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From: lgm@cbnewsc.att.com (lawrence.g.mayka)
Newsgroups: comp.object
Subject: Re: What is Objective C?
Summary: Garbage collection in real-time systems, et al.
Message-ID: <1990Sep12.000528.4726@cbnewsc.att.com>
Date: 12 Sep 90 00:05:28 GMT
References: <3864@bingvaxu.cc.binghamton.edu> <77500056@m.cs.uiuc.edu> <1990Sep11.030523.19830@cbnewsl.att.com>
Organization: AT&T Bell Laboratories
Lines: 40

In article <1990Sep11.030523.19830@cbnewsl.att.com>, psrc@cbnewsl.att.com (Paul S. R. Chisholm) writes:
> But if I was writing an operating system, or a real time application,
> the (unpredictable) overhead of garbage collection might not be
> acceptable.  Then, I'd be willing to pay the (programmer) effort, and
> manage memory better than a garbage collector could.

Garbage collection need not be unpredictable, with some degree of
hardware support.  Lisp machines are certainly able to manage their
garbage collection quite precisely, and some research indicates that
simple per-page read/write protection constitutes sufficient hardware
support on which to build a predictable garbage collector (See the
algorithm of Appel, Ellis, and Li.)

> Somebody (Lawrence K-something, I think) said that good garbage
> collectors are hard to beat, unless you use a fixed-size memory pool or
> some such trick.  Funny, it sounded as if he was disagreeing with me.
> That's *exactly* the kind of technique needed.

The difficulty comes in passing around references to an element of
such a pool.  Do you

(a) Deallocate the element "out from under" its references,
essentially requiring a validity check before each use of such a
reference.  (And if processes are preemptible, the validity check must
be atomic with the access.)

(b) Attempt to keep track of such references, through some inefficient
or error-prone scheme.

(c) Forbid such multiple references entirely.

Advocates of garbage collection consider all these alternatives
decidedly unattractive, to say the least.


	Lawrence G. Mayka
	AT&T Bell Laboratories
	lgm@iexist.att.com

Standard disclaimer.