Application root in garbage collection
Finalization may introduce complications limiting its usability, such as intolerable latency between disuse and reclaim of especially limited resources, or a lack of control over which thread performs the work of reclaiming. Many computer languages require garbage collection, either as part of the language specification e. Other languages were designed for use with manual memory management, but have garbage collected implementations available e.
While integrating garbage collection into the language's compiler and runtime system enables a much wider choice of methods, [ citation needed ] post hoc GC systems exist, including some that do not require recompilation. Post-hoc GC is sometimes distinguished as litter collection. The garbage collector will almost always be closely integrated with the memory allocator.
Garbage collection frees the programmer from manually dealing with memory deallocation.
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As a result, certain categories of bugs are eliminated or substantially reduced:. Tracing garbage collectors are the most common type of garbage collector. They first determine which objects are reachable or potentially reachable , and then discard all remaining objects. Informally, an object is reachable if it is referenced by at least one variable in the program, either directly or through references from other reachable objects.
More precisely, objects can be reachable in only two ways:. The reachability definition of "garbage" is not optimal, insofar as the last time a program uses an object could be long before that object falls out of the environment scope. A distinction is sometimes drawn between syntactic garbage , those objects the program cannot possibly reach, and semantic garbage , those objects the program will in fact never again use. For example:.
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The problem of precisely identifying semantic garbage can easily be shown to be partially decidable : a program that allocates an object X , runs an arbitrary input program P , and uses X if and only if P finishes would require a semantic garbage collector to solve the halting problem. Although conservative heuristic methods for semantic garbage detection remain an active research area, essentially all practical garbage collectors focus on syntactic garbage.
Another complication with this approach is that, in languages with both reference types and unboxed value types , the garbage collector needs to somehow be able to distinguish which variables on the stack or fields in an object are regular values and which are references: in memory, an integer and a reference might look alike.
The garbage collector then needs to know whether to treat the element as a reference and follow it, or whether it is a primitive value. One common solution is the use of tagged pointers. The garbage collector can reclaim only objects that have no references. However, there can exist additional references that, in a sense, do not matter, which are called weak references. In discussions about weak references, ordinary references are sometimes called strong references.
An object is eligible for garbage collection if there are no strong i. A weak reference is not merely just any pointer to the object that a garbage collector does not care about. The term is usually reserved for a properly managed category of special reference objects which are safe to use even when the object disappears because they lapse to a safe value. An unsafe reference that is not known to the garbage collector will simply remain dangling by continuing to refer to the address where the object previously resided.
This is not a weak reference. In some implementations, notably in Microsoft. NET, [ 3 ] the weak references are divided into two further subcategories: long weak references tracks resurrection and short weak references. Objects which maintain collections of other objects can also be devised which have weak tracking features.
For instance, weak hash tables are useful. Like a regular hash table, a weak hash table maintains an association between pairs of objects, where each pair is understood to be a key and value.
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However, the hash table does not actually maintain a strong reference on these objects. A special behavior takes place when either the key or value or both become garbage: the hash table entry is spontaneously deleted. There exist further refinements such as hash tables which have only weak keys value references are ordinary, strong references or only weak values key references are strong.
Weak hash tables are important for maintaining associations between objects, such that the objects engaged in the association can still become garbage if nothing in the program refers to them any longer other than the associating hash table. The use of a regular hash table for such a purpose could lead to a "logical memory leak": the accumulation of reachable data which the program does not need and will not use.
Tracing collectors are so called because they trace through the working set of memory. These garbage collectors perform collection in cycles. A cycle is started when the collector decides or is notified that it needs to reclaim memory, which happens most often when the system is low on memory [ citation needed ].
The original method involves a naïve mark-and-sweep in which the entire memory set is touched several times. In the naive mark-and-sweep method, each object in memory has a flag typically a single bit reserved for garbage collection use only. This flag is always cleared , except during the collection cycle. The first stage of collection does a tree traversal of the entire 'root set', marking each object that is pointed to as being 'in-use'.
All objects that those objects point to, and so on, are marked as well, so that every object that is ultimately pointed to from the root set is marked. Finally, all memory is scanned from start to finish, examining all free or used blocks; those with the in-use flag still cleared are not reachable by any program or data, and their memory is freed. For objects which are marked in-use, the in-use flag is cleared again, preparing for the next cycle.
This method has several disadvantages, the most notable being that the entire system must be suspended during collection; no mutation of the working set can be allowed. This will cause programs to 'freeze' periodically and generally unpredictably , making real-time and time-critical applications impossible.
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In addition, the entire working memory must be examined, much of it twice, potentially causing problems in paged memory systems. Because of these pitfalls, most modern tracing garbage collectors implement some variant of the tri-colour marking abstraction , but simple collectors such as the mark-and-sweep collector often do not make this abstraction explicit.
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Tri-colour marking works as follows:. The 3 sets partition memory; every object in the system, including the root set, is in precisely one set. This ensures that the white objects can be safely destroyed once the grey set is empty. Some variations on the algorithm do not preserve the tricolour invariant but they use a modified form for which all the important properties hold. The tri-colour method has an important advantage: it can be performed 'on-the-fly', without halting the system for significant time periods. This is accomplished by marking objects as they are allocated and during mutation, maintaining the various sets.
By monitoring the size of the sets, the system can perform garbage collection periodically, rather than as-needed. Also, the need to touch the entire working set each cycle is avoided. In order to implement the basic tri-colour algorithm, several important design decisions must be made, which can significantly affect the performance characteristics of the garbage collector. Once the unreachable set has been determined, the garbage collector may simply release the unreachable objects and leave everything else as it is, or it may copy some or all of the reachable objects into a new area of memory, updating all references to those objects as needed.
These are called "non-moving" and "moving" or, alternatively, "non-compacting" and "compacting" garbage collectors, respectively. At first, a moving GC strategy may seem inefficient and costly compared to the non-moving approach, since much more work would appear to be required on each cycle. In fact, however, the moving GC strategy leads to several performance advantages, both during the garbage collection cycle itself and during actual program execution:.
Application root in garbage collection
One disadvantage of a moving garbage collector is that it only allows access through references that are managed by the garbage collected environment, and does not allow pointer arithmetic. This is because any native pointers to objects will be invalidated when the garbage collector moves the object they become dangling pointers. For interoperability with native code, the garbage collector must copy the object contents to a location outside of the garbage collected region of memory.
An alternative approach is to pin the object in memory, preventing the garbage collector from moving it and allowing the memory to be directly shared with native pointers and possibly allowing pointer arithmetic. To further refine the distinction, tracing collectors can also be divided by considering how the three sets of objects white, grey, and black are maintained during a collection cycle. The most straightforward approach is the semi-space collector , which dates to In this moving GC scheme, memory is partitioned into a "from space" and "to space".
Initially, objects are allocated into "to space" until they become full and a collection is triggered. At the start of a collection, the "to space" becomes the "from space", and vice versa. The objects reachable from the root set are copied from the "from space" to the "to space". These objects are scanned in turn, and all objects that they point to are copied into "to space", until all reachable objects have been copied into "to space".
Once the program continues execution, new objects are once again allocated in the "to space" until it is once again full and the process is repeated. Voir tous les produits de gestion des services informatiques. Voir tous les produits de gestion des applications.