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GC biography, GC discography
Weiser
conservative garbage collector can
be used as a garbage collecting
replacement for C malloc or C++ new.It allows you to allocate memory basically as you normally would,
without explicitly deallocating memory that is no longer useful.The collector is also used by a number of programming language
implementations that either use C as intermediate code, want
to facilitate easier interoperation with C libraries, or
just prefer the simple collector interface.Alternatively, the garbage collector may be used as
a leak detector
for C or C++ programs, though that is not its primary goal.The arguments for and against conservative garbage collection
in C and C++ are briefly
discussed in
issues.Empirically, this collector works with most unmodified C programs,
simply by replacing
malloc with GC_malloc calls,
replacing realloc with GC_realloc calls, and removing
free calls.Where to get the collector
Typically several versions will be available.The latest experimental version of the source code is now maintained
on the SourceForge site (project "bdwgc").Just hit return in response to the password prompt.The source
code for that version is available for browsing
here.It may be used and copied without payment of a fee under minimal restrictions.See the README file in the distribution or the
license for more details.IT IS PROVIDED AS IS,
WITH ABSOLUTELY NO WARRANTY EXPRESSED OR IMPLIED.ANY USE IS AT YOUR OWN RISK.There are instructions for porting the collector
to a new platform.I'm not in a position to test under MacOS.Although I try to
incorporate changes, it is impossible for
me to update the project file.Our approach is discussed briefly in
scale.Linux, and Windows, with varying restrictions.It allows finalization code
to be invoked when an object is collected.The garbage collector distribution includes a C string
(cord) package that provides
for fast concatenation and substring operations on long strings.We also expect that in many cases any additional overhead
will be more than compensated for by decreased copying etc.Further Reading:
The beginnings of a frequently asked questions list for this
collector are here.The following provide information on garbage collection in general:
Paul Wilson's garbage collection ftp archive and GC survey.The Ravenbrook
Memory Management Reference.Richard Jones'
GC page and
his book.This is directed at an otherwise sophisticated
audience unfamiliar with memory allocation issues.The algorithmic details differ
from those in the implementation.There is a related letter to the editor and a minor
correction in the next issue.Proceedings of the Fifth ACM SIGPLAN International Conference on
Functional Programming, 2000, Montreal, Canada, pp.Includes some discussion of the
collector debugging facilities for identifying causes of memory retention.Discusses the parallel
collection algorithms, and presents some performance results.Includes a discussion of a collector facility to much more reliably test for
the potential of unbounded heap growth.The following papers discuss language and compiler restrictions necessary to guaranteed
safety of conservative garbage collection.Safety'', Proceedings
of the ACM SIGPLAN '96 Conference on Programming Language Design
and Implementation.The Detlefs, Dosser and Zorn's Memory Allocation Costs in Large C and C++ Programs.Slides for Hans Boehm's Allocation and GC Myths talk.Current users:
Known current users of some variant of this collector include:
The runtime system for GCJ,
the static GNU java compiler.The NAGWare f90 Fortran 90 compiler.Elwood Corporation's
Eclipse Common Lisp system, C library, and translator.The Bigloo
Scheme
and Camloo ML
compilers
written by Manuel Serrano and others.The University of Washington Cecil Implementation.The Toba Java Virtual
Machine to C translator.The
GNU Objective C runtime.The Vesta configuration management
system.Slides from an ISMM 2004 tutorial about the GC.FAQ (frequently asked questions) list.How to use the garbage collector as a leak detector.An overview of the implementation of the
garbage collector.More background information at this site
An attempt to establish a bound on space usage of
conservative garbage collectors.Pros and cons of conservative garbage collectors,
in comparison to other collectors.Issues related to garbage collection vs.An example of a case in which garbage collection
results in a much faster implementation as a result of reduced
synchronization.Slide set discussing Destructors, Finalizers, and Synchronization
(POPL 2003).Paper corresponding to above slide set.Slides for talk on memory allocation myths.We expect this to always remain a very low volume list.The gc list archive may also be read at
gmane.Its been an amazing year for all of us in GC land.Get ready for a new GC record and a ton of GC showzzz !!!!!!!!!!!!!!To celebrate the holidays and the success of Young Dre's new single "Workin" ft.Get these one of a kind gifts EXCLUSIVELY by going to KIISFM.MD to visit my family and friends for a week, ate lots of "Tofurkey", and got to play a few songs with my friends band, Beretta Jane.The event was hosted by Christ Kirkpatrick, always good to see him!Sorry its been so long since tour and i haven't posted in a while.Well Benj and myself just got back from a trip to Central African Republic, which is a small country in the middle of Africa.All the clothes are still there and still for sale.Please check your browser settings or contact your system administrator.This page uses frames, but your browser doesn't support them.GC advocates on behalf of small business in the federal procurement world.For Contracting Officers Best Practices in Mitigating the Effects of Contract Consolidation on Small Businesses U.This article needs additional citations for verification.In computer science, garbage collection (GC) is a form of automatic memory management.Garbage collection was invented by John McCarthy around 1959 to solve the problems of manual memory management in Lisp.Garbage collection is often portrayed as the opposite of manual memory management, which requires the programmer to specify which objects to deallocate and return to the memory system.However, many systems use a combination of the two approaches, and there are other techniques being studied (such as region inference) to solve the same fundamental problem.Generational GC (aka Ephemeral GC)
2.It also aids programmers in their efforts to make programs more stable, because it prevents several classes of runtime errors.The pointer still points to the location in memory where the object or data was, even though the object or data has since been deleted and the memory may now be used for other purposes, creating a dangling pointer.Newer Delphi versions support garbage collected dynamic arrays, long strings, variants and interfaces.GC systems exist, including ones that do not require recompilation.The garbage collector will almost always be closely integrated with the memory allocator.Benefits
Garbage collection frees the programmer from manually dealing with memory allocation and deallocation.Certain kinds of memory leaks, in which a program fails to free memory that is no longer referenced by any variable, leading over time to memory exhaustion.Researchers draw a distinction between "physical" and "logical" memory leaks.Novice programmers sometimes believe that garbage collection makes memory leaks impossible, not realizing that logical leaks are still possible.Anything referenced from a reachable object is itself reachable; more formally, reachability is a transitive closure.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.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; since in memory, an integer and a reference might look alike.One common solution is the use of Tagged pointers.These garbage collectors perform collection in cycles.In this method, each object in memory has a flag (typically a single bit) reserved for garbage collection use only.All objects transitively accessible from the root set are marked, as well.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.Create initial white, grey, and black sets; these sets will be used to maintain progress during the cycle.Initially the white set or condemned set is the set of objects that are candidates for having their memory recycled.The black set is the set of objects that cheaply can be proven to have no references to objects in the white set; in many implementations the black set starts off empty.Pick an object from the grey set.Repeat the previous step until the grey set is empty.No black object points directly to a white object.This ensures that the white objects can be safely destroyed once the grey set is empty.This is accomplished by marking objects as they are allocated and during mutation, maintaining the various sets.GC must visit each unreachable object and somehow record that the memory it alone occupied is available.Similarly, new objects can be allocated very quickly.Since large contiguous regions of memory are usually made available by the moving GC strategy, new objects can be allocated by simply incrementing a 'free memory' pointer.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).In this moving GC scheme, memory is partitioned into a "from space" and "to space".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 to "to space", until all reachable objects have been copied to "to space".Once the program continues execution, new objects are once again allocated from the "to space" until it is once again full and the process is repeated.This approach has the advantage of conceptual simplicity (the three object color sets are implicitly constructed during the copying process), but the disadvantage that a (possibly) very large contiguous region of free memory is necessarily required on every collection cycle.In a "mark and don't sweep" system, all reachable objects are always black.This momentarily breaks the invariant that reachable objects are black, but a full marking phase follows immediately, to mark them black again.Once this is done, all unreachable memory is white.Generational GC (aka Ephemeral GC)
It has been empirically observed that in many programs, the most recently created objects are also those most likely to become unreachable quickly (known as infant mortality or the generational hypothesis).GC divides objects into generations and, on most cycles, will place only the objects of a subset of generations into the initial white (condemned) set.Furthermore, the runtime system maintains knowledge of when references cross generations by observing the creation and overwriting of references.This technique permits very fast incremental garbage collection, since the garbage collection of only one region at a time is all that is typically required.Generational garbage collection is a heuristic approach, and some unreachable objects may not be reclaimed on each cycle.It may therefore occasionally be necessary to perform a full mark and sweep or copying garbage collection to reclaim all available space.This has the obvious disadvantage that the program can perform no useful work while a collection cycle is running (sometimes called the "embarrassing pause").Incremental garbage collectors are designed to reduce this disruption by interleaving their work with activity from the main program.Careful design is necessary to ensure that the main program does not interfere with the garbage collector and vice versa; for example, when the program needs to allocate a new object, the runtime system may either need to suspend it until the collection cycle is complete, or somehow notify the garbage collector that there exists a new, reachable object.Finally, a concurrent garbage collector can run concurrently in real time with the main program on a shared memory multiprocessing machine.Nonetheless, concurrent GC may be desirable for multiprocessing applications with high performance requirements.Some collectors running in a particular environment can correctly identify all pointers (references) in an object; these are called "precise" (also "exact" or "accurate") collectors, the opposite being a "conservative" or "partly conservative" collector.Naturally 64bit systems suffer less from this problem than 32bit systems, since the possibility for a 64bit pattern to form a valid pointer is obviously smaller than for a 32bit pattern as the domain of a 64bit value is much greater than a 32bit value.Thus it is less probable for a "random" 64bit value to point to a region on system's memory.An example for this is the C++ programming language, in which multiple inheritance can cause pointers to base objects to have different addresses.Performance implications
Tracing garbage collectors require some implicit runtime overhead that may be beyond the control of the programmer, and can sometimes lead to performance problems.The performance of modern computer architectures is increasingly tied to caching, which depends on the assumption of locality of reference for its effectiveness.In addition, copying collectors may even improve locality by defragmenting memory and thus potentially keeping related data together.For example, in the best case for a garbage collecting system, allocation just increments a pointer, but in the best case for manual heap allocation, the allocator maintains freelists of specific sizes and allocation only requires following a pointer.The object's memory is reclaimed when the count reaches zero.However, optimizations to this are described in the literature.This is usually a very expensive operation.Escape analysis can be used to convert heap allocations to stack allocations, thus reducing the amount of work needed to be done by the garbage collector.In languages that do not have built in garbage collection, it can often be added through a library, as with the Boehm garbage collector for C and C++.Lisp, which introduced functional programming, is especially notable for introducing this mechanism.Other dynamic languages, such as Ruby (but not Perl, or PHP, which use reference counting), also tend to use GC.On early microcomputers, with their limited memory and slow processors, garbage collection could often cause apparently random, inexplicable pauses in the midst of program operation.However, garbage collectors compatible with such limited environments have been developed.NET Micro Framework and Java Platform, Micro Edition are embedded software platforms that, like their larger cousins, include garbage collection.Note that there is an ambiguity of terms, as theory often uses the terms manual garbage collection and automatic garbage collection rather than manual memory management and garbage collection, and does not restrict garbage collection to memory management, rather considering that any logical or physical resource may be garbage collected.Maebe, Ronsse, and De Bosschere, "Precise detection of memory leaks."Time Garbage Collection Framework for Embedded Systems".Dijkstra and Leslie Lamport and A.Time Garbage Collector Based on the Lifetimes of Objects by H.This page was last modified on 8 January 2009, at 07:09.
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