Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!samsung!viewlog.viewlogic.com!peter
From: peter@suntan.viewlogic.com (Peter Colby)
Newsgroups: comp.sys.mac.system
Subject: Re: SE/30 VM trouble in System 7
Message-ID: <1991May29.213900.725@viewlogic.com>
Date: 29 May 91 21:39:00 GMT
References: <2842d41f.4648@petunia.CalPoly.EDU> <1991May29.125650.16389@zardoz.eng.ohio-state.edu>
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Organization: Viewlogic Systems, Inc., Marlboro, MA
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In article <1991May29.125650.16389@zardoz.eng.ohio-state.edu>, gaynor@agvax2.ag.ohio-state.edu writes:
|> In article <2842d41f.4648@petunia.CalPoly.EDU>, mjohnson@cosmos.acs.calpoly.edu (Mark S. Johnson) writes:
|> >
|> >I'm running the release System 7 with no inits on a 5 Meg SE/30 with
|> >an Apple internal 40 Meg HD. When I turn on VM with the default
|> >settings and then reboot, "About This Macintosh" says:
|> 
|> [stuff about how 5MB of VM eats 10MB of disk space]
|> 
|> Yep, that's the way it is.  2 MB of VM eats up 4 MB on my HD.  (Aiigh! 
|> Acronyms!).  Now, I'm not a systems programmer (my knowledge is much more in
|> theory than in hard code), but I'd hazard that the extra disk space
|> is used to keep track of usage, what's where, and to generally speed up the
|> addressing of VM.  I'm sure that Apple could have implemented VM in a fashion
|> to devour less disk space, but such an implementation would probably eat up
|> more physical RAM.
|> 
|> Of course, I could be totally off-base.  <grin>  But VM does tend to eat up
|> around 2 MB of disk space for each MB of VM.
|> 
	Sorry, but the disk space IS your virtual memory. If you have a 1MB
Mac and you allocate 10MB of virtual memory you eat up 10MB of disk space.
Of course, you also won't get anything done because you'll spend your life
paging memory in and out from disk but hey...
	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.
	Just remember, if you are using virtual memory, the memory actually
sitting in your mac is purely a subway stop for the memory on your disk. It
doesn't add anything extra to what you assign.
	Oh, and by the way, if your mac has 8 1MB simms in it and you allocate
4MB virtual memory (assuming you could even do something so foolish), you lose
4MB. Not only that but you'll have just wasted 4MB of disk space since you'll
never page.
	PC
 
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