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From: gillies@m.cs.uiuc.edu
Newsgroups: comp.arch
Subject: Re: Re^2: Macintosh OS
Message-ID: <3300135@m.cs.uiuc.edu>
Date: 11 Jun 90 06:23:00 GMT
References: <355@three.MV.COM>
Lines: 32
Nf-ID: #R:three.MV.COM:355:m.cs.uiuc.edu:3300135:000:1733
Nf-From: m.cs.uiuc.edu!gillies    Jun 11 01:23:00 1990


> What an amazingly half-baked idea!  Oh, sorry. . .I just insulted the bakers
> of the world.  What you're talking about is placing an absolutely impossible
> task in front of the compiler writers (who certainly have enough to worry
> about already) in hopes of making life just a *tiny* bit easier for OS and
> application developers.  There is NO WAY that the compiler can have any but
> the foggiest idea concerning which points are "appropriate" for a possible
> context switch.  The application designer will certainly have a much better
> idea, or the OS can allow the user to choose, but the compiler is absolutely
> the worst place to attempt a solution.

This makes me chuckle, since people have been doing it for 25 years
without knowing "it's impossible" or "half baked".  The threaded TUTOR
interpreter that runs on PLATO (CDC Cybers) inserts checks for context
switches ("autobreaks") before every backwards branch.  It works quite
well, too, supporting hundreds of users with high efficiency, and load
balancing the system so nobody claims more than 10,000 instructions
per second.

As for those who complain about the slowdown of tight loops --- well,
tight loops can be unrolled, or countdown timers can be used to reduce
the frequency of checking to give up the processor.

I think in 5 or 10 years when everyone wants their desktop workstation
to outperform a Cray 5, then this strategy may come back into vogue.
Like everything else, it costs some speed to build a CPU that can
handle interrupts, and Seymour Cray is well aware of this.


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