netdev - Re: [PATCH] NET : change layout of ehash table

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Message-Id: <200702091006.24242.dada1@cosmosbay.com>
Date:	Fri, 9 Feb 2007 10:06:24 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
Cc:	ak@...e.de, linux@...izon.com, akepner@....com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] NET : change layout of ehash table

On Friday 09 February 2007 09:40, David Miller wrote:
> From: Andi Kleen <ak@...e.de>
> Date: 09 Feb 2007 10:18:03 +0100
>
> > David Miller <davem@...emloft.net> writes:
> > > I've applied this, but I _REALLY_ don't like the new multiply
> > > instructions that are used now in the hash indexing paths when
> > > CONFIG_SMP is set.
> > >
> > > I think that's a higher cost than the memory waste.
> >
> > You're serious? multiply on a modern CPU is _much_ cheaper than a cache
> > miss e.g. a K8 can do a arbitary 64bit multiplication in 3-7 cycles.
> > Any cache miss will be in the three to four digits at least.
>
> I'm not thinking of modern CPUs, I'm think of the little
> guys :-)

Yes, but a decent C compiler for such targets should not use a multiply 
instruction to perform a (idx * 12) operation... :)
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