netdev - Re: Extensible hashing and RCU

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Date:	Tue, 20 Feb 2007 12:10:22 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	David Miller <davem@...emloft.net>, akepner@....com,
	linux@...izon.com, netdev@...r.kernel.org, bcrl@...ck.org
Subject: Re: Extensible hashing and RCU

On Tuesday 20 February 2007 11:30, Evgeniy Polyakov wrote:
> On Tue, Feb 20, 2007 at 02:12:09AM -0800, David Miller (davem@...emloft.net) 
wrote:
> > From: Eric Dumazet <dada1@...mosbay.com>
> > Date: Tue, 20 Feb 2007 11:04:15 +0100
> >
> > > Using a jenkin's hash permits a better hash distribution for a litle
> > > cpu cost.  I will post later a distribution simulation based on the
> > > data gathered from the same real server.
> >
> > Actually someone (I think it was Evgeniy in fact) made such
> > comparisons and found in his studies that not only does the current
> > ehash xor hash function distribute about as well as jenkins, it's
> > significantly cheaper to calculate :-)
>
> Yep, it happend to be my tests :)
> Jenkins hash was slower and had significant artifacts for some usage
> cases ended up with extremely long chain length.
> One can find more details at
> http://tservice.net.ru/~s0mbre/blog/2006/05/14#2006_05_14
> http://tservice.net.ru/~s0mbre/blog/2006/06/01#2006_06_01

Please explain why you chose    h = jhash_2words(faddr, laddr, ports);
        h ^= h >> 16;
        h ^= h >> 8;

jhash is very good, no need to try to be smarter, shufling some bytes... and 
adding artifacts.

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