Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!uw-june!rik
From: rik@june.cs.washington.edu (Rik Littlefield)
Newsgroups: comp.arch
Subject: Re: DMA on RISC-based systems
Message-ID: <8479@june.cs.washington.edu>
Date: 7 Jun 89 17:58:55 GMT
References: <46500067@uxe.cso.uiuc.edu> <181@dg.dg.com> <185@dg.dg.com>
Organization: U of Washington, Computer Science, Seattle
Lines: 21

In article <185@dg.dg.com>, rec@dg.dg.com (Robert Cousins) writes:
< Given that a disk channel will be averaging 200K bytes/second ...

< [comparison of programmed I/O vs small dedicated buffer vs stupid DMA,
<  evaluated against cpu cost and speed]

< I am the first to admit that these models are simplistic, but they
< do represent valid considerations and reasonable approximations to 
< to the actual solutions.
< 
< Comments?

The methodology seems sound, but I question the numbers.  Just guessing,
but I suspect that workstation class systems have an *average* disk
throughput that is at least 10X lower than this number, even when
they are working full out.  (Remember that 200K bytes/second is
720 Mbytes/hour.)  If so, then the value of DMA is also 10X lower.  

Would someone with real utilization numbers care to fill us in?

--Rik