Re: Question about tcp hash function tcp_hashfn()

[David Miller <davem@davemloft.net>]

From: "Brian F. G. Bidulock" <bidulock@openss7.org>
Date: Wed, 31 May 2006 02:49:54 -0600

> This would be faster than a Jenkins hash approach because it
> would not be necessary to calculate the hash function at all for
> in-window segments.  Per packet overheads would decrease and
> better small packet performance would be experienced (i.e. your
> http server).  It has better hash coverage because MD4 and other
> cryptographic algorithms used for initial sequence number
> selection have far better properties than Jenkins.
> 
> What do you think?

I don't know how practical this is.  The 4GB sequence space
wraps very fast on 10 gigabit, so we'd be rehashing a bit
and 100 gigabit would make things worse whenever that shows
up.

It is, however, definitely an interesting idea.

We also need the pure traditional hashes for net channels.  I don't
see how we could use your scheme for net channels, because we are just
hashing in the interrupt handler of the network device driver in order
to get a queue to tack the packet onto, we're not interpreting the
sequence numbers and thus would not able to maintain the sequence
space based hashing state.

On a 3ghz cpu, the jenkins hash is essentially free.  Even on slower
cpus, jhash_2words for example is just 20 cycles on a sparc64 chip.
It's ~40 integer instructions and they all pair up perfectly to
dual issue.  We'd probably use jhash_3words() for TCP ipv4 which
would get us into the 30 cycle range.

A few years ago when I introduced jenkins into the tree, I thought
it's execution cost might be an issue.  I really don't anymore.