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From: amdcad!amdcad.AMD.COM!rpw3@ames.arc.nasa.gov (Rob Warnock)
Newsgroups: comp.parallel
Subject: Re: Amdahl's law(s)
Message-ID: <1442@hubcap.UUCP>
Date: 18 Apr 88 12:31:28 GMT
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Approved: parallel@hubcap.clemson.edu

In article <1354@hubcap.UUCP> acornrc!glassman@ames.arc.nasa.gov
(Steven Glassman) writes:
+---------------
| Yesterday, I heard a new law attributed to [Amdahl]. This one related the
| amount of I/O to the MIPS of the processor (unfortunately, I don't remember
| it exactly, but it was something like: one bit of I/O per ??? per MIP).
+---------------

This is actually the oldest "Amdahl's Law". It dates back to the IBM/360
days, and was a rule of thumb for balancing CPU, memory, and I/O...
with the caveat that it was intended for the 360 (now 370) instruction set,
when running business (read "COBOL") programs (mostly big sorting jobs).
It went (approximately -- sorry, no formal reference):

	"For each instruction/second of CPU, you need one byte
	of main memory, and one byte/second of I/O performance."

Using current terminolgy, that would say that, for example, a 15 VUPS machine
needs 15 Mbytes of RAM and 15 Mbyte/sec of I/O. For RISC machines, better add
some memory to account for the lesser code density. For Unix, add some memory
for disk buffer caches, network "mbuf"s, etc. For timesharing machines in
general, add more memory to give good interactive performance (hold down
swapping/paging). Enough memory (say, twice the amount called out above) lets
you skimp a little bit on I/O performance. Still, today's relatively low-priced
Unix systems (with a few notable exceptions among the higher-priced brands) tend
to be really bottlenecked on I/O.

Note that an Ethernet can't give a sustained Mbyte/sec, and an ST412 disk
can't do half that. RISC systems with 10-25 VUPS are gonna be hurting.
MASSIVE amounts of disk buffer cache (30-100 Mbyte) can help the disk
problem, though...


Rob Warnock
Systems Architecture Consultant

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