linux-kernel - Re: [PATCH] add b+tree library

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Message-ID: <20090110202315.GE20611@logfs.org>
Date:	Sat, 10 Jan 2009 21:23:16 +0100
From:	Jörn Engel <joern@...fs.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Johannes Berg <johannes@...solutions.net>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add b+tree library

On Sat, 10 January 2009 20:41:03 +0100, Andi Kleen wrote:
> Johannes Berg <johannes@...solutions.net> writes:
> 
> > Also, to elaborate on that answer, I'm going to use this as a sort of
> > hash table for wireless, where it ensures better scalability than a pure
> > hashtable from quiet environments (say wireless off on an airplane) to
> > your wireless test lab (100+ access points)
> 
> Is there any particular reason you can't use the standard rbtrees
> for that?

Can't think of any.  You can use linked lists as well.  Whether you want
to is a different matter.

Key difference is the number of cachelines you need to find a particular
entry.  rbtrees have a fanout of sqrt(3), so for a million elements (to
pick a random example) you need about 25 cachelines with rbtrees and
about 5-16 with btrees.  Closer to 5 if keys and pointers are small and
cachelines are large, closer to 16 if keys and pointers are large and
cachelines are small.

Jörn

-- 
The key to performance is elegance, not battalions of special cases.
-- Jon Bentley and Doug McIlroy
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