Date: Wed, 6 Mar 1996 18:53:39 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: wollman@lcs.mit.edu (Garrett A. Wollman) Cc: questions@freebsd.org Subject: Re: Bad Ethernet cards Message-ID: <199603061753.SAA08697@labinfo.iet.unipi.it> In-Reply-To: <9603061611.AA28988@halloran-eldar.lcs.mit.edu> from "Garrett A. Wollman" at Mar 6, 96 11:11:31 am
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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. Luigi ==================================================================== Luigi Rizzo Dip. di Ingegneria dell'Informazione email: luigi@iet.unipi.it Universita' di Pisa tel: +39-50-568533 via Diotisalvi 2, 56126 PISA (Italy) fax: +39-50-568522 http://www.iet.unipi.it/~luigi/ ====================================================================
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