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

H. Peter Anvin
2010-10-18T18:15:23Z

On 10/18/2010 11:11 AM, Ingo Molnar wrote:
>
> * H. Peter Anvin <hpa@zytor.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