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From: Terry Lambert <terry@lambert.org>
Message-Id: <199603071746.KAA14281@phaeton.artisoft.com>
Subject: Re: Bad Ethernet cards
To: luigi@labinfo.iet.unipi.it (Luigi Rizzo)
Date: Thu, 7 Mar 1996 10:46:02 -0700 (MST)
Cc: wollman@lcs.mit.edu, questions@FreeBSD.ORG
In-Reply-To: <199603061753.SAA08697@labinfo.iet.unipi.it> from "Luigi Rizzo" at Mar 6, 96 06:53:39 pm
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> > > sell for approx US$60 here. Programmed I/O on the PCI bus should not be
> > > slow at all.
> > 
> > It doesn't matter what sort of bus it's on; if the CPU has to do all
> > the work, when it could (should) be doing higher-level processing, then
> > performance is going to suck.  There is no way around it.
> 
> The bus' speed changes things a lot.
> 
> An ethernet has a bandwidth of 1MB/s.  I agree that on the ISA bus
> you are likely to move data at 2MB/s or so, thus the CPU overhead
> is 50% just for getting data from the board. But moving data on
> VLB or PCI bus with programmed I/O is much faster (possibly limited
> by the on-board memory speed, but many boards use 25ns or so SRAMS).
> On the VLB I have measured at least 10MB/s, on the PCI I expect
> the number to be higher, possibly around 20MB/s. Thus the CPU
> overhead is at most 5% per card, assuming you consume the whole
> bandwidth of the net segment.
> 
> This is something that I can afford, except perhaps for a multiport
> high speed router.

You forget that it takes 4 CPU clocks for one PCI clock for a 133MHz
P5.

Multiply your PIO overhead by 4 (or more, if you trigger any wait
states).


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.