Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!brutus.cs.uiuc.edu!rpi!batcomputer!riley
From: riley@batcomputer.tn.cornell.edu (Daniel S. Riley)
Newsgroups: comp.sys.dec
Subject: Re: Why RISC doesn't support large nuber of users?
Message-ID: <9679@batcomputer.tn.cornell.edu>
Date: 8 Feb 90 15:22:12 GMT
References: <455.25D0A8A9@blkcat.fidonet.org>
Reply-To: riley@tcgould.tn.cornell.edu (Daniel S. Riley)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 30

In article <455.25D0A8A9@blkcat.fidonet.org> Frank.Mallory@f123.n109.z1.fidonet.org (Frank Mallory) writes:
> YA> types of RISC machines I hear from most suppliers that RISC machine 
> YA> cannot
> YA> hold a large number of users, but very few suppliers claim they can.
>
>Essentially the same question was answered in this week's Digital Review.  The  
>answer is that the basis for the ratings is not the same.  You have to run the  
>same benchmark on both to determine the estimated transactions per second of  
>each and thus the relative ability to support a user load.

At the Anaheim DECUS last fall, there was a talk titled "Performance of
the DECstation 3100/3100S/5400", by James Seagraves of the Ultrix(tm)
Engineering Group.  The talk was basically a ton-and-a-half of the sorts
of graphs DEC makes to determine the recommended number of users for
various systems.  One thing that's very obvious looking at the graphs is
that the maximum number of users depends on the kind of load on the system.
For CPU intensive tasks, response time vs. number of users for a DECstation
is flat way past the point where a VAX 6310 has started to bog down.  For
I/O intensive tasks, a DECstation 3100 doesn't have much chance against a
6310 with a KDB50, but a DS5400 with a KDA50 looks pretty good.  So you
have to look at the I/O structure and memory bandwidth as well as the MIPS 
rating, taking the intended use for the system into account.

(I also got the impression that DEC was somewhat more conservative assigning 
a maximum number of users to the DECstation 3100.  In their load tests, the
RISC systems don't seem to bog down as dramatically as the VAXes do, so there
is much more latitude in setting the cutoff point.)

-Dan Riley (riley@tcgould.tn.cornell.edu, cornell!batcomputer!riley)
-Wilson Lab, Cornell University