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

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Message-ID: <20090111182317.GN26290@one.firstfloor.org>
Date:	Sun, 11 Jan 2009 19:23:17 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Theodore Tso <tytso@....edu>,
	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


I only listed the proposals I've heard about before, not necessarily 
endorsing them.

> The number of people that truly understand what Judy trees do may be
> single-digit.  Main disadvantage I see is that Judy trees heavily rely
> on repacking nodes over and over.  Part of Judy is a memory manager with
> essentially slab caches for nodes with 2, 4, 6, 8, 12, 16, 24, 32, 48,
> 64, 96, 128, 192, 256, 384 and 512 words.

Well complicated code is en vogue recently :-) 

> Splay trees are still binary trees, so the fan-out argument is identical
> to that against rbtrees.  If we have to pull in a cacheline, we might as
> well use all of it.
> 
> Skip lists are just a Bad Idea(tm).  In O(x) notation they behave like
> binary trees, waste cachelines left and right, use more memory, depend
> on a sufficiently good random() function,...  I guess you never closely
> looked at them, because anyone who does tries to forget them as fast as
> possible.

Using the radix trees more would be also an alternative.

I honestly don't know how they will all perform in the kernel that is why I 
thought it would be a good idea to just try them out. But I'm not 
volunteering to code it up, so it was more an idle thought.

Doing that would be a reasonable student project. In fact I've been asked 
about this sort of thing by students in the past.

Cleaning up the rbtree interface to be a little more abstract
would be probably a good idea in general. I never really
liked the open coded searches.

-Andi

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