Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!uxc!uxc.cso.uiuc.edu!mcdurb!aglew
From: aglew@mcdurb.Urbana.Gould.COM
Newsgroups: comp.arch
Subject: Re: One aspect of bandwidth (backpl
Message-ID: <28200304@mcdurb>
Date: 28 Apr 89 18:44:00 GMT
References: <2222@lethe.UUCP>
Lines: 32
Nf-ID: #R:lethe.UUCP:2222:mcdurb:28200304:000:1683
Nf-From: mcdurb.Urbana.Gould.COM!aglew    Apr 28 13:44:00 1989


>In article <17298@cup.portal.com> bcase@cup.portal.com (Brian bcase Case) writes:
>| Though it's done for different reasons, Macintosh is a sign of times to
>| come:  all the (high-speed) main memory is "special," and is right next
>| to the CPU/cache.  What do you do for multiprocessors? Build that ECL bus
>| and charge several $million.
>
>  Well, sort of.
>  You do charge several millions, but you don't so much build a bus as
>you do a star, with the memory in the middle and the processors out on
>the arms.  The thing in the middle is called a system controller on a 'bun
>and costs a non-trivial amount of money.
>
>  One of my dear friends still works on Honeywells and regularly gets
>into uncontrolled snickering fits when the magazine-types start talking
>about bus minis replacing mainframes because they have so many more more 
>mips...
>
>David Collier-Brown,  | {toronto area...}lethe!dave

Now that there is no bus to snoop on, cache coherence between caches at the 
ends of the arms of the star [?] becomes problematic.  Start looking at
software managed cache coherency, or strictly private memory. Start thinking
about making a second stage cache controller at the center of the star,
that copies the tags in the private caches at the ends of the star, and uses
that to control "selectcasts" for snooping. Start worrying about having more
tags than data in your cache - or start upping the page size to 32K - 1M.
Realize that interprocessor communication is *slow*; think about a special
synchronization bus...

Yep... I've been through it once. Now that I'm working with microprocessors
again, I'll not have to worry about it for, oh, say maybe a year or so.