linux-kernel - Re: ima: use of radix tree cache indexing == massive waste of memory?

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Date:	Mon, 18 Oct 2010 11:13:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Eric Paris <eparis@...hat.com>, Kyle McMartin <kyle@...artin.ca>,
	James Morris <jmorris@...ei.org>,
	Christoph Hellwig <hch@...radead.org>,
	kernel@...ts.fedoraproject.org, Mimi Zohar <zohar@...ibm.com>,
	warthog9@...nel.org, Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, Serge Hallyn <serue@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: ima: use of radix tree cache indexing == massive waste of memory?

On 10/18/2010 11:11 AM, Ingo Molnar wrote:
> 
> * H. Peter Anvin <hpa@...or.com> wrote:
> 
>> On 10/18/2010 09:48 AM, Eric Paris wrote:
>>
>>> 1) IMA uses radix trees which end up wasting 500 bytes per inode because the key 
>>> is too sparse.  I've got a patch which uses an rbtree instead I'm testing and 
>>> will send along shortly.  I found it funny working on the patch to see that 
>>> Documentation/rbtree.txt says "This differs from radix trees (which are used to 
>>> efficiently store sparse arrays and thus use long integer indexes to 
>>> insert/access/delete nodes)" Which flys in the face of this report.
>>
>> Radix trees can efficiently store data associated with sparse keys *as long as the 
>> keys are clustered*.  For random key distributions, they perform horribly.
> 
> For random key distributions hash and rbtree data structures are pretty good 
> choices.
> 
> But the (much) more fundamental question is to turn the non-trivial allocation 
> overhead of this opt-in feature into truly opt-in overhead.
> 

Yes, and not just the allocation overhead, but apparently locking
overhead, too.

	-hpa
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