Automatic garbage collection : Computer memory garbage collection 

Basic principle

1. determine what data objects in a program cannot possibly be needed.
2. reclaim the storage used by those objects.

(liveness vs reachability)

Garbage collectors use a reachability-based criterion for determining whether a data object will be needed (but see note about reference counting below).

Reachability of an object

1. A distinguished set of objects are assumed to be reachable, these are known as the roots. In a typical system these objects will be the machine registers, the stack, the instruction pointer, the global variables. In other words everything that a program can reach directly.
2. anything referenced from a reachable object is itself reachable.

In other words a reachable object is one that the program could get to by following a chain of pointer references.

Basic algorithm

1. Create initial white, grey, and black sets; these sets will be used to maintain progress during the cycle. Initially the black set is empty, the grey set consists of specially denoted objects known as the "roots" and possibly some additional objects chosen according to the particular algorithm used, and the white set is everything else. At any time in the algorithm a particular object will be in exactly one of the three sets. The set of white objects can be thought of as the set of objects that we are trying to reclaim the storage for; in the course of the cycle the algorithm will remove many objects from the white set, leaving behind a set of objects that it can reclaim the storage for.
2. (This step is repeated until there are no objects in the grey set.) Pick an object from the grey set, move that object from the grey set to the black set, move all the white objects that are referenced (reachable in one step of following pointers) from the selected object into the grey set.
3. When there are no more objects in the grey set, then all the objects remaining in the white set are not reachable and the storage occupied by them can be reclaimed.

The tricolour invariant is an important property of the objects and their colours. It is simply this:

No black object points directly to a white object.

Notice that the algorithm above preserves the tricolour invariant. The initial partition has no black objects, so the invariant trivially holds. Subsequently whenever an object is made black any white objects that it references are made grey, ensuring that the invariant remains true. When the last step of the algorithm is reached, because the tricolour invariant holds, none of the objects in the black set point to any of the objects in the white set (and there are no grey objects) so the white objects must be unreachable from the roots, and the system calls their finalisers and frees their storage.

Some variations on the algorithm do not preserve the tricolour invariant but they use a modified form for which all the important properties hold.

Variants of the basic algorithm

• There is the issue of whether the garbage collector moves objects in memory (that is, changes their address) or not: moving or non-moving GC.

• Some collectors can correctly identify all pointers (references) in an object; these are called "exact" collectors, the opposite being a "conservative" or "partly conservative" collector. "Conservative" collectors have to assume that any bit pattern in memory is a pointer if (when interpreted as a pointer) it would point into any allocated object. Thus, conservative collectors may have some false negatives, where storage isn't released because of accidental fake pointers. However, this isn't a big drawback in practice.

• There is the issue of whether the garbage collector can run interleaved or in parallel with any of the rest of the system: the simplest garbage collectors stop the rest of the system while they perform a cycle; they are non-incremental, whereas incremental collectors interleave their work with units of activity from the rest of the system. Some incremental collectors can run fully parallel in a separate thread; these can theoretically run on a separate CPU, but cache issues make this less practical than it may initially appear.

Mark and sweep
Collectors can also be divided by considering how the three sets (of white, grey, and black objects) are implemented. A Mark sweep GC[?] maintains a bit (or two) with each object to record whether it is white or black; the grey set is either maintained as a separate list or using another bit. A Copying GC[?] identifies grey and black objects by copying them to another area of memory (the copy space) and often distinguishes black from grey objects by dividing the copy space into two portions (in the simple case by maintaining a single pointer that marks the division between black and grey objects).

Generational GC
There is the issue of when to perform a cycle and what objects to place in the initial grey set. A simple collector will always put only the roots in the initial grey set, and everything else will be initially white.

Statistically speaking, the objects most recently created in the runtime system are also those most likely to quickly become unreachable. A Generational GC[?] divides objects into generations and will generally only place the objects of a subset of all the generations into the initial white set (the grey set being everything else). This can result in faster cycles.

Reference counting
Reference counting is commonly referred to as garbage collection, but this usage isn't accepted by all. It could probably be argued that reference counting is a class of conservative garbage collection. Some punters like to reserve the term garbage collection for algorithms and systems that determine the liveness of data based on reachability.

Reference counting has serious problems anyway, such as a general inability to deal with circular references and a high overhead in both space and time. On the other hand, it does dispose of most garbage fairly promptly, which can have advantages if there are finalisers to be run to free up scarce resources other than heap memory. Hybrid systems that use reference counting to achieve (mostly) immediate freeing of resources, and occasionally call a mark-sweep collector to free stuff containing reference loops, have been proposed and occasionally implemented. This gives the best of both worlds in some ways, but still has a performance overhead.

Languages which use automatic garbage collection

• Lisp
• Smalltalk
• Java
• Javascript[?]

Further Reading

• http://www.memorymanagement.org/
• Publications by the OOPS group at the University of Texas: http://www.cs.utexas.edu/users/oops/papers.html

"Ich? Wahrlich nicht! Und war.html">war auch nie dazu Nur diese duenne Scheidewand mich trennte--" "Gewichtiger, mein Sohn, als du es meinst, Zwar leicht, doch zentnerschwer fuer dein Gewissen." Der Juengling ging gedankenvoll nach Hause, Den Schlaf, er waelzt sich gluehend auf.html">auf dem Lager Fuehrt unfreiwillig ihn der scheue Tritt. Und mitten in das Innre der Rotonde Den Einsamen die.html">die.html">die.html">die.html">die.html">die.html">die.html">die.html">die lebenlose Stille, In den geheimen Grueften unterbricht Der Mond den bleichen, silberblauen Schein, Erglaenzt durch des Gewoelbes Finsternisse Schon will die freche Hand das Heilige beruehren, Und stoesst ihn weg mit unsichtbarem Arme. In seinem Innern eine treue Stimme. Kein Sterblicher, sprach des Orakels Mund, Doch setzte nicht derselbe Mund hinzu: "Sei hinter ihm, was will! Ich heb ihn auf." Schauen! Nun, fragt ihr, und was zeigte sich ihm hier? So fanden ihn am andern Tag die Priester Was er allda gesehen und erfahren, War seines Lebens Heiterkeit dahin, "Weh dem", dies war sein warnungsvolles Wort, "Weh dem, der zu der Wahrheit geht durch Schuld, Senke, strahlender Gott--die Fluren duersten Matter ziehen die Rosse-- Lieblich laechelnd dir winkt! Erkennt dein Herz sie.html">sie? Tethys, die goettliche, winkt. Schnell vom Wagen herab in ihre Arme Stille halten die Rosse, Kommt die duftende Nacht; ihr folgt die suesse Phoebus, der liebende, ruht. Die Antiken zu Paris Mag der Franke mit den Waffen Und in prangenden Museen Dem erstaunten Vaterland! Ewig werden sie ihm schweigen, In des Lebens frischen Reihn. .

 On wordlookup.net  

All is still licensed under the GNU FDL.
It uses material from the wikipedia.