Date: Thu, 7 Mar 1996 10:46:02 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: luigi@labinfo.iet.unipi.it (Luigi Rizzo) Cc: wollman@lcs.mit.edu, questions@FreeBSD.ORG Subject: Re: Bad Ethernet cards Message-ID: <199603071746.KAA14281@phaeton.artisoft.com> 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.
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