Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!m.cs.uiuc.edu!p.cs.uiuc.edu!gillies
From: gillies@p.cs.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Personal OS
Message-ID: <76700219@p.cs.uiuc.edu>
Date: 30 May 90 04:32:00 GMT
References: <9437@pt.cs.cmu.edu>
Lines: 45
Nf-ID: #R:pt.cs.cmu.edu:9437:p.cs.uiuc.edu:76700219:000:2308
Nf-From: p.cs.uiuc.edu!gillies    May 29 23:32:00 1990


> If you look at the software on the Alto, the system from which much of the
> one-space mythology is descended, you find that there is no notion of
> non-interfering parallel processes at all -- all parallelism has to be
> preplanned -- and the only interprocess-communication method that works
> between independently-written programs is the file system.

They solved this problem in the Xerox/Pilot OS, see "Experience with
Processes and Monitors in MESA, CACM, 1978".  Since every module in
the machine is a reentrant program, and every module may enforce
mutual exclusion via monitoring, sharing is well-supported.


It seems from looking at the source code in UNIX, that correctness and
robustness are halfway down the list of programmer priorities for that
system.  The programmer expects that the separate address spaces gives
him artistic license to write flakey software.  He expects to be saved
(from a system crash) by a process crash.  He has no clue that the
system can run out of (swap, file) space.

Sure you need separate address spaces under UNIX, because the system
software is of such low quality.


Do you presume that because there are 65 processes on your machine,
that they must all be necessary?  In fact, most of them are probably
kluges to deal with the separate address space problem in UNIX.  The
UNIX philosophy is, "we care enough to have a process sniffing at it
every half hour" (or N seconds).  While this would work in the old
days, when there was only a very slow Cron(8) demon, today
workstations talk to networks and are unbuffered interactive devices.
There is no sanctioned way (under UNIX) to write software that
notifies all interested parties (unknown at compile-time) in the
system.

On the other hand, in a single address-space machine, it is easy for
the I/O module to make a notification procedure call, which chains
along, notifying all interested entities in the address space.  On my
old DLion, if the prettyprinter wrote a new version of a MESA source
file, then all editors would notice and replot the file window
immediately (if no changes had been made).  Try THAT under UNIX.

Don W. Gillies, Dept. of Computer Science, University of Illinois
1304 W. Springfield, Urbana, Ill 61801      
ARPA: gillies@cs.uiuc.edu   UUCP: {uunet,harvard}!uiucdcs!gillies