Reference counting garbage collection is where each object has a count of the number of references to it. Garbage is identified by having a reference count of zero. An object's reference count is incremented when a reference to it is created, and decremented when a reference is destroyed. When the count reaches zero, th
Reference counting garbage collection is where each object has a count of the number of references to it. Garbage is identified by having a reference count of zero. An object's reference count is incremented when a reference to it is created, and decremented when a reference is destroyed. When the count reaches zero, the object's memory is reclaimed.[7]
As with manual memory management, and unlike tracing garbage collection, reference counting guarantees that objects are destroyed as soon as their last reference is destroyed, and usually only accesses memory which is either in CPU caches, in objects to be freed, or directly pointed by those, and thus tends to not have significant negative side effects on CPU cache and virtual memory operation.
There ar
As with manual memory management, and unlike tracing garbage collection, reference counting guarantees that objects are destroyed as soon as their last reference is destroyed, and usually only accesses memory which is either in CPU caches, in objects to be freed, or directly pointed by those, and thus tends to not have significant negative side effects on CPU cache and virtual memory operation.
There are a number of disadvantages to reference counting; this can generally be solved or mitigated by more sophisticated algorithms:
Escape analysis is a compile-time technique that can convert heap allocations to stack allocations, thereby reducing the amount of garbage collection to be done. This analysis determines whether an object allocated inside a function is accessible outside of it. If a function-local allocation is found to be accessible to another function or thread, the allocation is said to “escape” and cannot be done on the stack. Otherwise, the object may be allocated directly on the stack and released when the function returns, bypassing the heap and associated memory management costs.[14]
Heartbeat-based garbage collection is a technique to free heterogenous resources such as file handlers and network pointers, rather than unused memory like traditional garbage collection algorithms. An example is a network algorithm that sends a "heartbeat" signal to a monitor. Once the client fails to send a heartbeat to the monitor serve, the network connection is closed and the resources are freed. Timestamp methods can work as garbage collectors for collection of unused memory but have serious drawbacks for this task and are used when other methods are not practical (i.e. network tasks).
Generally speaking, higher-level programming languages are more likely to have garbage collection as a standard feature. In some languages that do not have built in garbage collection, it can be added through a library, as with the Boehm garbage collector for C and C++.
Most functional programming languages, such as ML, Haskell, and APL, have garbage collection built in. Lisp is especially notable as both the first functional programming language and the first language to introduce garbage collection.[15]
Other dynamic languages, such as Ruby and Julia (but not Perl 5 or PHP before version 5.3,[16] which both use reference counting), JavaScript and ECMAScript also tend to use GC. Object-oriented programming languages such as Smalltalk and Java usually provide integrated garbage collection. Notable exceptions are C++ and Delphi which have destructors.
Historically, languages intended for beginners, such as BASIC and Logo, have often used garbage collection for heap-allocated variable-length data types, such as strings and lists, so as not to burden programmers with manual memory management. On early microcomputers, with their limited memory and slow processors, BASIC garbage collection could often cause apparently random, inexplicable pauses in the midst of program operation.
Some BASIC interpreters, such as Applesoft BASIC on the Apple II family, repeatedly scanned the string descriptors for the string having the highest address in order to compact it toward high memory, resulting in O(n2) performance, which could introduce minutes-long pauses in the execution of string-intensive programs. A replacement garbage collector for Applesoft BASIC published in Call-A.P.P.L.E. (January 1981, pages 40–45, Randy Wigginton) identified a group of strings in every pass over the heap, which cut collection time dramatically. BASIC.System, released with ProDOS in 1983, provided a windowing garbage collector for BASIC that reduced most collections to a fraction of a second.
While the Objective-C traditionally had no garbage collection, with the release of OS X 10.5 in 2007 Apple introduced garbage collection for Objective-C 2.0, using an in-house developed runtime collector.[17] However, with the 2012 releas
Heartbeat-based garbage collection is a technique to free heterogenous resources such as file handlers and network pointers, rather than unused memory like traditional garbage collection algorithms. An example is a network algorithm that sends a "heartbeat" signal to a monitor. Once the client fails to send a heartbeat to the monitor serve, the network connection is closed and the resources are freed. Timestamp methods can work as garbage collectors for collection of unused memory but have serious drawbacks for this task and are used when other methods are not practical (i.e. network tasks).
Generally speaking, higher-level programming languages are more likely to have garbage collection as a standard feature. In some languages that do not have built in garbage collection, it can be added through a library, as with the Boehm garbage collector for C and C++.
Most functional programming languages, such as ML, Haskell, and APL, have garbage collection built in. Lisp is especially notable as both the first functional programming language and the first language to introduce garbage collection.[15]
Other dynamic languages, such as Ruby and Julia (but not Perl 5 or PHP before version 5.3,[16]Generally speaking, higher-level programming languages are more likely to have garbage collection as a standard feature. In some languages that do not have built in garbage collection, it can be added through a library, as with the Boehm garbage collector for C and C++.
Most functional programming languages, such as ML, Haskell< Most functional programming languages, such as ML, Haskell, and APL, have garbage collection built in. Lisp is especially notable as both the first functional programming language and the first language to introduce garbage collection.[15]
Other dynamic languages, such as Ruby and Julia (but not Perl 5 or PHP before version 5.3,[16] which both use reference counting), JavaScript and ECMAScript also tend to use GC. Object-oriented programming languages such as Smalltalk and Java usually provide integrated garbage collection. Notable exceptions are C++ and Delphi which have destructors.
Historically, languages intended for beginners, such as BASIC and Logo, have often used garbage collection for heap-allocated variable-length data types, such as strings and lists, so as not to burden programmers with manual memory management. On early microcomputers, with their limited memory and slow processors, BASIC garbage collection could often cause apparently random, inexplicable pauses in the midst of program operation.
Some BASIC interpreters, such as Applesoft BASIC on the Apple II family, repeatedly scanned the string descriptors for the string having the highest address in order to compact it toward high memory, resulting i Some BASIC interpreters, such as Applesoft BASIC on the Apple II family, repeatedly scanned the string descriptors for the string having the highest address in order to compact it toward high memory, resulting in O(n2) performance, which could introduce minutes-long pauses in the execution of string-intensive programs. A replacement garbage collector for Applesoft BASIC published in Call-A.P.P.L.E. (January 1981, pages 40–45, Randy Wigginton) identified a group of strings in every pass over the heap, which cut collection time dramatically. BASIC.System, released with ProDOS in 1983, provided a windowing garbage collector for BASIC that reduced most collections to a fraction of a second.
While the Objective-C traditionally had no garbage collection, with the release of OS X 10.5 in 2007 Apple introduced garbage collection for Objective-C 2.0, using an in-house developed runtime collector.[17]
However, with the 2012 release of OS X 10.8, garbage collection was deprecated in favor of LLVM's automatic reference counter (ARC) that was introduced with OS X 10.7.[18] Furthermore, since May 2015 Apple even forbids the usage of garbage collection for new OS X applications in the App Store.[19][20] For iOS, garbage collection has never been introduced due to problems in application responsivity and performance;[6][21] instead, iOS uses ARC.[22][23]
Garbage collectors available in Java JDKs inc Garbage collectors available in Java JDKs include:
Compile-time garbage collection is a form of static analysis allowing memory to be reused and reclaimed based on invariants known during compilation.
This form of garbage collection has been studied in the Mercury programming language,[26] and it saw greater usage with the introduction of LLVM's automatic reference counter (ARC) into Apple's ecosystem (iOS and OS X) in 2011.[22][23]This form of garbage collection has been studied in the Mercury programming language,[26] and it saw greater usage with the introduction of LLVM's automatic reference counter (ARC) into Apple's ecosystem (iOS and OS X) in 2011.[22][23][19]
Incremental, concurrent, and real-time garbage collectors have been developed, such as Baker's algorithm or Lieberman's algorithm.[27][28][29]
In Baker's algorithm, the allocation is done in either half of a single region of memory. When it becomes half full, a garbage collection is performed which moves the live objects into the other half and the remaining objects are implicitly deallocated. The running program (the 'mutator') has to check that any object it references is in the correct ha In Baker's algorithm, the allocation is done in either half of a single region of memory. When it becomes half full, a garbage collection is performed which moves the live objects into the other half and the remaining objects are implicitly deallocated. The running program (the 'mutator') has to check that any object it references is in the correct half, and if not move it across, while a background task is finding all of the objects.[30]
Generational garbage collection schemes are based on the empirical observation that most objects die young. In generational garbage collection two or more allocation regions (generations) are kept, which are kept separate based on object's age. New objects are created in the "young" generation that is regularly collected, and when a generation is full, the objects that are still referenced from older regions are copied into the next oldest generation. Occasionally a full scan is performed.
Some high-level language computer architectures include hardware support for real-time garbage collection.
Most implementations of real-time garbage collectors use tracing.[citation needed] Such real-time garbage collectors meet hard real-time constraints when used with a real-time operating system.[31]
Limited environments