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

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Message-Id: <200702090957.25110.ak@suse.de>
Date:	Fri, 9 Feb 2007 09:57:24 +0100
From:	Andi Kleen <ak@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	dada1@...mosbay.com, 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 :-)

Even for them it's true, although the ratios are smaller (unless you go back to 
m68000 or VAX days) 

-Andi
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