Abstract:
A method for garbage collection in which resources used by data objects in a computer program that are not accessed in the future are reclaimed, the method consists of the following steps of 1. Providing a tracing garbage collector which is both parallelized and incremental; 2. Integrating the collector with the computer program to create the mutator; 3. Instructing the mutator to scan and collect resources used by data objects in a computer program that are not accessed in the future simultaneously on all threads in small amounts; and 4. Interleaving the mutator scanning and collection with unrelated processing by the computer program such that all of the threads in the application are never stopped simultaneously for any portion of the collection cycle.

Description:
FIELD OF THE INVENTION 
     The invention relates to automated memory management processes known as garbage collection in the field of computer science, and more particularly to a method and device for performing parallelized, incremental garbage collection in order to prevent the occurrence of stop the world-type computer thread processing and minimize synchronization thereof. 
     BACKGROUND OF THE INVENTION 
     In computer science, garbage collection (GC) is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory used by objects that will never be accessed or mutated again by the application or computer program. Garbage collection was invented by John McCarthy around 1959 to solve the problems of manual memory management in Lisp. 
     Garbage collection is 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. 
     A typical tracing garbage collector maintains a set, U, of all memory objects known to the collector. During a collection cycle, the collector&#39;s task is to categorize all objects as either mutable or immutable. Mutable objects are objects that the mutator, i.e., the computer program, is able to read from or write to because the mutator has retained references, to the object in somewhere in memory. Objects that have no remaining references in the application&#39;s domain are considered immutable. The programmer provides the collector with a set of objects that are to be prejudged as mutable. The collector uses this set, the root set, as roots of a graph composed of objects, i.e., the vertices, and references, i.e., the edges. As the collector traverses this graph, objects are added to the mutable set M. When the graph has been completely traversed, all of the objects in U that do not also belong to M, i.e., U-M, are considered immutable and safe to collect. 
     Many applications or computer programs, such as those with real-time constraints and those that maintain large heaps, cannot afford to stop processing long enough so that the collector can compute all immutable objects in memory. Incremental garbage collection addresses this issue by splitting the work of a single collection cycle into small parts, interrupting the application/computer program frequently for small periods of time instead of interrupting the application/computer program relatively infrequently for potentially long periods of time. 
     Previous attempts to parallelize garbage collection incur a bottleneck, commonly referred to as “stop the world,” meaning that all threads in the application must be stopped for a portion of the collection cycle. Long pauses are anathema to parallelism, as shown in  FIG. 4 . The task of parallelizing incremental collection represents even more of a challenge to implement without defeating the benefits of parallelization completely. 
     Tri-Color Marking 
     Most modern tracing garbage collectors implement some variant of the tri-color marking abstraction, but simple collectors, such as the mark-and-sweep collector, often do not make this abstraction explicit. 
       FIG. 1  (prior art) is a schematic view of a“tri-color” garbage collector algorithm  80 . Tri-color marking works as follows: 
     1. Create initial white W, grey G, and black B sets; these sets will be used to maintain progress during the cycle. Initially the white W set or condemned set is the set of objects that are candidates for having their memory recycled. The black B set is the set of objects that can be proven to have no references to objects in the white W set; this diagram in  FIG. 1  (prior art) demonstrates an implementation that starts each collection cycle with an empty black B set. The grey G set is all the remaining objects that may or may not have references to objects in the white W set and elsewhere. These sets partition memory; every object in the system, including the root set, is in precisely one set. 
     2. Mark the root set grey. This step is important since both the black and the grey sets start off empty. 
     3. Pick an object from the grey G set. Blacken this object, i.e., move it to the black B set, by greying all the white W objects it references directly. 
     4. Repeat the previous step until the grey G set is empty. 
     5. When there are no more objects in the grey G set, then all the objects remaining in the white W set are safe to consider unreachable and the storage occupied by them can be reclaimed safely. 
     The tri-color marking algorithm preserves an important invariant: “No black B object points directly to a white W object.” This ensures that the white W objects can be safely destroyed once the grey G set is empty. 
     The tri-color 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. 
       FIG. 2  (prior art) is a schematic view of the traditional actor model  90 . The actor model has been described with respect to parallel programming in computing. In a network of active objects, all processes run concurrently and communicate through messaging. As an example, the internet is a model network in which each computer is an actor and all actors interact together in essentially real-time. Each actor has both a state and a thread of execution. The degree of parallelism is related to the degree of time-sharing, and not all messages can receive an immediate response. It will be understood, therefore, that with synchronous communication, there is the need to stop a thread in anticipation of a response, but with asynchronous processing, there is no need to wait. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     It is an object and advantage of the present invention to provide a garbage collector which is both parallelized and incremental. 
     It is another object and advantage of the present invention to provide a garbage collector which never “stops the world”. The present invention also resolves the impedance problem between parallel and incremental collection so that garbage collection can be used in multithreaded applications or computer programs with large heaps and in multithreaded applications within the soft real-time and real-time spectrum. It will be understood that while prior art including U.S. Pat. No. 6,199,075, mentions “multiple processing units” instead of “threads.” The present invention describes the process in terms of threads because that&#39;s a paradigm in which programmers work, the term is also less abstract. The term “multiple processing units” is potentially a broader term than “threads”, since a “processing unit” could be a CPU, a thread, or an actor. It will be understood that in the present invention, a “thread” is not defined as a CPU or an actor. 
     It is yet another object and advantage of the present invention to provide a programmable garbage collector which does not create threads internally, thus leaving the programmer to determine which thread topology best suits the needs of the mutator. 
     It is an object and advantage of the present invention to provide a garbage collector that minimizes synchronization. 
     It is an object and advantage of the present invention to provide a garbage collector which is portable to any platform that supports simple synchronization primitives such as mutexes. 
     It is yet a further object and advantage of the collector of the present invention to integrate such collector into a language or virtual machine that uses garbage collection, for example JAVA, NET, or a CPU emulator. The collector of the present invention can be integrated through the interpreter or virtual machine or integrated directly into the program generated by a compiler. 
     An embodiment of the present invention comprises management of database resources such as in the use of the algorithm applied to file systems, as well as to collection of objects in memory. 
     The collector of the present invention also intends to satisfy real-time constraints of parallelized, incremental garbage collection. 
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  (prior art) is a schematic view of a“tri-color” garbage collector algorithm  80 . 
         FIG. 2  (prior art) is a schematic view of the traditional actor model  90 . 
         FIG. 3  is a schematic view of the message flow in the garbage collector process  100  of the present invention. 
         FIG. 4  is a graph showing schematic views of synchronization patterns used to parallelize the collection algorithm according to the garbage collector process  100  of the present invention. 
         FIG. 5  is a flowchart showing “Send a Message” function steps used by various interface functions to send a message. 
         FIG. 6  is a flowchart showing “Broadcast a Message” function steps used by various interface functions to broadcast a message. 
         FIG. 7  is a flowchart showing “Introduce” function steps used to introduce a handle to the collector. 
         FIGS. 8A and 8B  collectively show a flowchart showing “Nom” function steps used to collect incrementally. 
         FIG. 9  is a flowchart showing “Pin” function steps used to pin a handle. 
         FIG. 10  is a flowchart showing “Unpin” function steps used to unpin a handle. 
         FIG. 11  is a flowchart showing “Identify” function steps used to identify a referenced handle. 
         FIG. 12  is a flowchart showing the process “Pin” message and response steps used to pin a handle as identified in Table III. 
         FIG. 13  is a flowchart showing the process “Unpin” message and response steps used to unpin a handle as identified in Table III. 
         FIG. 14  is a flowchart showing the process “Bleach” message and response steps used to initiate the bleach phase as identified in Table III. 
         FIGS. 15A ,  15 B and  15 C collectively show a flowchart showing the process “Scan” message and response steps used to initiate the scan phase as identified in Table III. 
         FIG. 16  is a flowchart showing the process “Blacken” message and response steps used to color a handle black and scan it as identified in Table III. 
         FIG. 17  is a flowchart showing the process “Sweep” message and response steps used to initiate the sweep phase as identified in Table III. 
         FIG. 18  is a flowchart showing “Sample Collection Thread” function steps. 
         FIG. 19  is a flowchart showing “Sample Scan Function” function steps. 
         FIGS. 20A ,  20 B,  20 C and  20 D collectively show a flowchart showing “Sample Mutator” function steps. 
         FIG. 21  is a block diagram showing the mutator and collector functions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The description that follows is presented to enable one skilled in the art to make and use the present invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be apparent to those skilled in the art, and the general principals discussed below may be applied to other embodiments and applications without departing from the scope and spirit of the invention. Therefore, the invention is not intended to be limited to the embodiments disclosed, but the invention is to be given the largest possible scope which is consistent with the principals and features described herein. 
     It will be understood that in the event parts of different embodiments have similar functions or uses, they may have been given similar or identical reference numerals and descriptions. It will be understood that such duplication of reference numerals is intended solely for efficiency and ease of understanding the present invention, and are not to be construed as limiting in any way, or as implying that the various embodiments themselves are identical. 
     Terminology: 
     The garbage collector described in this document will be referred to as the collector or the algorithm. 
     The application or computer program integrated with the collector is called the mutator. 
     The programmer is the person that integrates the mutator with the collector. 
       FIG. 3  is a schematic view of the message flow in the garbage collector process  100  of the present invention. 
       FIG. 4  is a graph showing schematic views of synchronization patterns used to parallelize the collection algorithm according to the garbage collector process  100  of the present invention. The X axis represents increasing amounts of contention and the Y axis represents an increasing quantity of processing time that must be serialized between competing threads. The closest schematic to the origin represents the ideal where no process synchronization is needed and no bottleneck exists. 
     Properties: 
     The collector is both parallelized and incremental. The mutator can scan and collect simultaneously on all threads and do so in small amounts, interleaved with unrelated processing. 
     The collector never “stops the world.” The collector does not create threads internally, leaving the programmer to determine which thread topology best suits the needs of the mutator. The programmer can choose to call the incremental processing function while processing an allocation call, or can decide to dedicate a number of threads to the task of garbage collection. The programmer also has the option to eschew threading altogether, though this collector would not perform as well as traditional, single-threaded incremental collectors. 
     The collector minimizes synchronization, which is limited to queue operations, a single counter, and a hook whose synchronization needs, if any, are dependent upon the mutator&#39;s needs. 
     The collector does not implement a specialized, internal allocator. Instead, the collector is designed to be used with any allocator that uses common allocate and deallocate semantics. The collector does not need to be modified if the programmer wishes to use memory pooling or to address the issue of memory fragmentation though an optimized allocator. 
     The collector should be portable to any platform that supports simple synchronization primitives such as mutexes. 
     Interface: 
     This algorithm collects handles, which represent objects in memory that are potentially collectible. Exactly what a handle is is determined by how the programmer integrates the collector with the mutator and in most situations it is expected that a handle will simply be defined as the address of a structure in memory whose definition is well known to the programmer. 
     The programmer can also request that the collector pin (or unpin) handles. Pinned handles are considered noncollectable and use a reference counter to determine how long a handle should be pinned. At least one handle must be pinned for the collector to be function. 
     Introducing a new handle to the collector produces a pinned handle. This is necessary because the collector is capable of identifying objects as immutable before they have been referenced for the first time. The programmer is responsible for unpinning the handle once it is referenced by another handle known to be mutable. 
     Table 1 summarizes the basic interface the collector exposes to the programmer. Functions with an asterisk (*) are only intended to be called by the scanning hook: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Summary of Interface Functions 
               
             
          
           
               
                 Name 
                 Description 
                 Effect 
               
               
                   
               
               
                 Introduction 
                 Introduces a handle to the 
                 The collector initializes the handle&#39;s collection state 
               
               
                   
                 collector. 
                 and pins the object. The programmer is expected to 
               
               
                   
                   
                 unpin the handle when it is known to be referenced 
               
               
                   
                   
                 by another handle that is already known to the 
               
               
                   
                   
                 collector. 
               
               
                 Quit 
                 Shut down the collector. 
                 The collector sends a quit message to each actor. 
               
               
                 Nom 
                 Collect incrementally 
                 If an actor is on the ready queue, the caller&#39;s thread 
               
               
                   
                   
                 processes a single message from the actor&#39;s queue. 
               
               
                   
                   
                 If no outstanding messages remain in the current 
               
               
                   
                   
                 phase, send the message that initiates the next phase 
               
               
                   
                   
                 to each actor. 
               
               
                 Pin 
                 Pin a handle 
                 The collector sends a pin message to the actor that 
               
               
                   
                   
                 the handle is bound to. 
               
               
                 Unpin 
                 Unpin a handle 
                 The collector sends an unpin message to the actor 
               
               
                   
                   
                 that the handle is bound to. 
               
               
                 Identify* 
                 Identify a referenced 
                 The collector sends a blacken message to the actor 
               
               
                   
                 handle. 
                 the handle is bound to. 
               
               
                   
               
             
          
         
       
     
     The programmer is responsible for providing associated storage for a handle&#39;s color, reference count, and for an additional value that aids the collector in binding a handle to a message processor. It is reasonable to pack these values into a single 32-bit number or use lookup tables to retrieve values for the collector. 
     The programmer must supply an implementation of a scanning hook that determines how the collector is to discover mutable handles given a handle that is already known to be mutable (or uncollectable). This function must be thread-safe: the collector must be able to scan the handle&#39;s associated data for references without the danger of the data being simultaneously changed to by another thread. The collector does not write, so read-only data requires no synchronization. 
     It will be understood that the idea of never stopping two threads simultaneously is a theoretical ideal. Sometimes it can be accomplished, however sometimes it cannot. When it cannot, the collector is designed in such a way that the number of threads simultaneously stopped on a given occasion is minimized. This is called minimization of contention. Furthermore, the amount of time a given thread maintains exclusive access to a resource—this is called lock scope—is also minimized. As best shown in  FIG. 4 , it depicts a literal representation of contention but not of lock scope, both contention and lock scope contribute to how long a given number of threads are simultaneously stopped. 
       FIG. 5  is a flowchart showing “Send a Message” function  500  steps used by various interface functions to send a message.  FIG. 6  is a flowchart showing “Broadcast a Message” function steps used by various interface functions to broadcast a message. “Send a Message” and “Broadcast a Message” are subroutines that are used by the other flowcharts, i.e., there are references to both sending and broadcasting message references in the interface function subroutine blocks. “Send a Message” also demonstrates how the collector knows which actor is responsible for a given handle, i.e., which actor the handle is bound to. 
     “Send a Message” function  500  subroutine is initiated by Start step  502 . The number n is set to be equal to the number of actors in step  504 . In step  50 , i is equivalent to handle affinity modulo n in step  508 , actor i is retrieved. Step  510  comprises placing the message in the actor&#39;s queue. NOTE: Only one thread may access an actor&#39;s message queue at a time. In step  512 , a query is made as to whether or not the queue has exactly one message in it. NOTE: Only one thread may access the ready queue at a time. Additionally, the ready queue may not hold more than one reference to a specific actor. If the result of step  512  is yes, then step  514  consists of placing the actor in the ready queue. If the result of step  512  is no, then step  516  is termination of the subroutine. 
     “Broadcast a Message” function  600  subroutine is initiated by Start step  602 . The first actor in the collector&#39;s set is retrieved in step  604 . In step  606 , the message is placed in the collector&#39;s set. NOTE: Only one thread may access an actor&#39;s message queue at a time. In step  608 , a query is made as to whether or not the queue has exactly one message in it. NOTE: Only one thread may access the ready queue at a time. Additionally, the ready queue may not hold more than one reference to a specific actor. If the result of step  608  is yes, then step  610  consists of placing the actor in the ready queue and proceeding to step  612 . If the result of step  608  is no, then it will directly proceed to step  612  wherein a query is made as to whether or not the actor is the last actor in the set. If the result of step  612  is yes, step  616  is termination of the subroutine. If the result of step  612  is no, it will loop back to step  606 . 
       FIG. 7  is a flowchart showing “Introduce” function steps used to introduce a handle to the collector. “Introduce” function  700  subroutine is initiated by Start step  702 . A random number n is generated in step  704 . In step  706 , the handle&#39;s affinity is set to n. Step  708  comprises sending a pin handle message. Then step  710  is termination of the subroutine. 
       FIGS. 8A and 8B  collectively show a flowchart showing “Nom” function steps used to collect incrementally. As best shown in  FIG. 8A , “Nom” function  800  subroutine is initiated by Start step  802 . In step  804 , a query is made as to whether or not there is an actor in the ready queue. NOTE: Only one thread may access the ready queue at a time. If the result of step  804  is yes, then step  806  consists of retrieving an actor from the ready queue. Then in step  808 , a message from the actor&#39;s queue is retrieved. NOTE: Only one thread may access an actor&#39;s message queue at a time. Subsequently in step  810 , the message to the appropriate processing routine is dispatched and it will proceed to step  812 , as best shown in  FIG. 8B . However, if the result of step  804  is no, it will then proceed directly to step  812 . As best shown in  FIG. 8B , In step  812 , a query is made as to whether or not the message counter equals to zero. NOTE: Only one thread may access the collector&#39;s message counter at a time. If the result of step  812  is no, then step  832  is termination of the subroutine. If the result of step  812  is yes, then in step  814  the number n is set to the number of actors. The in step  816 , the message counter will be incremented by n. In step  818 , a query is made as to whether or not the next phase is the bleach phase. If the result of step  818  is yes, it will proceed to step  820  in which a bleach message is broadcast and then it proceeds to connector shape A  824  which is a go to function to shape A  834  which connects to end routine  832 . If the result of step  818  is no, it will proceed to step  826  wherein a query is made as to whether or not the subsequent phase is the scan phase. If the result of step  826  is yes, it will proceed to step  828  in which a scan message is broadcast and then it proceeds to connector shape A  824  which is a go to function to shape A  834  which connects to end routine  832 . If the result of step  826  is no, it will proceed to step  830  in which a sweep message is broadcast and then it proceeds to connector shape A  824  which is a go to function to shape A  834  which connects to end routine  832 . 
       FIG. 9  is a flowchart showing “Pin” function steps used to pin a handle. “Pin” function  900  subroutine is initiated by Start step  902 . In step  904 , a pin handle message is sent, then step  906  is termination of the subroutine. 
     Implementation: 
     This collector uses a two-color system to find immutable objects. At the beginning of a collection cycle, white objects are not known to be mutable or immutable. As the collector traverses the graph of mutable objects, objects are colored black as they are discovered. At the end of a cycle, handles that are still white are considered immutable and are scheduled to be collected. 
     Table II shows each collection cycle divided into three phases, i.e., Bleach, Scan and Sweep phases: 
     
       
         
               
             
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Collection Cycle Phases: 
               
             
          
           
               
                 Name 
                 Description 
               
               
                   
               
               
                 Bleach 
                 Actors prepare their handles for the new collection cycle by 
               
               
                   
                 coloring all handles that are not known to be uncollectable 
               
               
                   
                 white. Actors also color handles within the root, recently 
               
               
                   
                 pinned, and recently unpinned sets black. 
               
               
                 Scan 
                 Each actor prepares a new root set and beginning with this set 
               
               
                   
                 scans each handle for references to white handles and blackens 
               
               
                   
                 them. 
               
               
                 Sweep 
                 All mutable handles have been colored black. Any handle that is 
               
               
                   
                 still colored white is collected. 
               
               
                   
               
             
          
         
       
     
     Once the final phase (sweep) is complete, the collector begins with the first phase (bleach) of the next cycle. 
     The collector divides work between specialized processing units called actors, which communicate through message passing. An actor with a message in its queue is placed on the ready queue. When a thread wishes to devote a small amount of time to collection, it invokes the nom function, which pulls a single actor off of the ready queue and processes the message in the actor&#39;s queue. In this fashion, the act of collection can be performed incrementally, one message at a time if necessary. 
     The collector maintains a count of outstanding unprocessed messages during a given phase. When this counter reaches zero, the collector knows to send the message that starts the next phase. This counter must be shared between threads and must therefore be synchronized. The production and consumption of “pin” and “unpin” messages does not affect the value of this counter because they can be sent during any phase of the collection cycle. 
     Each actor is assigned a unique number that is used to bind handles to it. The collector binds a handle to a specific actor for the lifetime of the object so that all messages with respect to that handle will be dispatched to the same actor. This guarantees that operations on the same handle will be serialized without the use of handle-specific synchronization objects and permits the use of passive operations where messaging would otherwise be necessary. The collector uses a random number generator to assign affinity values to actors. The collector is able to determine which actor the handle is bound to using by performing a modulo operation on the affinity and given a sufficiently high-quality random number generator, the collector maintains a uniform distribution of handles across all of its actors, independent of mutator&#39;s memory allocation patterns. 
     Each actor maintains a reference color, which is a record of the most recent value used to represent the color white. This value is a single bit that alternates between 1 and 0 from collection cycle to collection cycle as part of the bleach phase. 
     Each actor maintains a subset of handles that the have been introduced into the collector. Within this subset, the actor maintains several subsets that categorize the collection state of the handle. First, there are two sets of handles that represent colored objects that do not have a special collection state. Three sets are used to track handles with a special state: the root set, the recently pinned set, and the recently unpinned set. The recently pinned and unpinned sets contain those handles that are transitioning into and out of the root set. A handle cannot belong to multiple sets and being individually bound to a specific actor, moves from set to set passively as related messages are processed and the handle&#39;s associated collection state changes. 
     Table III summarizes the messages that each actor understands and its associated response. All messages are asynchronous. Messages marked with an asterisk (*) do not require a handle argument and are sent to every actor in the collector. Messages that require a handle argument are dispatched to the actor the handle is bound to. Messages marked with a tick symbol (′) do not require the outstanding messages counter to be adjusted as they are produced and consumed. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Summary of the messages that each actor understands and its associated response: 
               
             
          
           
               
                 Name 
                 Description 
                 Response 
               
               
                   
               
               
                 Pin&#39; 
                 “Pin” a handle 
                 1. increment the reference counter associated with the 
               
               
                   
                   
                 handle. 
               
               
                   
                   
                 2. if the reference counter is now 1, color the handle 
               
               
                   
                   
                 black and move it into the recently pinned set. 
               
               
                 Unpin&#39; 
                 “Unpin” a handle 
                 1. decrement the reference counter associated with the 
               
               
                   
                   
                 handle. 
               
               
                   
                   
                 2. if the reference counter is now 0, color the handle 
               
               
                   
                   
                 black and move it into the recently unpinned set. 
               
               
                 Bleach* 
                 Initiates the bleach phase 
                 1. invert the reference color. 
               
               
                   
                   
                 2. color all handles in the recently pinned set black. 
               
               
                   
                   
                 3. color all handles in the recently unpinned set black. 
               
               
                   
                   
                 4. color all handles in the root set black. 
               
               
                 Scan* 
                 Initiates the scan phase 
                 1. color the handles in the recently unpinned set black. 
               
               
                   
                   
                 2. scan the recently unpinned set. 
               
               
                   
                   
                 3. move the handles in the recently unpinned set into 
               
               
                   
                   
                 the black set. 
               
               
                   
                   
                 4. move recently pinned objects into the root set. 
               
               
                   
                   
                 5. scan the root set. 
               
               
                 Blacken 
                 Color a handle black and 
                 1. drop the message if the handle is already colored 
               
               
                   
                 scan it. 
                 black. 
               
               
                   
                   
                 2. color the handle black. 
               
               
                   
                   
                 3. move the handle into the black set. 
               
               
                   
                   
                 4. scan the handle. 
               
               
                 Sweep* 
                 Initiates the sweep phase 
                 Collect all handles remaining in the white set. 
               
               
                 Quit* 
                 Cease collection 
                 Signal that message processing should cease. 
               
               
                   
               
             
          
         
       
     
       FIG. 10  is a flowchart showing “Unpin” function steps used to unpin a handle. “Unpin” function  1000  subroutine is initiated by Start step  1002 . In step  1004 , an unpin handle message is sent, then step  1006  is termination of the subroutine. 
       FIG. 11  is a flowchart showing “Identify” function steps used to identify a referenced handle. “Identify” function  1100  subroutine is initiated by Start step  1102 . In step  1104 , the message counter is incremented by 1. NOTE: Only one thread may access the collector&#39;s message counter at a time. In step  1106 , a “blacken handle” message is sent. Then step  1108  is termination of the subroutine. 
       FIG. 12  is a flowchart showing the process “Pin” message and response steps used to pin a handle as identified in Table III. “Process Pin Message” function  1200  subroutine is initiated by Start step  1202 . The handle&#39;s reference counter is incremented by 1 in step  1204 . In step  1206 , a query is made as to whether or not the reference count equals to 1. If the result of step  1206  is yes, it will proceed to step  1210  wherein the handle is colored black. Subsequently in step  1212 , the handle is moved into the pinned set. Then step  1208  is termination of the subroutine. If the result of step  1206  is no, it will proceed directly to step  1208  to terminate the subroutine. 
       FIG. 13  is a flowchart showing the process “Unpin” message and response steps used to unpin a handle as identified in Table III. “Process Unpin Message” function  1300  subroutine is initiated by Start step  1302 . In step  1304 , white is assigned as the reference color and black is assigned the not white color. Then in step  1306 , the handle&#39;s reference count is reduced by 1. In step  1308 , a query is made as to whether or not the reference count equals to 1. If the result of step  1308  is yes, it will proceed to step  1312  wherein the handle is colored black. Subsequently in step  1314 , the handle is moved into the unpinned set. Then step  1310  is termination of the subroutine. If the result of step  1308  is no, it will proceed directly to step  1310  to terminate the subroutine. 
       FIG. 14  is a flowchart showing the process “Bleach” message and response steps used to initiate the bleach phase as identified in Table III. “Process Bleach Message” function  1400  subroutine is initiated by Start step  1402 . In step  1404 , the reference color is inverted. NOTE: This has the effect of reversing the roles of the sets that hold black and white handles. The empty set that previously held white handles is now ready to hold black handles. The set that previously held black handles now holds white handles. Then in step  1406 , white is assigned as the reference color and black is assigned the not white color. Then in step  1408 , each handle in the pinned set is colored black. In step  1410 , each handle in the unpinned set is colored black. Subsequently, each handle in the root set is colored black in step  1412 . In step  1414 , the collector&#39;s message counter is reduced by 1. NOTE: Only one thread may access the collector&#39;s message counter at a time. Then step  1416  is termination of the subroutine.  FIGS. 15  A,  15 B and  15 C collectively show a flowchart showing the process “Scan” message and response steps used to initiate the scan phase as identified in Table III. As shown in  FIG. 15A , “Process Scan Message” function  1500  subroutine is initiated by Start step  1502 . In step  1504 , white is assigned as the reference color and black is assigned the not white color. The handle from the unpinned set is removed in step  1506 . Then the handle is colored black in step  1508 . In step  1510 , the handle is passed into the scanning hook. Then in step  1512 , the handle is added to the black set. In step  1514 , a query is made as to whether or not the unpinned set is empty yet. If the result of step  1514  is no, another handle will removed from the unpinned set in step  1516  and the subroutine will then be looped back to step  1508 . As best shown in  FIGS. 15A and 15B , if the result of step  1514  is yes, it will proceed to step  1518  wherein a handle from the pinned set is removed. Subsequently in step  1520 , the handle is added to the root set. As best shown in  FIG. 15B , then in step  1522 , a query is made as to whether or not the pinned set is empty yet. If the result of step  1522  is no, another handle from the pinned set is removed in step  1524  and the subroutine will then be looped back to step  1520 . If the result of step  1522  is yes, it will proceed to step  1526  wherein the first handle from the root set is retrieved. As best shown in  FIG. 15C , subsequently in step  1528 , the handle is colored black and then in step  1530  the handle is passed into the scanning hook. Then in step  1532 , a query is made as to whether or not there is another handle in the root set. If the result of step  1532  is yes, the next handle from the root set is retrieved in step  1534  and the subroutine will then be looped back to step  1528  wherein the handle is colored black. If the result of step  1532  is no, the collector&#39;s message counter is reduced by 1 in step  1536 . NOTE: Only one thread may access the collector&#39;s message counter at a time. Then step  1538  is termination of the subroutine. 
       FIG. 16  is a flowchart showing the process “Blacken” message and response steps used to color a handle black and scan it as identified in Table III. “Process Blacken Message” function  1600  subroutine is initiated by Start step  1602 . In step  1604 , white is assigned as the reference color and black is assigned the not white color. In step  1606 , a query is made as to whether or not the handle is colored white. If the result of step  1606  is yes, the handle is colored black in step  1608  and then the handle is moved into the black set in step  1610 . In step  1612 , the handle is passed into the scanning hook. Then the collector&#39;s message counter is reduced by 1 in step  1614 . NOTE: Only one thread may access the collector&#39;s message counter at a time. The subroutine will be then be terminated in step  1616 . However, if the result of step  1606  is no, it will proceed to directly to step  1614  wherein the collector&#39;s message counter is reduced by 1 and then the subroutine is terminated in step  1616 . 
       FIG. 17  is a flowchart showing the process “Sweep” message and response steps used to initiate the sweep phase as identified in Table III. “Process Sweep Message” function  1700  subroutine is initiated by Start step  1702 . In step  1704 , white is assigned as the reference color and black is assigned the not white color. Then in step  1706 , a handle is removed from the white set. Subsequently the handle is destroyed in step  1708 . In step  1710 , a query is made as to whether or not the white set is empty yet. If the result of step  1710  is no, another handle from the white set is removed in step  1712  and the subroutine is looped back to step  1708  wherein the handle is destroyed. If the result of step  1710  is yes, the collector&#39;s message counter is reduced by 1 in step  1714 . NOTE: Only one thread may access the collector&#39;s message counter at a time. The subroutine will be then be terminated in step  1716 . 
       FIG. 18  is a flowchart showing “Sample Collection Thread” function steps. It will be understood that in an implementation of the garbage collector  100  of the present invention, collection threads run in the background and collect garbage resources simultaneously with the operation of the application or computer program integrated with the collector  100 . “Sample Collection Thread” function  1800  subroutine is initiated by start step  1802 . In step  1804 , a query is made as to whether or not a quit message has been sent to an actor. If the result of step  1804  is no, it will proceed to step  1808  wherein the collector&#39;s nom function is invoked and the process is looped back to step  1804 . If the result of step  1804  is no, it will proceed directly to step  1806  to terminate the subroutine. 
       FIG. 19  is a flowchart showing “Sample Scan Function” function steps. “Sample Scan” function  1900  subroutine is initiated by start step  1902 . In step  1904 , a query is made as to whether or not the handle refers to a dictionary object. NOTE: The handle refers to a read-only string or number object, neither of which requires synchronization nor holds references to other handles. If the result of step  1904  is no, it will proceed directly to step  1914  to terminate the subroutine. If the result of step  1904  is yes, then step  1906  consists of retrieving the first key-value pair from the dictionary and proceeding to step  1908 . NOTE: Writers must be restricted from modifying the dictionary during a scan (with a mutex, for example). In step  1908 , the key is identified. Subsequently in step  1910 , the value is identified. Then in step  1912 , a query is made as to whether or not the there is another key-value pair in the dictionary. If the result of step  1912  is yes, it will loop the process back to step  1906  wherein the first key-value pair from the dictionary will be retrieve. If the result of step  1912  is no, it will proceed directly to step  1914  to terminate the subroutine. 
       FIGS. 20A ,  20 B,  20 C and  20 D collectively show a flowchart showing “Sample Mutator” function steps. It will be understood that the garbage collector  100  of the present invention can be configured for different thread topologies. As best shown in FIG.  20 A,“Sample Mutator” function  2000  subroutine is initiated by Start step  2002 . In step  2004 , the value of THREAD_COUNT is set to be equal to the number of background threads to use. Then in step  2006 , a new collector is created using the “Sample Scan Function”  1900  as best shown in  FIG. 19 . Then in step  2008 , a query is made as to whether or not the value of THEAD_COUNT is greater than zero. If the result of step  2008  is yes, i.e., if there are any threads to collect on, it will proceed to step  2010  wherein it will jump start the THREAD_COUNT collection by threads as best shown in  FIG. 18 . Again, collection by threads such as shown in  FIG. 18  is a process which is asynchronous with the running of the application or computer program within which the collector  100  is integrated. If the result of step  2010  is no, i.e., if the number of threads is ZERO, the collector  100  will skip step  2012  and proceed directly to step  2012  in which the value of root is made equal to the address of empty dictionary object. In step  2014 , root is introduced to the collector. In step  2016 , n is set to be a number object containing the value 1. Then in step  2018 , n is introduced to the collector. In step  2020 , m is set to be a number object containing the value  2 . Then it proceeds to step  2022  in  FIG. 20B . As shown in  FIG. 20B , in step  2022 , m is introduced to the collector. Subsequently, in step  2024 , k is set to be a number object containing the value 3. In step  2026 , k is introduced to the collector. In step  2028 , q is set to be a number object containing the value 4. In step  2030 , q is introduced to the collector. Then in step  2032 , one is set to be a string object containing the value one. In step  2034 , one is introduced to the collector. In step  2036 , two is set to be a string object containing the value two. In step  2038 , two is introduced to the collector. In step  2040 , d is set to be a an empty dictionary object. In step  2042 , d is introduced to the collector. In step  2044 , root [one] is set to be equal to n. Then in step  2046 , root [two] is set to be equal to d. In step  2048 , d[one] is set to be equal to n and in step  2050 , d[two] is set to be equal to root. Then in step  2052 , d[two] is set to be equal to +q. Then in proceed to step  2054  in  FIG. 20C . As shown in  FIG. 20C , in step  2054 , n is unpinned. In step  2056 , m is unpinned and in step  2058 , k is unpinned. In step  2060 , +q is unpinned and in step  2062 , one is unpinned. In step  2064 , two is unpinned. In step  2066 , d is unpinned. Subsequently in step  2068 , a query is made as to whether or not the value of THEAD_COUNT is equal to zero. If the result of step  2068  is yes, it will proceed to step  2070  in which the collector&#39;s nom function is invoked  100  times. Then it will proceed to step  2074 . If the result of step  2068  is no, it will proceed to step  2072  the process will be put to sleep for 10 seconds and then proceed to step  2074  also in which root [two] is erased. Subsequently it proceeds to step  2076  as best shown in  FIG. 20D , in step  2076  a query is made as to whether or not the value of THEAD_COUNT is equal to zero. If the result of step  2076  is yes, it will proceed to step  2078  in which the collector&#39;s nom function is invoked 100 times. Then it will proceed to step  2082 . If the result of step  2076  is no, it will proceed to step  2080  the process will be put to sleep for 10 seconds and then proceed to step  2082  also in which the collector&#39;s quit function is invoked. Then it will proceed directly to step  2084  to terminate the subroutine. 
       FIG. 21  is an overall block diagram  2100  showing the mutator  2110  and collector  2120  functions. As described above, the application or computer program  2112  that requires collection integrated with the collector  2120  is called the mutator  2110 , and the programmer is the person that integrates the mutator  2110  with the collector  2120 . The programmer decides the parameters of the scan function  2114  that scans the predetermined set of objects, as shown in either  FIG. 15  or  19 . The programmer also develops a threading strategy  2116 , prior art examples of which are shown in  FIG. 4 . 
     Furthermore, collector  2120  is comprised of a lowest level operating system layer called the system allocator  2122 . System allocator  2122  can be both allocate and free. It will further be understood that allocators in the diagram with manual memory management semantics can be labeled “manual” instead of “allocate and free”. In embodiments of the present invention, allocate function can both allocate memory to needed resources, as well as free up memory where no longer needed. Low-latency allocator  2124  can be built off the system allocator  2122  and can be considered to be an intermediate level in the operating system. At higher levels of operating, the facade provides a function call by which a programmer can submit asychronous messages to a dispatcher routine  2132 , which is where the correct actor and subroutine are selected to process the instructions called out by a programmer through the facade  2130 , and actors, each which has a state and are where instructions called out by a programmer through the facade  2130  are implemented and executed in parallel. 
     Thus, the mutator  2110  comprises not only application logic  2112  which needs to be collected periodically or as desired, but so also scan function  2114  and threading strategy  2116 , whereby integration of the application  2112  with the collector  2120  becomes possible. 
     It will be understood, as stated above, that the collector of the present invention also intends to satisfy real-time constraints of parallelized, incremental garbage collection. Optimization of the collector  100  to make it friendly to real-time collection processing could require changes or modifications, which changes and modifications are expressly included within the scope of the present invention. As an example, during the “bleach phase”, a goal is to clean up objects and make them safe for reclamation. An embodiment of the present invention sends a message, reclaims each reclaimable object sequentially and in linear fashion. Furthermore, to optimize the collector to operate in real-time and eliminate the purely linear response, minor changes and/or modifications to the algorithm which are trivial and would be obvious to one skilled in the art would render the algorithm associated with the collector  100  of the present invention friendly to real-time as well as “soft” real-time processing. 
     As stated above, the collector of the present invention can be integrated into a language or virtual machine that uses garbage collection, for example JAVA, NET, or a CPU emulator. The collector of the present invention can be integrated through the interpreter or virtual machine or integrated directly into the program generated by a compiler. 
     Because this collector  100  algorithm is based on asynchronous messaging, which can be passed over the network, it can be applied to a distributed system with shared resources. The manner in which the resources, i.e., handles, are bound to actors is particularly suited for this because the algorithm does not require adding a load balancing algorithm, i.e., an algorithm that moves resources around to eliminate unbalanced load on the system. It will be understood that such load-balancing algorithm would increase complexity and be performance prohibitive in distributed systems. In short, the algorithm can be applied to a distributed computer program without much modification. Databases would fall into this category. 
     As discussed, garbage collectors can be adapted to improve the performance of programs that were not designed to be used with a collector. Traditionally, this has applied to a small subset of programs due to the unsuitability of collectors for parallelism, large heaps, and real-time constraints. The garbage collector  100  of the present invention can be adapted to improve the reliability of a separate program that was not designed to be used with a collector. Products like this have existed in the past, but are of limited use for multithreaded applications because of the performance cost, which the present algorithm eliminates. 
     Thus, it will be understood that garbage collectors can be adapted to detect memory leaks in a separate program that was not designed to be used with a collector. Several prior art products, such as IBM Rational&#39;s Purify (trademark) use this technique. However, the lack of a parallel, incremental collector degrades performance to the point where these tools are less useful than they could be if the parallelism didn&#39;t have to be serialized. 
     It will be understood that an aspect of the invention, such as in a prototype or for use in developing and operating the algorithm, is the logger. It is not part of the collector algorithm, however in an embodiment it is the tool which can be used to diagnose flaws in the algorithm. The logger is a debugging tool which makes a record or log of what the collector did. The logger can be synchronized or non-synchronized with the computer program and/or the collector. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Although any methods and materials similar or equivalent to those described can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications and patent documents referenced in the present invention are incorporated herein by reference. 
     While the principles of the invention have been made clear in illustrative embodiments, there will be immediately obvious to those skilled in the art many modifications of structure, arrangement, proportions, the elements, materials, and components used in the practice of the invention, and otherwise, which are particularly adapted to specific environments and operative requirements without departing from those principles. The appended claims are intended to cover and embrace any and all such modifications, with the limits only of the true purview, spirit and scope of the invention.