Copyright (C) 1993, Digital Equipment Corporation           

All rights reserved. | See the file COPYRIGHT for a full description. | Last modified on Tue Nov 9 12:28:51 PST 1993 by mcjones | modified on Mon Feb 22 09:13:09 PST 1993 by jdd | modified on Thu Jan 28 15:14:28 PST 1993 by kalsow | modified on Wed Jul 3 04:15:39 1991 by muller

RTCollector provides control over the Modula-3 garbage collector. \index{collector} \index{garbage collector!control over}

The purpose of a garbage collector is to reclaim unreachable nodes on the traced heap; most Modula-3 programs could not run very long without a collector. Even so, automatic garbage collection has some practical drawbacks.

\begin{enumerate} \item

The collector might move heap nodes to different addresses. This is usually unnoticable to programs, but can cause problems when programs must work with the addresses of heap nodes, since it is not guaranteed that ADR(x^) is a constant over the lifetime of x^. There are two main cases when programs must work with such addresses.

\begin{enumerate} \item

To implement hash tables, etc.


To pass addresses to procedures written in other languages, which is inherently unportable.



Unsafe code can put the traced heap temporarily into an inconsistent state. If the collector happens to run then, it might delete nodes that seem unreachable but that in fact are accessible. Of course, unsafe code itself is inherently unportable.


This interface allows the program to control the Modula-3 collector to avoid such problems, as well as to pass hints to improve performance.

\paragraph*{Disabling the collector.}

The collector is initially enabled; the collector can reclaim storage, and move nodes in memory. While the collector is disabled, there will be no time spent in the collector. Allocation in the traced heap may proceed normally, although the heap will grow without bound. Nodes unreachable by the Modula-3 rules will not be reclaimed, and no nodes will move.

PROCEDURE Disable();
Disable the collector.

Reenable the collector if Enable has been called as many times as Disable. It is a checked runtime error to call Enable more times than Disable.

\paragraph*{Disabling motion.}

Disabling motion gives fewer guarantees than disabling the collector; while motion is disabled, it is guaranteed only that no nodes will move. Disabling motion is no more expensive than disabling the entire collector, and may be cheaper in some implementations.

PROCEDURE DisableMotion();
Disable motion. While motion is disabled, no nodes will move.

PROCEDURE EnableMotion();
Reenable motion if EnableMotion has been called as many times as DisableMotion, and Enable has been called as many times as Disable. It is a checked runtime error to call EnableMotion more times than DisableMotion.


Calling Collect is a hint from the program that now would be a good time for a collection (for example, if a large amount of storage has become unreachable, or if the program expects to wait some time for an external event).

PROCEDURE Collect();
Maybe collect now.

END RTCollector.

\paragraph*{Implementation notes.}

This section describes the implementation of the SRC Modula-3 collector, as a guide to SRC Modula-3 programmers and as an indication of how this interface is matched to a particular implementation. Portable programs must not take advantage of implementation details of the SRC Modula-3 collector.

The SRC Modula-3 collector is an incremental, generational, conservative mostly-copying collector that uses VM protection on heap pages to be notified of certain heap accesses. \index{garbage collector!properties of}

Because the SRC collector is conservative, an inaccessible node may be considered reachable if a bit-pattern either on a thread's stack or in its registers might be a reference to or into the node. Experience to date has not shown accidental node retention to be a problem.

The SRC collector will not collect or move a node while any thread's stack or registers contains a reference to or into the node. The SRC Modula-3 system guarantees that this will include references passed as value parameters. This guarantee is useful for calling foreign procedures.

Disable completes the current incremental collection, if any, and unprotects all heap pages, so that no page faults will occur while collection is disabled. No new collections will start while collection is disabled. The next collection after collection is reenabled will be total, as opposed to partial, since unprotecting the heap loses generational information.

DisableMotion disables further collections from beginning. DisableMotion does not finish the current incremental collection, since the collector already guarantees that the program will not see addresses in the previous space. No new collections will start while motion is disabled, so that the current space will not become the previous space. It is not necessary to unprotect the heap.

Collect completes the current incremental collection, if any, then performs a total collection before returning to the caller.

The @M3nogc flag performs an initial call to Disable. \index{{\tt @{}M3nogc} command-line flag}

The SRC collector also supports additional operations for controlling the frequency of collection, disabling and reenabling incremental and generational collection, reporting on collector performance, and so on. These operations are accessible through the implementation-dependent RTCollectorSRC interface. \index{RTCollectorSRC interface@{\tt RTCollectorSRC} interface}