Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!sun-barr!texsun!texbell!uhnix1!sugar!karl
From: karl@sugar.hackercorp.com (Karl Lehenbauer)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Virtual Memory / doable 1.4 request
Message-ID: <3856@sugar.hackercorp.com>
Date: 23 May 89 02:08:47 GMT
References: <8905200449.AA20664@postgres.Berkeley.EDU>
Organization: Sugar Land Unix - Houston
Lines: 46

I think VIRTUAL is an attribute that should be settable for tasks' code
segments.  (The means of defining working space size and such have yet to
be defined.  Perhaps it should be inherited from the CLI?  Yeah, and it
could be in the icon info for workbench launches.)

It has been mentioned that MEMF_VIRTUAL requests must default to physical
memory on machines without MMUs and that the existence of machines without
MMUs is a compelling argument for why requests for virtual memory should be 
explicit.  I am in wholehearted agreement.

I think also that virtual memory under AmigaDOS is something that should mostly
be explicitly under the control of the programmer.  Many AmigaDOS tasks are
highly realtime (games, music programs, *anything* that twiddles the display
in an animated/copper list manner) and will simply totally screw up if they
take page faults.  But even a heavily realtime program could use VM judiciously
if it was under explicit control (some requests are for virtual memory, others
are not) of the program and the programmer had carefully considered what could
safely be virtual.

Mapped files would about always be OK as one has to do disk I/O for disk files 
anyway.  (It goes without saying that only new programs could make use of this,
but I'll say it anyway to prevent a followup that says it.)

Also you need really excellent disk performance to do serious VM; that is, to 
take more than a few page faults a second and not thrash.  Many MMU-equipped
systems will fall down in this area, including mine.

Finally, I think VM will be of limited usefulness to most people.  While
OS/2 weenies blather that they can have four gigabyte virtual tasks, they 
don't have the storage or the application to make use of it.  Likewise, the 
Amiga is readily and reasonably affordably expandable to many megabytes and 
there are few programs that have need of that much.  (I bet most people who 
want VM already have at least 3 MB of RAM.)  I am not saying that VM will not 
be useful or that it should not be done, quite the contrary I think it is an 
interesting exercise, practically mandated by the existence of the 68851 on 
the A2620 (and on-chip MMU on the 68030s), merely that most Amigas will not
have MMUs and fast-enough disks to see its widespread use.

Bummer.  You can't do mapped files without an MMU.  That means mapped files
would be not work with non-MMU Amigas.  OK, you *could* do it if you didn't
demand page the file in on non-MMU machines but rather copied the whole
thing in on first open and then wrote the whole thing out on last close and 
maybe at checkpoint intervals or whatever.  It's sick, I know.
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