linux-kernel - Re: [RFC] B+Tree library

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Date:	Sat, 1 Nov 2008 10:17:41 +0000
From:	Sean Young <sean@...s.org>
To:	Joern Engel <joern@...fs.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] B+Tree library

On Sat, Nov 01, 2008 at 12:36:15AM +0100, Joern Engel wrote:
> On Fri, 31 October 2008 20:17:45 +0000, Sean Young wrote:
> > On Sun, Oct 26, 2008 at 01:46:44PM +0100, Joern Engel wrote:
> > > General advantages of btrees are memory density and efficient use of
> > > cachelines.  Hashtables are either too small and degrade into linked
> > > list performance, or they are too large and waste memory.  With changing
> > > workloads, both may be true on the same system.  Rbtrees have a bad
> > > fanout of less than 2 (they are not actually balanced binary trees),
> > > hence reading a fairly large number of cachelines to each lookup.
> > 
> > Which reminds me:
> > 
> > find_vma() uses rbtrees. Now I assume find_vma() is called far more than
> > mmap() and friends. Since avltree are balanced (unlike rbtrees) lookups
> > will be faster at the expense of extra rotations during updates.
> 
> Maybe I should have been clearer.  Rbtrees _are_ balanced trees.  They
> are not balanced _binary_ trees, but balanced 234-trees in a binary
> representation.

I should have been more clearer. avltrees are more rigidly balanced than
rbtrees, making them faster for lookups but slower for modification:

http://en.wikipedia.org/wiki/AVL_tree#Comparison_to_other_structures

The difference between the shortest path to a leaf node and the longest 
path to a leaf node is +1 for avl and *2 for red-black.


Sean
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