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From: dplatt@ntg.uucp (Dave Platt)
Newsgroups: comp.sys.mac.hardware
Subject: Re: System 7.0 and Additional RAM
Message-ID: <58@goblin.ntg.uucp>
Date: 12 Apr 91 18:46:52 GMT
References: <1991Apr8.034446.14836@murdoch.acc.Virginia.EDU>
Reply-To: dplatt@ntg.UUCP (Dave Platt)
Distribution: usa
Organization: New Technologies Group, Inc.  Palo Alto CA
Lines: 50

In article <1991Apr8.034446.14836@murdoch.acc.Virginia.EDU> mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes:
>
>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?  

Well, think of it this way.  When you access real RAM in a random
fashion, it has an access time that falls somewhere in the 80-150
nanosecond range.  When you access a page of virtual memory that's out
on disk, it has an access time on the order of tens of milliseconds.
This is many _thousands_ of times slower!

Virtual memory is a wonderful feature... within certain limits.
Ideally, you want to have enough physical RAM to keep _all_ of your
"working set" (the memory you're accessing frequently) in RAM at one
time.  Whenever your working-set size exceeds the amount of RAM
available, you're going to start paging to disk, and this will slow your
machine down quite a lot.  The rule I learned back in college, lo those
many years ago, is "If your working-set size exceeds three times your
physical memory allocation, you're doomed to slow death by swapping."

If you're running several different applications, and switching between
them infrequently, and there's not a lot of heavy processing and memory-
accessing being done by the background applications, then you can get by
with a modest amount of RAM.  You'll probably see a burst of disk
activity, and some slugging behavior, when you switch between
applications... the application you've brought forwards will be paged in
off of the disk, and another application will be paged out.

If you're running several large, memory-intensive applications, with
frequent switching between them and/or lots of background processing,
then you'll really want to have enough RAM to hold most of the
applications in memory.  Otherwise, your disk will be eternally paging,
and you'll spend most of your time waiting for the disk.

As far as I know, System 7's VM paging system is entirely synchronous.
You cannot have one program using the CPU while VM is paging another
process out to disk.  This is true even on a IIfx, which is (in
principle) capable of using DMA for SCSI data transfers... the Mac OS
doesn't take advantage of this capability.

Having more RAM is almost always a win.




-- 
Dave Platt                                                VOICE: (415) 813-8917
                    UUCP: ...apple!ntg!dplatt
 USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303