Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!samsung!uunet!sco!erics
From: erics@sco.COM (eric smith)
Newsgroups: comp.sys.mac.system
Subject: Re: Virtual Memory
Message-ID: <12383@scolex.sco.COM>
Date: 1 Jun 91 17:21:18 GMT
References: <1991May30.060108.5637@cs.yale.edu>
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Distribution: usa
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bitting-douglas@cs.yale.edu (Douglas Bitting) writes:

>Virtual Memory, like many other things in an OS, can be implemented in many
>ways.  It all boils down to the design decision made by the OS programmers.
>Basically, the trade off in implementing VM as Apple has is speed over disk
>space.  When the entire Virtual Memory space is mapped to a disk, the operating
>system is assured that any page in memory that is clean (ie, unchanged since
>read in from the disk) is on the disk somewhere.  This means that when said
>page is to be swapped out, it does not have to be written to the disk first.
>This results in an *immense* speedup.

Ah, but pages do have to be swapped out at some point if the data they
contain has changed. I"m not running System 7.0 yet (soon though, I hope),
so I haven't observed how Apple has implemented VM, but I assume they swap
pages that have changed on some time interval and certainly when the page
is swapped out. While having the entire contents of VM on disk may save
some disk IO (because not every page's data changes), the savings will vary
greatly depending on the type of applications running. I suspect that the
main reason to have the entire VM on disk is to greatly simplify the page
management algorithm and eliminate fragmentation of memory contents on disk.

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