Newsgroups:  comp.sys.mac.system
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From:        "John R. Delaney" <delaney@xn.ll.mit.edu>
Subject:     VM rule of thumb (sic)
Message-ID: <1991May30.132020.23146@xn.ll.mit.edu>
Sender: usenet@xn.ll.mit.edu
Organization: MIT Lincoln Laboratory
Date: Thu, 30 May 91 13:20:20 GMT

>	The rule of thumb for allocating swap space (virtual memory to you
>mac hacks) is 2MB of virtual to 1MB of physical. Anything more than that
>and you start beating the disk to death.

I am getting tired of hearing this "rule of thumb" restated as if carved on
a bloody tablet in words of fire. I am composing this message on a TI
Explorer with 8 MB of actual memory and 128 MB of virtual memory. Sure, if
I run a memory-intensive application or jump rapidly from one normal
application to another, I get pronounced delays due to paging. And when the
garbage collector cuts in, its time for a tea break. But if I use one
application for a while, then another, and so on, there may be a brief
burst of paging when I change over. But even that does not always happen.
The right ratio of virtual to actual memory depends on the hardware you
use, the operating system's design, and your pattern of usage.

The 2-to-1 rule may be appropriate for System 7.0; Apple may have tuned it
with that in mind. But until I have tried something greater than 2-to-1 for
a while on my SE/30, I am going to withold judgement. I ain't holy writ,
gang!

And by the way, what we have on the Mac's right now strikes me as
demand-paged memory, not virtual memory. But please, let's not start a
"true virtual memory" thread to go along with the "true multi-tasking"
thread.

John



