Re: Question about tcp hash function tcp_hashfn()

[Evgeniy Polyakov]

On Wed, May 31, 2006 at 02:58:18PM +0400, Evgeniy Polyakov (johnpol@xxxxxxxxxxx) wrote:
> On Wed, May 31, 2006 at 03:51:24AM -0600, Brian F. G. Bidulock (bidulock@xxxxxxxxxxx) 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
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