Date: Thu, 7 Mar 1996 22:41:03 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: terry@lambert.org (Terry Lambert) Cc: terry@lambert.org, wollman@lcs.mit.edu, questions@FreeBSD.ORG Subject: Re: Bad Ethernet cards Message-ID: <199603072141.WAA11583@labinfo.iet.unipi.it> In-Reply-To: <199603072047.NAA14745@phaeton.artisoft.com> from "Terry Lambert" at Mar 7, 96 01:47:01 pm
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> > I did my computation as follows: assume the CPU has to transfer 1MB/s > > from the board. Say the available bandwidth on the PCI bus, doing > > programmed I/O, is X MB/s. Then the CPU uses 1/X of its time just to > > transfer that 1MB/s. For X=20, that makes 5% of the CPU time > > unavailable for other things. Anything wrong ? > > The CPU spends 1/X * clock_differential_for_bus_access of its time; > it will be in "bus wait" on clock_differential_for_bus_access - 1 > of those clocks that it ordinarily would be using to run instructions > in its L1 cache. The bandwidth I call "X" is the available bandwidth in accessing that particular device, which varies from device to device. What you call X is probably something diffetent. > I intensely dislike clock multiplied chips for precisely this reason; If you have to drive the I/O pins at 200 MHz instead of 66, then your CPU will likely consume 100W or more, provided you are able to make it work reliably and cool it down to a reasonable temperature. I suppose the little daemon would feel very comfortable on such a CPU :) 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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