Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!apple!apple.com!desnoyer
From: desnoyer@apple.com (Peter Desnoyers)
Newsgroups: comp.arch
Subject: Re: Multics & Memory mapped files
Message-ID: <6662@internal.Apple.COM>
Date: 13 Feb 90 18:04:43 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 17
References:<5180@crdgw1.crd.ge.com> <12780026@acf4.NYU.EDU>

If I remember correctly from my computer architecture class several years 
back, there is a substantial penalty in kernel complexity to implement a 
paging scheme which can page from arbitrary files instead of a 
pre-allocated contiguous chunk of disk. (The comparison was between 
Multics and some other system of the same vintage - the Multics code was 
about 10X longer.) You also end up with a much larger body of code that 
must be protected from paging.

It is quite possible to design a VM system that will not allow 
memory-mapped file access. The problem is that this system may end up 
being faster, simpler,  and more robust than one which is able to map user 
files into its memory space.


                                             Peter Desnoyers
                                             Apple ATG
                                             desnoyer@apple.com