6323-2-git-send-email-mel@csn.ul.ie>
References: <1291137339-6323-1-git-send-email-mel@csn.ul.ie>
	 <1291137339-6323-2-git-send-email-mel@csn.ul.ie>
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Date: Wed, 01 Dec 2010 10:13:56 +0800
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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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