linux-kernel - Re: [PATCH 1/6] mm: kswapd: Stop high-order balancing when any suitable zone is balanced

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Message-Id: <20101214143306.485f2c7c.akpm@linux-foundation.org>
Date:	Tue, 14 Dec 2010 14:33:06 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Simon Kirby <sim@...tway.ca>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] mm: kswapd: Stop high-order balancing when any
 suitable zone is balanced

On Fri, 10 Dec 2010 15:46:20 +0000
Mel Gorman <mel@....ul.ie> 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 within a smaller zone discarding an excessive number of
> pages.

Why was it doing this?  

> The user-visible behaviour is that kswapd is awake and reclaiming
> even though plenty of pages are free from a suitable zone.

Suitable for what?  I assume you refer to a future allocation which can
be satisfied from more than one of the zones?

But what if that allocation wanted to allocate a high-order page from
a zone which we just abandoned?

> This patch alters the "balance" logic for high-order reclaim allowing kswapd
> to stop if any suitable zone becomes balanced to reduce the number of pages

again, suitable for what?

> it reclaims from other zones. kswapd still tries to ensure that order-0
> watermarks for all zones are met before sleeping.

Handling order-0 pages differently from higher-order pages sounds weird
and wrong.

I don't think I understand this patch.

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