134 lines
6.6 KiB
HTML
134 lines
6.6 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>Conservative GC (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Conservative GC (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Conservative GC (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Data-Representation.html" rel="up" title="Data Representation">
|
||
|
|
<link href="The-SCM-Type-in-Guile.html" rel="next" title="The SCM Type in Guile">
|
||
|
|
<link href="Cheaper-Pairs.html" rel="prev" title="Cheaper Pairs">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="subsection-level-extent" id="Conservative-GC">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="The-SCM-Type-in-Guile.html" accesskey="n" rel="next">The SCM Type in Guile</a>, Previous: <a href="Cheaper-Pairs.html" accesskey="p" rel="prev">Cheaper Pairs</a>, Up: <a href="Data-Representation.html" accesskey="u" rel="up">Data Representation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<h4 class="subsection" id="Conservative-Garbage-Collection"><span>9.2.4 Conservative Garbage Collection<a class="copiable-link" href="#Conservative-Garbage-Collection"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<p>Aside from the latent typing, the major source of constraints on a
|
||
|
|
Scheme implementation’s data representation is the garbage collector.
|
||
|
|
The collector must be able to traverse every live object in the heap, to
|
||
|
|
determine which objects are not live, and thus collectable.
|
||
|
|
</p>
|
||
|
|
<p>There are many ways to implement this. Guile’s garbage collection is
|
||
|
|
built on a library, the Boehm-Demers-Weiser conservative garbage
|
||
|
|
collector (BDW-GC). The BDW-GC “just works”, for the most part. But
|
||
|
|
since it is interesting to know how these things work, we include here a
|
||
|
|
high-level description of what the BDW-GC does.
|
||
|
|
</p>
|
||
|
|
<p>Garbage collection has two logical phases: a <em class="dfn">mark</em> phase, in which
|
||
|
|
the set of live objects is enumerated, and a <em class="dfn">sweep</em> phase, in which
|
||
|
|
objects not traversed in the mark phase are collected. Correct
|
||
|
|
functioning of the collector depends on being able to traverse the
|
||
|
|
entire set of live objects.
|
||
|
|
</p>
|
||
|
|
<p>In the mark phase, the collector scans the system’s global variables and
|
||
|
|
the local variables on the stack to determine which objects are
|
||
|
|
immediately accessible by the C code. It then scans those objects to
|
||
|
|
find the objects they point to, and so on. The collector logically sets
|
||
|
|
a <em class="dfn">mark bit</em> on each object it finds, so each object is traversed
|
||
|
|
only once.
|
||
|
|
</p>
|
||
|
|
<p>When the collector can find no unmarked objects pointed to by marked
|
||
|
|
objects, it assumes that any objects that are still unmarked will never
|
||
|
|
be used by the program (since there is no path of dereferences from any
|
||
|
|
global or local variable that reaches them) and deallocates them.
|
||
|
|
</p>
|
||
|
|
<p>In the above paragraphs, we did not specify how the garbage collector
|
||
|
|
finds the global and local variables; as usual, there are many different
|
||
|
|
approaches. Frequently, the programmer must maintain a list of pointers
|
||
|
|
to all global variables that refer to the heap, and another list
|
||
|
|
(adjusted upon entry to and exit from each function) of local variables,
|
||
|
|
for the collector’s benefit.
|
||
|
|
</p>
|
||
|
|
<p>The list of global variables is usually not too difficult to maintain,
|
||
|
|
since global variables are relatively rare. However, an explicitly
|
||
|
|
maintained list of local variables (in the author’s personal experience)
|
||
|
|
is a nightmare to maintain. Thus, the BDW-GC uses a technique called
|
||
|
|
<em class="dfn">conservative garbage collection</em>, to make the local variable list
|
||
|
|
unnecessary.
|
||
|
|
</p>
|
||
|
|
<p>The trick to conservative collection is to treat the C stack as an
|
||
|
|
ordinary range of memory, and assume that <em class="emph">every</em> word on the C
|
||
|
|
stack is a pointer into the heap. Thus, the collector marks all objects
|
||
|
|
whose addresses appear anywhere in the C stack, without knowing for sure
|
||
|
|
how that word is meant to be interpreted.
|
||
|
|
</p>
|
||
|
|
<p>In addition to the stack, the BDW-GC will also scan static data
|
||
|
|
sections. This means that global variables are also scanned when looking
|
||
|
|
for live Scheme objects.
|
||
|
|
</p>
|
||
|
|
<p>Obviously, such a system will occasionally retain objects that are
|
||
|
|
actually garbage, and should be freed. In practice, this is not a
|
||
|
|
problem, as the set of conservatively-scanned locations is fixed; the
|
||
|
|
Scheme stack is maintained apart from the C stack, and is scanned
|
||
|
|
precisely (as opposed to conservatively). The GC-managed heap is also
|
||
|
|
partitioned into parts that can contain pointers (such as vectors) and
|
||
|
|
parts that can’t (such as bytevectors), limiting the potential for
|
||
|
|
confusing a raw integer with a pointer to a live object.
|
||
|
|
</p>
|
||
|
|
<p>Interested readers should see the BDW-GC web page at
|
||
|
|
<a class="uref" href="http://www.hboehm.info/gc/">http://www.hboehm.info/gc/</a>, for more information on conservative
|
||
|
|
GC in general and the BDW-GC implementation in particular.
|
||
|
|
</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="The-SCM-Type-in-Guile.html">The SCM Type in Guile</a>, Previous: <a href="Cheaper-Pairs.html">Cheaper Pairs</a>, Up: <a href="Data-Representation.html">Data Representation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|