Abstract:
A method of reclaiming memory space allocated to a data structure comprising data objects ( 910-960 ) linked by identifying pointers, in which the memory allocated to data objects is reclaimed using two systems: a first system ( 980 ), by which a selected part of the data structure is traversed by following the pointers, one of at least two identifiers being allocated to the data objects, a first identifier which indicates that the data object has been traversed so that the data objects referenced by the pointers of that data object have been identified, and a second identifier which indicates that the data object is referenced by a pointer, but the data object has not yet been traversed; and a second system ( 990 ), by which an individual data object is selected for deletion to enable the associated memory space to be reclaimed. The second system ( 990 ) reads the first system identifier for the individual data object, and if the first identifier is present deletes the data object thereby reclaiming the associated memory space. If the second identifier is present, it allocates a third identifier, where the first system ( 980 ) operates to reclaim the memory space allocated to data objects having the third identifier.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a memory reclamation method, in particular one in which conflicting deletion attempts for a stored data object may be made. 
     Garbage collection is the automated reclamation of system memory space after its last use by a programme. A number of examples of garbage collecting techniques are discussed in “Garbage Collection- Algorithms for Automatic Dynamic Memory Management” by R. Jones et al, pub. John Wiley &amp; Sons 1996, ISBN 0-471-94148-4, at pages 1 to 18, and “Uniprocessor Garbage Collection Techniques” by P. R. Wilson, Proceedings of the 1992 International Workshop on Memory Management, St. Malo, France, September 1992. Whilst the storage requirements of many computer programs are simple and predictable, with memory allocation and recovery being handled by the programmer or a compiler, there is a trend toward functional languages having more complex patterns of execution such that the lifetimes of particular data structures can no longer be determined prior to run-time and hence automated reclamation of this storage, as the program runs, is essential. 
     A common feature of a number of garbage collection reclamation techniques, as described in the above-mentioned Wilson reference, is incrementally traversing the data structure formed by referencing pointers carried by separately stored data objects. The technique involves first marking all stored objects that are still reachable by other stored objects or from external locations by tracing a path or paths through the pointers linking data objects. 
     This may be followed by sweeping or compacting the memory—that is to say examining every object stored in the memory to determine the unmarked objects whose space may then be reclaimed. 
     Normally, the garbage collection and reclamation process runs on the computer in parallel to a program process, the garbage collection and reclamation process operating on the heap (memory area) occupied by data objects of the program process, so that garbage from the program process can be detected as soon as possible and the appropriate resources reclaimed. A result of the two processes running in parallel to each other and operating the same memory area is that they could both be operating on the same data objects at the same time. In the event of only a single processing thread being available to the two processes, steps from the garbage collection and reclamation process are interleaved with steps of the program process. A single step of the garbage collection and reclamation process may not necessarily completely process a data object and those data objects that it has pointers referenced to. 
     Many computer programming languages offer functions for manual reclamation of memory used by data objects. As opposed to automatic garbage collection, to manually reclaim memory used by a data object a programmer must explicitly execute an appropriate function from within the program that created the data object. Indeed, a popular feature of Object-Oriented programming languages such as C++ is the ability to define a destructor method for an object class. A destructor method is a function written by a programmer specifically for an object class. The default operation performed by a destructor method is to delete the object it is associated with from the heap. The programmer can include calls to other functions from within a destructor method so that when the destructor method is executed for a data object created from the object class, resources held by the data object can be reclaimed, such as file handles and other objects referenced by pointers from the object, before the data object is automatically deleted from the computer&#39;s memory heap. This method is particularly popular for reclamation of linked list and tree data structures where destructors for each object in the linked list can be recursively called until the end of the list is reached, thereby reclaiming the whole linked list from a single destructor method call. In this manner, a programmer, knowing that a program has finished with a data object, could execute the destructor method from the program so that the object and its resources can be reclaimed in an orderly fashion as soon as the object is finished with. 
     It will be apparent, however, that such manual memory reclamation methods conflict with the garbage collection and reclamation methods described above. If a garbage collection and reclamation process is part way through processing a series of linked data objects, when the program process manually reclaims the resources held by the data objects, the next time the garbage collection and reclamation process resumes, it will attempt to process a data object that no longer exists resulting in an error state being generated and potentially a system failure. 
     In Java (® Sun Microsystems Inc.) virtual machines there is no provision for manual memory reclamation methods. All memory reclamation must be performed by an automatic garbage collector. However, as Java (® Sun Microsystems Inc.) supports multiple processing and different garbage collection mechanisms have different strengths and weaknesses (for example reference counting can quickly identify 0-referenced data objects as garbage but cannot identify cyclic data structures as garbage, whilst the mark-sweep algorithm can identify most types of garbage but takes much longer to identify it), it is desirable that a number of automatic garbage collectors operate concurrently on the heap occupied by data objects. In order for garbage collectors to operate concurrently they must have some conflict resolution mechanism in the event that one garbage collector tries to reclaim a data object that another garbage collector is currently processing. 
     Before arriving at the solution of the present invention, a number of other possible solutions were explored. Data objects that are currently being processed by the garbage collection and reclamation process can be marked such that before a manual reclamation operation, or the data object being processed by another garbage collection and reclamation process, a mark is checked for and the operation can be aborted if the data object is found to be marked. However, by aborting the operation, reclamation of the resources held by the data object becomes reliant on the garbage collection and reclamation process recognising them as garbage and reclaiming them in the future—this reclamation may be at some indefinite time in the future or, if the program process is permanently running and keeps a valid pointer referencing the data object, the data object may never be recognised as garbage and would therefore never be reclaimed. 
     Alternatively the garbage collection and reclamation process could place a mutual exclusion lock (by a method such as semaphores) on the data object(s) it is processing. Such a lock, however, prevents the program process from accessing and manipulating the data objects and would at least halt the execution of the program process at the point it requires access to the data object(s) until the lock(s) are released and could cause the program process to fail. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method of reclaiming memory space allocated to a data structure comprising data objects linked by identifying pointers, in which the memory allocated to data objects is reclaimed using two systems: 
     a first system, by which a selected part of the data structure is traversed by following the pointers, one of at least two identifiers being allocated to the data objects, a first identifier which indicates that the data object has been traversed so that the data objects referenced by the pointers of that data object have been identified, and a second identifier which indicates that the data object is referenced by a pointer, but the data object has not yet been traversed; and 
     a second system, by which an individual data object is selected for deletion to enable the associated memory space to be reclaimed, 
     wherein the second system reads the first system identifier for the individual data object, and if the first identifier is present deletes the data object thereby reclaiming the associated memory space, and if the second identifier is present, allocates a third identifier, and wherein the first system operates to reclaim the memory space allocated to data objects having the third identifier. 
     An advantage of the present invention is that the second system does not delete the data object if the first system has not finished traversing it, but adds a marker so that deletion will only take place when the first system is ready. 
     Preferably, the first system comprises an automatic garbage collection system. The invention thus eliminates the conflict between automatic garbage collection and manual memory reclamation. Preferably, the first system also operates to reclaim the memory space allocated to data objects having no identifier. The second system may be the manual deletion of data objects. This may be in the form of the deletion of data objects by a creating program of the objects when the objects are no longer needed. Alternatively and preferably, the second system comprises an automatic garbage collection system. The invention thus eliminates the conflict between the two automatic garbage collection systems. Preferably, the second system also reclaims memory space allocated to data objects referenced by pointers from the data object having the third identifier. 
     The invention also provides a data processing apparatus for implementing the method of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a data processing system suitable to embody the present invention; 
     FIGS. 2 to  4  represent a group of partially linked data objects as tracing proceeds, using a tricolour marking scheme; 
     FIG. 5 represents the group of partially linked data objects of FIG. 3 including a data object for which a deletion attempt conflicting with a garbage collection process has been made; 
     FIGS. 6 and 7 represents a data object for which a deletion is processed by a method of reclaiming memory space embodying the present invention; 
     FIG. 8 is a flow chart describing features of a method of reclaiming memory space embodying the present invention; and 
     FIG. 9 represents a group of partially linked data objects processed by a program, a first garbage collection process and a second garbage collection process embodying the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 represents a data processing system, such as a personal computer, which acts as host for a number of software utilities which may, for example, configure the system as a browser for data defining a virtual environment. The system comprises a central processing unit (CPU)  10  coupled via an address and data bus  12  to random-access (RAM) and read-only (ROM) memories  14 ,  16 . These memories may be comprised of one or several integrated circuit devices and may be augmented by a system hard-disk as well as means to read from additional (removable) memory devices, such as a CD-ROM. The present invention is particularly embodied in memory management for a working area of the RAM  14  under control of the CPU  10 ; a controlling program for this may initially be held in ROM  16  and loaded up with the operating system on power-up. Also coupled to the CPU  10  via bus  12  are first and second user input devices  18 ,  20  which may suitably comprise a keyboard and a cursor control and selection device such as a mouse or trackball. Audio output from the system is via one or more speakers  22  driven by an audio processing stage  24 . Video output from the system is presented on display screen  26  driven by display driver stage  28  under control of the CPU  10 . A further source of data for the system is via online link to remote sites, for example via the Internet, to which end the system is provided with a network interface  30  coupled to the bus  12 . 
     FIGS. 2 to  4  schematically illustrate the basic principles of tracing in incremental garbage collecting using a tricolour marking scheme, the terminology of which will be used in the following examples. A particular feature of incremental collectors is that the procedure is not carried out contiguously but in discrete steps interspersed with periods of program execution to avoid introducing lengthy breaks to the execution. The tricolour scheme is a convenient method to describe the process of marking as it spreads through the “network” formed by memory objects linked by pointers. In the scheme, objects which have been reached and passed by the traversal process (i.e. the traversal has reached all linked immediate descendants) are coloured black; those objects reached by the traversal whose actual or potential descendants have not been reached are coloured grey, and all other objects are coloured white. 
     In the partially-linked collection of objects of FIGS. 2 to  4 , FIG. 2 shows the status after the first incremental stage of the tracing procedure starting from object  35  and spreading outwards/downwards, with object  35  marked black and the three objects  36 , 38 , 39  to which it carries pointers being marked grey. FIG. 3 represents the status after a further incremental stage with the descendant objects  40 , 41 , 42 , 44 , 45  of objects  36 , 38 , 39  having been reached by the traversal and accordingly marked grey. As all their descendants have been reached, objects  36 , 38 , 39  are marked black at this stage. FIG. 4 represents the final stage at which it has been determined that there are no remaining linked objects, and objects  40 , 41 , 42 , 44 , 45  are marked black. This concludes the mark phase and shows that there are two “unreachable” objects  37 , 43  to which there are no pointers. These objects are still marked white, indicating them to be garbage and thus available for deletion. 
     The tricolour marking scheme is discussed in greater detail in the previously cited Wilson reference including various techniques for handling the incremental nature of the procedure, where the layout of the data structure may change during traversal, with the interspersed segments of program execution shifting the arrangement of placed pointers. One particular point to note is that the hierarchical layout of objects in FIGS. 2 to  4  is purely for the purposes of illustration: in actuality, the objects may be spread throughout the available memory with the hierarchy only determined by the arrangement of linking pointers. 
     The problem of conflicting deletion attempts is explained with reference to FIG. 5 which represents the group of partially linked data objects of FIG.  3 . The incremental garbage collector has processed data objects  35 ,  36 ,  38  and  39  and has references to data objects  40 ,  41 ,  42 ,  44  and  45  which are due to be processed when the garbage collector next resumes. However, between the garbage collector processing data object  38  and data object  42 , a process external to the garbage collector executes a destructor operation for data object  38 , which in turn executes a destructor operation for data object  42 . When the garbage collector subsequently resumes and follows its reference to what it expects to be a data object it may find no data object at all in which case an error condition is generated, alternatively the process external to the garbage collector may have created another data object in the memory area previously occupied by data object  42 , in which case the garbage collector will incorrectly believe that the new data object is linked to data object  38  and in turn to data object  35 . 
     The method of reclaiming memory space embodying the present invention is explained with reference to FIG. 6. A program has created in the heap data object  50  linked to descendent data object  52  which is in turn linked to descendent data object  54  which is linked to data object  56 . The program uses the data objects  50 - 56  to store its data and subsequently manipulates the objects and stored data during its execution. In this example the garbage collector has processed data objects  50  and  52  and is due to process data object  54  when it next resumes. During its period of execution, the program determines that data object  52  and its descendants are no longer needed and executes the destructor method for data object  52  which has been written to recursively call the destructor for each of its descendants. To the extent described above this is conventional in the art. 
     The present invention effectively modifies known manual memory reclamation operations, such as destructor methods, to make them compatible with automatic garbage collection. The modified destructor method determines the marking status of data object  52  from a data field of the object. On finding the status to be black, the destructor recursively calls the destructor for each descendant data object, in this case data object  54 . The destructor for data object  54  determines its marking status to be grey, indicating a conflict between the destructor and the garbage collector. The destructor for data object  54  therefore, instead of deleting the data object  54 , changes the marking status of the data object  54  to red, indicating a request to delete the object. The destructor for data object  54  then recursively calls the destructor for data object  56  which, determining the marking status of data object  56  to be white, deletes the data object  56  and returns to the destructor for data object  54 . This destructor in turn ends its execution returning to the destructor for data object  52  which ends by deleting data object  52  from the heap. If all the data objects had a black or white marking status, there would have been no conflict and the data objects could have all been recursively deleted. Instead, data object  54  remains, with a red marking status. 
     The garbage collector resumes its marking of the data objects shown in FIG.  7 . When processing each referenced data object, its respective marking status is checked. Upon processing data object  54 , the garbage collector finds its status to be red. In the same way as the status of data objects marked grey are changed to black when their descendent data objects are processed, data objects marked red are changed to white. This is performed as an atomic test-and-set operation (if (state=red) then state=white). The data object is then reclaimed with other garbage during the reclamation phase of the garbage collection. 
     In a preferred embodiment of the present invention, any red status objects processed by the garbage collector during the marking phase are immediately deleted by the garbage collector without waiting for the reclamation phase. 
     The operation of destructors or other manual reclamation functions embodying features of the present invention will depend on the structures of data objects they are written for reclaiming. For example, if a data object is the descendant of two parent data objects (such as where the data objects are linked in a cyclic structure), only one of which is deleted, or requested to be deleted, it may be desired that the data object is not deleted in which case the manual reclamation function for the parent data objects would be written without code recursively calling for the manual reclamation of the descendant data object. Indeed, it may be preferable that only the highest level data object is deleted or marked red, indicating a request for deletion, so that the automatic garbage collector recognises the orphaned descendants as garbage on its next mark phase through the memory heap. 
     In a further example of manual reclamation embodying features of the present invention, when a data object is marked red indicating a deletion request conflicting with garbage collection, the red mark is propagated down the tree or list to any other descendants of the red marked data object. 
     FIG. 8 is a flowchart describing features of the method of the present invention. In step  800 , a program that has created a number of data objects in the form of data structures on the heap, some of which it no longer uses, is interrogated to obtain a reference to each top level (root) data object for data structures it is currently using. These references are stored in a pool. As will be understood from the following description, the pool is used to store data objects being processed by the garbage collector—references are added to and removed from the pool as the garbage collector traverses the data structures. During the processing according to the method of the present invention, references to descendant data objects will be added to the pool and, as each data object is completely processed its respective reference is removed from the pool. The marking status of all data objects in the heap is made white in step  810 . For each data object referenced in the pool, if its marking status is found to be white, it is changed to grey in step  820  and a reference to each, if any, descendent data object is added to the pool in step  830 . The reference to the data object is kept in the pool so that it is subsequently processed again. If the marking status of a data object is found to be grey, it is changed to black in step  840  and the reference to that data object is removed from the pool in step  850 . If the marking status of a data object is found to be red, it is changed to white in step  860  and the reference to the data object is removed from the pool in step  870 . As discussed previously with reference to FIGS. 6 and 7, an object can only be assigned a red marking status by a manual deletion operation conflicting with a grey marking status. The change of marking status from grey to red is performed during the manual deletion operation. 
     In this manner, once all references to data objects in the pool are exhausted, all data objects in the heap will have a marking status of black, if they are linked to a root data object held by the program and have not been requested to be deleted, or white otherwise. The subsequent reclamation phase of the garbage collection may then reclaim memory held by data objects having a white marking status by a method such as sweeping, compaction etc. 
     It will be apparent to the skilled person that the deletion request mechanism of the present invention may be implemented in a number of ways dependent on the programming language and compiler used. 
     For those Object-Oriented languages permitting the use of destructor methods, the deletion of a data object is automatic once its destructor method has been called. In order to implement the present invention in this case, the compiler could be modified so that destructor methods could be aborted if a red marking of an object is found. Alternatively, it may be permitted to include a finaliser method within destructors able to resurrect data objects in which case the finaliser can resurrect the data object if a red marking of that data object is found. 
     For less sophisticated languages, allowing delete or de-allocation functions to be used on data objects, the programmer could write an appropriate function which checks the marking status of a data object before deleting or de-allocating it. 
     Other operations performed by deletion functions or destructor methods, such as recursive deletion of lists or trees would be dependent on the programmer calling the appropriate operations from the respective function. 
     FIG. 9 represents a group of partially linked data objects processed by a program, a first garbage collection process and a second garbage collection process embodying the present invention. A Java (® Sun Microsystems Inc.) virtual machine has a program process  900  running in a first thread. The program process  900  has created a number of data objects  910 - 960  on the heap  905 . Some of the data objects are linked together by pointers to form tree  915 - 935  and list  950 - 960  data structures. The program process accesses the data objects via root data objects  910 ,  915  and  970 . Those data objects not reachable from the root data objects, such as objects  940  and  950 - 960  are no longer accessible to the program process and are therefore garbage. A first garbage collection process  980 , running in a second thread, is operating on the heap. The garbage collection process  980  uses a reference-count algorithm to detect and delete unreferenced data objects. A second garbage collection process  990 , running in a third thread, is also operating on the heap. The garbage collection process  990  uses a mark-sweep algorithm, as has been previously described with reference to FIG. 8 to detect and reclaim garbage. On processing the heap, the first garbage collection process  980  detects and attempts to delete unreferenced data object  940 . However, the second garbage collection process  990  is currently processing this object (and has therefore marked it grey). The first garbage collection process detects the grey marking of the data object and, instead of deleting it, changes the marking status to red. The second garbage collection process  990  then continues, as has been described with reference to FIG. 8 whilst the first garbage collection process  980  would delete the red marked data object when it resumes, as has been previously described. 
     Although defined principally in terms of a software implementation, the skilled reader will be well aware that the above-described functional features could equally well be implemented in hardware, or in a combination of software and hardware. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of data processing and storage apparatus and devices and component parts thereof and which may be used instead of or in addition to features already described herein.