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From: johnson@uiucdcsp.cs.uiuc.edu
Newsgroups: comp.lang.smalltalk
Subject: Re: Smalltalk for (embedded) real-time
Message-ID: <80500017@uiucdcsp>
Date: Wed, 9-Sep-87 09:42:00 EDT
Article-I.D.: uiucdcsp.80500017
Posted: Wed Sep  9 09:42:00 1987
Date-Received: Sat, 12-Sep-87 06:02:19 EDT
References: <1881@tekig4.TEK.COM>
Lines: 37
Nf-ID: #R:tekig4.TEK.COM:1881:uiucdcsp:80500017:000:1780
Nf-From: uiucdcsp.cs.uiuc.edu!johnson    Sep  9 08:42:00 1987


	In fact, most Smalltalk implementations employ
	BOTH reference counting AND garbage collection.

Actually, most of the Smalltalk interpreters build recently do not
employ reference counting.  Most use generation scavenging, a particular
garbage collection algorithm.

	Reference counting is used to detect "dead" objects (i.e. objects
	which are no longer referenced by any other object).

This is exactly what reference counting does.

	Eventually, fragmentation resulting from allocation and reallocation
	of a given segment of memory may require garbage collection (i.e. the
	relocation of "live" objects to coalesce the free memory of the 
	segment into one block) in order to keep the system from going belly
	up.

This is compaction, not garbage collection.  Garbage collection is detecting
all objects that can no longer be accessed and recovering their space. An
object might be referenced by another "dead" object and so not be considered
garbage by a reference counting algorithm.  A cycle of these dead objects
will not be recovered by a reference counting algorithm and so a reference
counting algorithm is usually backed up by a mark-and-sweep garbage collection
algorithm or something like that.

Reference counting is really a particular form of garbage collection
that is fairly cheap but that occasionally misses garbage.  Generation
scavenging is better, at least for Smalltalk.  The original article on
generation scavenging was by Dave Ungar and is reprinted in the May 84
issues of SIGPLAN Notices and Software Engineering News.

Tektronix uses generation scavenging in their interpreter.  The only 
company that uses reference counting in their Smalltalk interpreter is
Xerox, and I have heard that they are planning on going to generation
scavenging.