Re: 'conservative' GC == 'risky' GC

markt@harlequin.co.uk (Mark Tillotson)
Thu, 26 May 1994 15:38:52 GMT

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Newsgroups: comp.compilers
From: markt@harlequin.co.uk (Mark Tillotson)
Keywords: GC
Organization: Harlequin Limited, Cambridge, England
References: 94-05-084 94-05-091
Date: Thu, 26 May 1994 15:38:52 GMT

jgmorris+@cs.cmu.edu (Greg Morrisett) wrote:
> Actually, it's just as much of a misnomer to call a copying or mark-sweep
> or tracing garbage collector "accurate" or "non-convservative". It is
> generally uncomputable whether an object is garbage (i.e. is going to be
> accessed in the future or not.) We've grown accustomed to using
> "pointer-reachability" as an approximation as to what must be preserved,
> but this is only a _conservative_ approximation.


If you have a debugger and value-inspector in your system, then
pointer-reachability is what the user wants and expects, because the user
view is of a graph of nodes, not purely the semantics of the original
program....


Maybe a separate space-leak tool is the answer to uncover unnecessarily
preserved pointers---it is a significantly harder problem to crack than
recycling of disconnected graph nodes. It all depends on one's definition
of "garbage" I suppose, and where you draw the abstraction barrier.


M. Tillotson Harlequin Ltd.
markt@harlqn.co.uk Barrington Hall,
+44 223 873829 Barrington, Cambridge CB2 5RG
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