Path: utzoo!attcan!uunet!mmlai!burzio
From: burzio@mmlai.UUCP (Tony Burzio)
Newsgroups: comp.sys.dec
Subject: Re: Why RISC doesn't support large nuber of users?
Summary: RISC to VAX users
Message-ID: <673@mmlai.UUCP>
Date: 5 Feb 90 14:18:49 GMT
References: <25c936b0141@vms.huji.ac.il> <35387@mips.mips.COM>
Organization: Martin Marietta Labs, Baltimore, MD
Lines: 64

In article <35387@mips.mips.COM>, mark@mips.COM (Mark G. Johnson) writes:

I hope the original poster was joking, but with all those gullible
VAX architecture users out there, one can hardly take the chance :-)

> The problem with Reduced Instruction Set Computers is that the
> instruction set determines how many users a computer can handle.

Perhaps one should also look at the bus structure and I/O bandwidth
in the quest for number of users.

> Some instruction sets (for example, the VAX instruction set) can
> handle a lot of users, while others (like, the i80286 instruction
> set) cannot.  Reduced Instruction Set Computers ("RISC") just don't
> have enough instructions to adequately take care of a large
> set of simultaneous users.   L0(

Odd one that.  How then, do you explain the HP computer (RISCish) that
can handle 500+ users doing transaction processing?  No, the number of
instructions has only minor import.  Perhaps what you are seeing is the
well known Sun saw that says: "RISC = 15 MIPS downhill with a stiff
tail wind" :-).  When measuring speed, first use MIPS, then number of
users.  The kernal can be tuned for low number of users and get higher
performance benchmark numbers.

> Think about it; some people say that the Crays were the first RISC
> machines, yet Crays are just batch scheduled compute engines; they
> handle effectively ONE user (!).

Could this be that CRAY computer memory is very expensive, and much faster
than disk swapping?  Forcing a CRAY to swap removes the advantage of
this super-fast memory...

> has 500 instructions --- and, best of all, it has microcode and
> microinstructions, which count for more.  L0(

Hmmm...  Our HP835 (~14 MIPS) handles under load much better than
our VAX3xxx.  The HP835 is rated 30 users actually DOING something,
not like our VAXen which just sit around with 40 users idling waiting
for mail.  A 1 week job on a RISC machine is a 4 week job on a VAX :-)

> the VAX has more instructions than the PDP-11 and it can consequently
> handle more users than the PDP-11 did.

Perhaps modern chip sets could help a PDP-11 blow away a VAX?  But
then, one could ask "Why?"

> Summary: (# of users) = (1/5) * (# of instructions).
>          This is why RISCs will always be inferior to the VAX and
>          the S/370 in multiuser capability.

Hmmm...  Not according to Digital Review, which talked about new 500+
user RISC machines.  One could load down a RISC machine so that it
had the response time of a VAX, but I have found that people get RISC
machines to get AWAY from slow computers like the VAX.  We have found
that there is a general rebellion among our users who are getting their
own RISC fast machines to be done with central VAXen...  The users
are always right :-)

*********************************************************************
Tony Burzio               * Man the pumps!
Martin Marietta Labs      *
mmlai!burzio@uunet.uu.net *
*********************************************************************