linux-kernel - Re: [PATCH 2/2] Rewrite jump_label.c to use binary search

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Date:	Wed, 22 Sep 2010 13:54:23 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Andi Kleen <andi@...stfloor.org>, jbaron@...hat.com,
	rostedt@...dmis.com, linux-kernel@...r.kernel.org, mingo@...e.hu,
	tglx@...utronix.de, roland@...hat.com, rth@...hat.com,
	mhiramat@...hat.com, fweisbec@...il.com, avi@...hat.com,
	davem@...emloft.net, vgoyal@...hat.com, sam@...nborg.org,
	tony@...eyournoodle.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] Rewrite jump_label.c to use binary search

On 09/22/2010 01:41 PM, Mathieu Desnoyers wrote:
>>
>> In the case of multiple instances of the same key you want the perfect
>> hash to point to the cluster of solutions -- a list.  Since this is by
>> simply be an array.
> 
> Yep, and sorting the section seems like a very natural way to create
> these arrays. So to summarize:
> 
> - We add a post-linking step to core image and module build in
>   modpost.c.
> - This step accesses exception tables, tracepoint and static jump
>   sections.
>   - Both tracepoint and static jump need to be sorted.
>   - For each of the 3 sections, a perfect hash is computed (creation
>     must have the property to always succeed). The perfect hash creation
>     should only take into account the first entry of duplicate keys.
>   - Each of these perfect hash would translate into C code that would
>     need to be compiled in a post-link phase.
>   - Then we can link the perfect hash objects with the rest of the code,
>     filling in one symbol per considered section (function pointer to
>     the perfect hash function) and setting function pointers in struct
>     module for modules.
> 
> I'm mostly writing this down as food for thoughts, since my own
> implementation time is currently focused on other things.
> 

For what it's worth, here is a working (verified and in use) perfect
hash generator written in Perl:

http://repo.or.cz/w/nasm.git/tree/HEAD:/perllib

Like most other perfect hash generators it needs a prehash: the prehash
should be parameterizable (seedable) and produce 2 ceil(log n) bits of
hash material and cannot have collisions.  The actual phash algorithm
compresses it down to a perfect hash.  The prehash is typically
generated via a pseudorandom algorithm: the particular implementation
pointed to uses one based on CRC64 because it's fast to compute but has
a finite probability of not existing; a universal prehash is guaranteed
to exist but is much more expensive.  In practice a very simple prehash
is usually sufficient, and one goes for speed.

For binary numbers being input, an even simpler prehash based on
multiplies or rotates is generally more than sufficient.

	-hpa
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