Re: [PATCH] Define hash_mem in lib/hash.c to apply hash_long to an arbitraty piece of memory.

[Neil Brown]

On Monday January 6, [e-mail] wrote:
> On Tue, Jan 07, 2003 at 04:03:28PM +1100, Neil Brown wrote:
> > I did a little testing and found that on a list of 2 million 
> > basenames from a recent backup index (800,000 unique):
> > 
> >  hash_mem (as included here) is noticably faster than HASH_HALF_MD4 or
> >  HASH_TEA: 
> > 
> >   hash_mem:		10 seconds
> >   DX_HASH_HALF_MD4:	14 seconds
> >   DX_HASH_TEA:		15.2 seconds
> 
> I'm curious how the hash at
> http://www.burtleburtle.net/bob/hash/doobs.html would fare. He has a
> 64-bit version at
> http://www.burtleburtle.net/bob/c/lookup8.c.

Performing the same tests: producing 8 bit hashes from 800,000
filenames.

Speed is 10 seconds, comarable to hash_mem

normalised standard deviation of frequencies is 0.0171039
which is is the same ball park as the hashes ext3 uses
(they gave 0.0169 and 0.0182.  hash_mem gave 0.02255).

So (on this set of values at least) it does seem to be a better hash
function. 

I might look more closely at it.

Thanks,
NeilBrown