linux-kernel - Re: [PATCH 7/9] swap_info: swap count continuations

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Message-Id: <20091016130622.8c096641.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 16 Oct 2009 13:06:22 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, hongshin@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/9] swap_info: swap count continuations

On Fri, 16 Oct 2009 03:24:57 +0100 (BST)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:

> On Fri, 16 Oct 2009, KAMEZAWA Hiroyuki wrote:
> > > 
> > My concern is that small numbers of swap_map[] which has too much refcnt
> > can consume too much pages.
> > 
> > If an entry is shared by 65535, 65535/128 = 512 page will be used.
> > (I'm sorry if I don't undestand implementation correctly.)
> 
> Ah, you're thinking it's additive: perhaps because I use the name
> "continuation", which may give that impression - maybe there's a
> better name I can give it.
> 
> No, it's multiplicative - just like 999 is almost a thousand, not 27.
> 
> If an entry is shared by 65535, then it needs its original swap_map
> page (0 to 0x3e) and a continuation page (0 to 0x7f) and another
> continuation page (0 to 0x7f): if I've got my arithmetic right,
> those three pages can hold a shared count up to 1032191, for
> every one of that group of PAGE_SIZE neighbouring pages.
> 
Ah, okay. I see. thank you for explanation.

Regards,
-Kame

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