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

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Date:	Sat, 10 Jan 2009 14:23:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Theodore Tso <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	Johannes Berg <johannes@...solutions.net>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add b+tree library

On Sat, 10 Jan 2009 23:01:35 +0100 J__rn Engel <joern@...fs.org> wrote:

> One key difference is that rbtrees maintain the tree within objects and
> btrees maintain the tree externally.  So btrees have to allocate memory
> on insertion, where rbtrees have the necessary memory as part of the
> object.

This is a major disadvantage of the btrees.

See all the hoops we ended up jumping through with things like
radix_tree_preload() and idr_pre_get().

The IDR code there wasn't very well designed and still has holes.  The
radix-tree code afaik is solid, but look at all the stuff it does!

>  With mempools the memory allocation should be reasonably safe,
> so maybe this is a bit of a red herring now.

No, mempools won't help, particularly if items are being added from
atomic contexts.

This is a major major shortcoming which greatly limits the usefulness
of btrees and which greatly reduces the chances of anyone migrating any
rbtree users over to btrees.
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