Re: I'm a total push-over..

From: Dmitry Potapov <dpotapov@gmail.com>
Date: Jan 23, 2008, 17:10 UTC
Message-ID: <20080123171004.GS14871@dpotapov.dyndns.org>
In-Reply-To: <4797095F.9020602@op5.se>

On Wed, Jan 23, 2008 at 10:31:11AM +0100, Andreas Ericsson wrote:

Show 7 quoted lines

> ---
> FNV Hash
> 
> I need to fill this in. Search the web for FNV hash. It's faster than my 
> hash on Intel (because Intel has fast multiplication), but slower on most 
> other platforms. Preliminary tests suggested it has decent distributions. 
> ---

I believe that under words "my hash", Bob Jenkins meant lookup2, which was significant slower.

> 
> My tests ran on Intel.

Please, could you specify your CPU model.

Show 5 quoted lines

> I also noticed I had a few hashes commented out when
> doing the test, one of them being Paul Hsie's. For some reason, Jenkin's and
> Hsie's didn't perform well for me last time I used the comparison thing (I
> did a more thorough job back then, with tests running for several minutes
> per hash and table-size, so I commented out the poor candidates).

I expected that Paul Hsieh's hash may not do well on some architecture, though it seems it did even worse than I expected.

> 
> I still believe that for this very simple case, the lookup3.c case is not
> very practical, as the code is that much more complicated, which was my
> main point with posting the comparison.

I would not describe lookup3 as impractical. It is widely used and well tested. Perhaps, for some Intel CPUs, the difference in speed is not so big, and FNV hash is much smaller and simpler, so FNV is a reasonable choice, but the hash is twice slower on my AMD processor and I suspect it may be even worse on other CPUs, where integer multiplication is slow. Besides, it may turn out that hashing filename may be not only case where a fast hash is needed.

Dmitry