netdev - Re: ipv4: Simplify ARP hash function.

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Message-ID: <20110708200848.4529bb0c@s6510.ftrdhcpuser.net>
Date:	Fri, 8 Jul 2011 20:08:48 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	roland@...estorage.com, johnwheffner@...il.com, mj@....cz,
	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

On Fri, 08 Jul 2011 16:47:51 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Fri, 8 Jul 2011 16:41:28 -0700
> 
> > What about using murmur hash which has a four byte pass as well.
> >   https://sites.google.com/site/murmurhash/
> 
> I'm trying to avoid multiplies that are not done in hardware on some
> cpus.
> 
> Right now I'm looking at one of Thomas Wang's hashes, referenced on
> Bob Jenkin's hash analysis page:
> 
> u32 hashint(u32 a)
> {
> 	a += ~(a<<15);
> 	a ^=  (a>>10);
> 	a +=  (a<<3);
> 	a ^=  (a>>6);
> 	a += ~(a<<11);
> 	a ^=  (a>>16);
> 
> 	return a;
> }
> 
> It's 15 instructions, and produces better entropy in the low bits of
> the result than the high bits, which is fine for how we'll use this
> thing.

Ok. but you really have sell those Sparc's while they are still
worth something on Ebay :-)
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