Re: Question about tcp hash function tcp_hashfn()

Evgeniy Polyakov
2006-05-31T11:05:21Z

On Wed, May 31, 2006 at 02:58:18PM +0400, Evgeniy Polyakov (johnpol@2ka.mipt.ru) wrote:
> On Wed, May 31, 2006 at 03:51:24AM -0600, Brian F. G. Bidulock (bidulock@openss7.org) wrote:
> > Evgeniy,
> >
> > On Wed, 31 May 2006, Evgeniy Polyakov wrote:
> > > 2. Compared Jenkins hash with XOR hash used in TCP socket selection code.
> > > http://tservice.net.ru/~s0mbre/blog/2006/05/14#2006_05_14
> >
> > Two problems with the comparison:
> >
> > Port numbers can be collected into a 32 bit register in network
> > byte order directly from the TCP packet without taking two 16 bit
> > values and shifting and or'ing them.
>
> They are.
>
> u32 ports;
>
> ports = lport;
> ports <<= 16;
> ports |= fport;

Using network or host byte order does not affect hash distribution,
that shifting was coded to simulate other types of mixing ports,
which actually never showed different results.

> > Worse: he folded the jenkins algorith result with
> >
> > h ^= h >> 16;
> > h ^= h >> 8;
> >
> > Destroying the coverage of the function.
>
> It was done to simulate socket code which uses the same folding.
> Leaving 32bit space is just wrong, consider hash table size with that
> index.
>
> > I, for one, am not suprised that artifacts appeared in the comparison
> > as a result of this destruction of the coverage of the hashing function.
>
> It is comparison of the approach used in TCP hashing code, it is not full
> mathematical analysis. And in that case jenkins hash already not good.
> I'm sure it can be tuned, but it does require a lot of iterations, while
> XOR one "just works".

--
Evgeniy Polyakov