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From: ech@cbnewsk.att.com (ned.horvath)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.misc,comp.sys.mac.system
Subject: Re: System 7.0 and RAM
Message-ID: <1991Apr11.193950.7332@cbnewsk.att.com>
Date: 11 Apr 91 19:39:50 GMT
References: <1991Apr8.010338.14202@murdoch.acc.Virginia.EDU>
Distribution: usa
Organization: AT&T Bell Laboratories
Lines: 31

From article <1991Apr8.010338.14202@murdoch.acc.Virginia.EDU>, by mil@mendel.acc.Virginia.EDU (Maria I. Lasaga):

> Given the virtual memory capacities of system 7.0, will there be any
> need to purchase additional RAM?  If not, will it even be advantageous
> to have more RAM rather than less?  

You can never have enough RAM!  Seriously, though, in the early days of 
VM (late 60's -- yes, I KNOW the Atlas was running in '57!) empirical studies
gave a rule of thumb of about a factor of two between real and virtual
memory; try to squeeze it more than that, and you begin to spend all your
time in disk-wait.  That is likely to be even MORE true for the graphics-
intensive applications we run now than it was then.

A bit more sophisticated notion is that of a working set, which is a fancy
way of saying that a program needs to have rapid access to the data
and code it's actively using in order to perform well.  If real memory is
less than the sum of the working sets for the active programs, you're going
to see severe performance degradation.  VM systems characteristically have a
"knee" in the performance curve at that sum-of-working-sets point: add just
one process, and the disklights go full on, and performance goes into the
tank.

Of course, your mileage may vary; if you run lots of applications that
don't maintain open windows, and don't do a lot of processing in background,
you may find that VM really helps a lot.  Swapping in working sets is
cheaper than launching the program.  If you run with MPW running C++
makes and a terminal emulator doing up/downloads and the printmonitor
punching away and the kids using your Mac as a server, all in the
background, you may conclude that VM is a waste of time...

=Ned Horvath=