Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!sun-barr!olivea!oliveb!amdahl!terry
From: terry@uts.amdahl.com (Lewis T. Flynn)
Newsgroups: comp.arch
Subject: Amdahl's Rule
Message-ID: <48qO01yL75GS00@amdahl.uts.amdahl.com>
Date: 17 May 91 21:51:52 GMT
Organization: Amdahl Corporation, Sunnyvale CA
Lines: 41

There was a discussion in this group recently concerning Amdahl's Rule
and whether it should be updated, if it still held, etc. Some portions
of what was being quoted didn't ring exactly true so I did some
homework. First, I checked Hennessey and Patterson to see what was
said on the subject there and it mostly agreed with what was on the
net. So rather than do more second hand research, I called Gene to
ask him about it. As it happened, he was out of the country, but, being
the gentleman he is, he returned my call when he got back.

As to the 1 megabit of I/O capacity per MIPS rule: he and his people
did a lot of research on what workloads were being run on the then
current machines (709/7090) and found that, in general, these workloads
used 1 bit of I/O per instruction. Thus, in order to run these
workloads, you must be able to sustain this rate. Peak I/O capability
must be much higher to handle variations in demand. The actual data
supporting this was from several sources and included a 2 month study
at Lawrence Livermore Laboratories. The numbers from Livermore were
1.01 bits per instruction during the day and 1.1 to 1.2 bits per
instruction for night time processing (my notes are a little fuzzy, so
I may have reversed the day versus night). He didn't say so, but from
the time frame and from the extensive study done at Livermore, I would
speculate that this work was done as part of the research for the
Stretch project.

As to the 1 megabyte per MIPS rule: he stated that this was influenced
by the cost of memory for the 709 series. He later found that on the
7090 that 2 megabytes/MIPS was more reasonable and this value held when
he was doing 360/370 design at IBM and Amdahl. He said it really showed
when the 370s hit the 16meg limit (24 bit addressing) and faster
processors got no better throughput.

I guess the most useful thing to conclude from this is that the first
rule of processor design is "Know Thy Workload". The second most useful
thing is that there's no substitute for lots of research and
measurement.

Terry

Disclaimer: Dr. Gene Amdahl no longer has any affiliation with Amdahl
Corporation and my discussion with him was completely unofficial. I had
met Dr. Amdahl in the course of my duties at a previous company.