Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!uw-beaver!tektronix!orca!frip!andrew
From: andrew@frip.WV.TEK.COM (Andrew Klossner)
Newsgroups: comp.arch
Subject: Re: DMA on RISC-based systems
Message-ID: <3480@orca.WV.TEK.COM>
Date: 2 Jun 89 19:49:44 GMT
References: <46500067@uxe.cso.uiuc.edu> <181@dg.dg.com> <1989May31.163057.543@utzoo.uucp>
Sender: nobody@orca.WV.TEK.COM
Organization: Tektronix, Wilsonville, Oregon
Lines: 24

[]

	"basic requirements ... which must be met to be considered
	state-of-the art ... [include] dedicated LAN controllers to
	handle the low levels of the LAN protocol ..."

This can backfire on you.  I've seen more than one example of a very
smart LAN interface board which actually slowed down system throughput,
because its Chevy on-board processor couldn't do nearly as fast a job
as the Formula 1 dragcar that was the main CPU, and the single active
process was blocked waiting for LAN completion.

Ranging a bit, I've also dealt with systems with a fast main CPU, a
SCSI channel, and a wimpy Z8 in the on-disk SCSI controller.  Yep, the
Z8 was the overall system bottleneck -- lots of time wasted while it
slooooowly processed all the messages that SCSI bus master and slave
must exchange.

If you're going to buy into off-CPU agents to move I/O around, make
sure that those agents will improve as fast as the CPU, or your future
generation machines will be crippled.

  -=- Andrew Klossner   (uunet!tektronix!orca!frip!andrew)      [UUCP]
                        (andrew%frip.wv.tek.com@relay.cs.net)   [ARPA]