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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.os.minix
Subject: Re: File system/virtual memory integration
Message-ID: <7730@utzoo.UUCP>
Date: Mon, 2-Mar-87 20:59:34 EST
Article-I.D.: utzoo.7730
Posted: Mon Mar  2 20:59:34 1987
Date-Received: Mon, 2-Mar-87 20:59:34 EST
References: <1276@husc6.UUCP>, <1057@ius2.cs.cmu.edu>
Organization: U of Toronto Zoology
Lines: 28
Keywords: file system,virtual memory,Multics

> I was wondering how difficult it might be to 'integrate' virtual memory
> with the file system process.  I'm told that the Multics operating system
> mapped a file into the virtual address space when it was opened.

Multics did indeed do that.  The problem is that the ability to do tricks
like that is *very* dependent on good memory-management hardware.  That's
why Unix doesn't do it.  An 8088- or 8086-based machine has no hope of
doing it.  Nor does a bare 68000 as in the ST.  The somewhat better MMU
in the 80286 just might make it possible in a sleazy and inefficient way
on an AT.  You really need paging to do it right, and it better be a well-
designed paging box at that.

> Would this be of any advantages (smaller code size, simpler semantics?)
> in a Minix-based system?

If anything it complicates the code and the semantics.  For one thing, it's
not clear how you can do such mapping for non-ordinary files -- directories,
devices, and so forth.  For another thing, even with a cooperative MMU it
becomes very difficult to maintain a file size down to the byte when the
MMU only works down to the page.

In short, it's an interesting concept but a whole new can of worms.  If you
are looking for a neat experiment, go ahead.  If you are looking for a
simple way to improve Minix without introducing serious incompatibilities,
forget it.
-- 
"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology
the dust.  Which shall it be?"	{allegra,ihnp4,decvax,pyramid}!utzoo!henry