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From: bzs@bu-cs.UUCP (Barry Shein)
Newsgroups: net.arch
Subject: Memory Law
Message-ID: <764@bu-cs.UUCP>
Date: Wed, 13-Nov-85 23:08:55 EST
Article-I.D.: bu-cs.764
Posted: Wed Nov 13 23:08:55 1985
Date-Received: Fri, 15-Nov-85 08:07:34 EST
Organization: Boston Univ Comp. Sci.
Lines: 40

Someone suggested that they put a lot of memory on their VAX to help their
load here. I noticed that raising our 750 to 8MB seemed not to help much.
As a matter of fact, under some common conditions I suspect it might have
made it worse although I haven't tried to measure it yet (ie. memory managing
overhead has gone way up, tables etc, sort of making promises you cannot
keep cause you are actually flat out of CPU, not memory.)

Sooooo....I have been trying to come up with a reasonable rule of thumb
for how much memory is too much (?!) It will go something like this:

	Don't buy more memory than your CPU can zero out in N seconds.

For example, the 750 with 8MB running flat out a little loop like:

		clr	r1
	loop:
		clr	(r1)+
		jbr	loop

I figure it would take about, oh, wild guess, 20-30 seconds, maybe a little
less to zero out all of memory. Our 3081 with 24MB of memory, about 2-3 secs
(dual processor, which is fair to use as they will both use the memory,
I figure 7.5Mips/processor, 15mips.) A Cray-II with 4 processors, 2GB memory,
I dunno, total of 500MIPS for just running that loop? So, 8-10 seconds.

I think somewhere in there lies a rule, any good suggestions what N is?
(I know, given enough memory you could use it as a RAMDISK, but let's
forget that kind of usage for now and assume at some point, particularly
on a time sharing system, you would be better off telling a user to come
back later than committing more physical memory and the overhead to manage
it (is this linear? I bet not completely.)

Don't flame my guesses about how much time here (they don't look too good
to me either, but I think they are within reason for the purpose of making
a point.) Just curious if there is a rule of thumb that could be useful.

	-Barry Shein, Boston University

Of course, if it is brilliant, it is "Shein's Rule of Memory", if it's
dumb I'll deny I ever sent this message :-)