Patent Publication Number: US-9418005-B2

Title: Managing garbage collection in a data processing system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of patent application U.S. Ser. No. 12/173,107, filed Jul. 15, 2008, now U.S. Pat. No. 8,286,134 entitled: Call Stack Sampling for a Multi-Processor System, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to an improved data processing system and in particular to a method and apparatus for processing data. Still more particularly, the present disclosure relates to a computer implemented method, apparatus, and computer program code for managing garbage collection in a data processing system. 
     2. Description of the Related Art 
     Garbage collection is a form of automatic memory management. A garbage collector or other process attempts to reclaim memory used by objects in the memory that will never be accessed or modified by an application or other process. 
     Garbage collection frees a programmer from having to worry about releasing objects that are no longer needed when writing applications. Further, garbage collection also may aid programmers in their efforts to make programs more stable because garbage collection may prevent various runtime errors that could occur. Examples of errors that may occur include, for example, dangling pointer bugs and double free bugs. 
     A dangling pointer bug may occur when a piece of memory is freed while pointers are still pointing to the memory and one of the pointers is used. A double free bug may occur when a region of memory is free and an attempt is made by a program to free that region of memory. Also, memory leaks that occur when a program fails to free memory that is no longer accessed also may be reduced and/or eliminated through garbage collection. 
     Garbage collection may be used in various environments including in a Java Virtual Machine (JVM). Garbage collection also may be available with other environments including, for example, C and C++. 
     BRIEF SUMMARY OF THE INVENTION 
     The different illustrative embodiments provide a computer implemented method, apparatus, and computer program product for managing garbage collection. Monitoring is performed for a garbage collection state in a virtual machine. Responsive to detecting the garbage collection state, a priority for a set of garbage collection threads is increased. 
     In another illustrative embodiment, a computer comprises a bus; a storage device connected to the bus, wherein program code is stored on the storage device; and a processor unit is connected to the bus. The processor unit executes the program code to monitor for a garbage collection state within an execution environment, and increase a priority of a set of garbage collection threads in response to detecting the garbage collection state. 
     In still another illustrative embodiment, computer program product for managing garbage collection comprising a computer recordable storage medium and program code stored on the computer recordable storage medium. Program code is present for monitoring for a garbage collection state within an execution environment. Program code is also present, responsive to detecting the garbage collection state, for increasing a priority of a set of garbage collection threads. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram of a data processing system in accordance with an illustrative embodiment; 
         FIG. 2  is a diagram illustrating components that may be used to manage garbage collection in accordance with an illustrative embodiment; 
         FIG. 3  is a diagram of components used in garbage collection in accordance with an advantageous embodiment; 
         FIG. 4  is a flowchart of a process that may be initiated for other processing when garbage collection is occurring in accordance with an illustrative embodiment; and 
         FIG. 5  is a flowchart of a process for increasing garbage collection performance in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As will be appreciated by one skilled in the art, the present invention may be embodied as a system, method, or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.), or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module,” or “system.” Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium. 
     Any combination of one or more computer usable or computer readable medium(s) may be utilized. The computer usable or computer readable medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CDROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. 
     Note that the computer usable or computer readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer usable or computer readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer usable medium may include a propagated data signal with the computer usable program code embodied therewith, either in baseband or as part of a carrier wave. The computer usable program code may be transmitted using any appropriate medium, including, but not limited to, wireless, wireline, optical fiber cable, RF, etc. 
     Computer program code for carrying out operations of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as, for example, Java, Smalltalk, C++, or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer, or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     The present invention is described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. 
     These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer program instructions may also be stored in a computer readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     Turning now to  FIG. 1 , a diagram of a data processing system is depicted in accordance with an illustrative embodiment. In this illustrative example, data processing system  100  includes communications fabric  102 , which provides communications between processor unit  104 , memory  106 , persistent storage  108 , communications unit  110 , input/output (I/O) unit  112 , and display  114 . 
     Processor unit  104  serves to execute instructions for software that may be loaded into memory  106 . Processor unit  104  may be a number of processors, depending on the particular implementation. A number of items, as used herein, refers to one or more items. For example, a number of processors is one or more processors. These processors may be separate chips or may be cores within a multi-processor core. In other words, a processor may be a processor such as a central processing unit and/or a core within a multi-core central processing unit. Further, processor unit  104  may be implemented using one or more heterogeneous processor systems in which a main processor is present with secondary processors on a single chip. As another illustrative example, processor unit  104  may be a symmetric multi-processor system containing multiple processors of the same type. 
     Memory  106  and persistent storage  108  are examples of storage devices. A storage device is any piece of hardware that is capable of storing information either on a temporary basis and/or a permanent basis. Memory  106 , in these examples, may be, for example, a random access memory or any other suitable volatile or non-volatile storage device. Persistent storage  108  may take various forms depending on the particular implementation. For example, persistent storage  108  may contain one or more components or devices. For example, persistent storage  108  may be a hard drive, a flash memory, a rewritable optical disk, a rewritable magnetic tape, or some combination of the above. The media used by persistent storage  108  also may be removable. For example, a removable hard drive may be used for persistent storage  108 . 
     Communications unit  110 , in these examples, provides for communications with other data processing systems or devices. In these examples, communications unit  110  is a network interface card. Communications unit  110  may provide communications through the use of either or both physical and wireless communications links. 
     Input/output unit  112  allows for input and output of data with other devices that may be connected to data processing system  100 . For example, input/output unit  112  may provide a connection for user input through a keyboard and mouse. Further, input/output unit  112  may send output to a printer. Display  114  provides a mechanism to display information to a user. 
     Instructions for the operating system and applications or programs are located on persistent storage  108 . These instructions may be loaded into memory  106  for execution by processor unit  104 . The processes of the different embodiments may be performed by processor unit  104  using computer implemented instructions, which may be located in a memory, such as memory  106 . These instructions are referred to as program code, computer usable program code, or computer readable program code that may be read and executed by a processor in processor unit  104 . The program code in the different embodiments may be embodied on different physical or tangible computer readable media, such as memory  106  or persistent storage  108 . 
     Program code  116  is located in a functional form on computer readable media  118  that is selectively removable and may be loaded onto or transferred to data processing system  100  for execution by processor unit  104 . Program code  116  and computer readable media  118  form computer program product  120  in these examples. In one example, computer readable media  118  may be in a tangible form, such as, for example, an optical or magnetic disc that is inserted or placed into a drive or other device that is part of persistent storage  108  for transfer onto a storage device, such as a hard drive that is part of persistent storage  108 . In a tangible form, computer readable media  118  also may take the form of a persistent storage, such as a hard drive, a thumb drive, or a flash memory that is connected to data processing system  100 . The tangible form of computer readable media  118  is also referred to as computer recordable storage media. In some instances, computer readable media  118  may not be removable. 
     Alternatively, program code  116  may be transferred to data processing system  100  from computer readable media  118  through a communications link to communications unit  110  and/or through a connection to input/output unit  112 . The communications link and/or the connection may be physical or wireless in the illustrative examples. The computer readable media also may take the form of non-tangible media, such as communications links or wireless transmissions containing the program code. 
     The different components illustrated for data processing system  100  are not meant to provide architectural limitations to the manner in which different embodiments may be implemented. The different illustrative embodiments may be implemented in a data processing system including components in addition to or in place of those illustrated for data processing system  100 . Other components shown in  FIG. 1  can be varied from the illustrative examples shown. 
     As one example, a storage device in data processing system  100  is any hardware apparatus that may store data. Memory  106 , persistent storage  108  and computer readable media  118  are examples of storage devices in a tangible form. 
     In another example, a bus system may be used to implement communications fabric  102  and may be comprised of one or more buses, such as a system bus or an input/output bus. Of course, the bus system may be implemented using any suitable type of architecture that provides for a transfer of data between different components or devices attached to the bus system. Additionally, a communications unit may include one or more devices used to transmit and receive data, such as a modem or a network adapter. Further, a memory may be, for example, memory  106  or a cache such as found in an interface and memory controller hub that may be present in communications fabric  102 . 
     With reference to  FIG. 2 , a diagram illustrating components to initiate garbage collection management is depicted in accordance with an illustrative embodiment. In the depicted example, the components are examples of hardware and software components found in the data processing system, such as data processing system  100  in  FIG. 1 . 
     These components include processor unit  200 , operating system  202 , virtual machine  204 , device driver  206 , deferred procedure call handler  208 , profiler  210 , threads  212 , sampling threads  214 , device driver work area  216 , and data area  218 . 
     Processor unit  200  is similar to processor unit  104  in  FIG. 1  and may generate interrupts, such as interrupts  220  and  222  from processors within processor unit  200 . These interrupts may be, for example, without limitation, timer interrupts. 
     In particular, interrupt  220  and interrupt  222  may be generated based on timed interrupts that may be initiated for all of the processors within processor unit  200 . In these examples, this type of interrupt may be generated using an advanced programmable interrupt controller within each processor and processor unit  200 . Processing of interrupt  220  and interrupt  222  may initiate garbage collection management processes in these illustrative embodiments. 
     Alternatively, virtual machine  204  initiates garbage collection by detecting that the amount of objects in heap  223 , a work area for objects in object  219 , exceeds a threshold. In one embodiment, virtual machine  204  notifies profiler  210  that it has started garbage collection for heap  223 . Profiler  210  notifies device driver  206  that virtual machine  204  has entered the garbage collection state. In this embodiment, virtual machine  204  notifies profiler  210  that it has completed garbage collection. Profiler  210  then notifies device driver  206  that virtual machine  204  has completed the garbage collection and is no longer in a garbage collection state. 
     Profiler  210  may use interfaces to virtual machine  204  to identify garbage collection threads  221 , in threads  212 , and operating system interfaces for operating system  202  to change the priorities of garbage collection threads  221  instead of requiring support from device driver  206 . In another embodiment, virtual machine  204  uses operating system interfaces to control the priorities of garbage collection threads  221 . 
     The interrupts may be passed to device driver  206  in a number of different ways. For example, interrupt  220  is passed to device driver  206  through call  224 . Alternatively, interrupt  222  is passed directly to device driver  206  via an Interrupt Vector Table (IVT). After receiving an interrupt, device driver  206  may process the interrupt using a deferred procedure call (DPC) to deferred procedure call handler  208  located within device driver  206 . Of course, other routines or processes may be used to process these interrupts. The deferred procedure call initiated by device driver  206  is used to continue processing interrupt information from interrupt  222 . Device driver  206  determines the interrupted process and thread and may use this information by applying policy  228 . 
     This determination may be made using policy  228 . Policy  228  may be a set of rules identifying what actions to take. The rules may include identification of one or more garbage collection thread identifications used to identify an interrupted thread as a garbage collection thread, or checking an indication of garbage collection mode set by virtual machine  204  or profiler  210 . In response to determining the garbage collection state, other actions may be taken. These actions may include changing a priority of garbage collection threads. Whether changes to the priority of garbage collection threads occur may depend on what threads are currently executing, what memory ranges are being accessed, what processes are executing, and/or some other suitable criteria. 
     In these examples, the interrupt handler  229  may identify the address interrupted or the data address being accessed at the time of interrupt  222 . For example, a user may identify a set of routines of interest. Profiler  210  may identify the address ranges for a set of routines by obtaining loaded module information or by monitoring addresses of JITed methods to form address ranges  227 . Profiler  210  passes address ranges  227  to device driver  206 , which places address ranges  227  into device driver work area  216 . 
     In a similar manner, a user may specify a specific object class or object instance meeting specific criteria or a data area referenced by a lock or monitor using profiler  210 . Profiler  210  may obtain the data information area from virtual machine  204  and pass this information to device driver  206 . In turn, device driver  206  places this information into device driver work area  216  as address ranges  227 . In this manner, the interrupt handler may compare the identified address with the set of address ranges stored in device driver work area  216 . 
     Deferred procedure call handler may decide that garbage collection management processes are to be executed using policy  228 . For example, policy  228  may state that priority for garbage collection threads should be increased if a garbage collection state is present when interrupt  222  is received. In other illustrative embodiments, policy  228  may include rules stating that garbage collection priority may change if a particular thread is being executed, a particular address range is being accessed, and/or when some other suitable criteria is present. 
     With reference now to  FIG. 3 , a diagram of components used in managing garbage collection is depicted in accordance with an advantageous embodiment. In this example, garbage collection environment  300  is an example of an environment in which garbage collection may be managed when a condition occurs in which garbage collection state is present. This garbage collection management may be performed for execution environment  301 . Execution environment  301  is any environment in which the execution of code may result in the use and/or creation of objects containing data. In this example, execution environment  301  includes virtual machine  302 , which includes heap  304 . 
     Heap  304  contains objects  306 . These objects may be allocated during the execution of threads  308 . Threads  308  may access objects  306 . When a thread within threads  308  accesses an object within objects  306 , a lock is obtained for that object from locks  310 . This lock prevents other threads from accessing the same object. Once the thread releases the lock for the object, then that object may be accessed by another thread. Of course, in other illustrative embodiments, execution environment  301  may include environments other than virtual machines. 
     For example, execution environment  301  may be any environment in which threads execute and use memory that requires periodic garbage collection as part of the supported infrastructure, typically when there are data structures that are no longer being used or can be reallocated if needed. When garbage collection is supported, any threads within threads  308  that need to allocate objects must release their locks and wait for garbage collection threads  312  to acquire and release locks from locks  310 . 
     During the phase of acquiring ownership of locks  310  by garbage collection threads  312 , it is advantageous for any of threads  308  currently owning at the lock within locks  310  to complete processing as quickly as possible to allow garbage collection threads  312  to acquire locks  310  and begin processing of heap  304 . Once garbage collection threads  312  own locks  310 , it is advantageous to allow garbage collection threads  312  to execute as fast as possible without interference from threads  308 . It is also desirable for threads  308  to stay inactive until garbage collection is completed by garbage collection threads  312 . 
     Some of this type of processing is performed automatically by operating system  314  as a part of normal lock handling processing. The length of time required to perform garbage collection, however, may be longer and require more resources than other types of processing handled by other uses of locks  310 . For example, traversing heap  304  accesses more virtual storage. This situation is typical for large multi-gigabyte heaps. As a result, the illustrative embodiments recognize that effective garbage collection by garbage collection threads  312  may be improved through specialized handling. 
     In these different examples, operating system  314  has garbage collection interface  316 . In this example, this garbage collection interface may support registering garbage collection threads in thread registration  318 . As a result, when a garbage collection thread within garbage collection threads  312  obtains a lock from locks  310 , thread registration  318  may be used to identify the lock as a garbage collection lock. In other words, a garbage collection thread registered in thread registration  318  may be identified when that thread obtains a lock from locks  310 . 
     With this information, operating system  314  may identify a number of different phases for a garbage collection state. In these examples, these phases include starting garbage collection  320 , entered garbage collection  322 , and completed garbage collection  324 . Starting garbage collection  320  may be identified when a garbage collection thread within garbage collection threads  312  obtains a lock from lock  310 . Entered garbage collection  322  occurs when all of threads  308  have released any locks from locks  310 . Completed garbage collection  324  occurs when garbage collection threads  312  release all of locks  310 . 
     In these examples, when operating system  314  detects starting garbage collection  320 , operating system  314  may change the priority of garbage collection threads  312 . In particular, the priority of garbage collection threads  312  may be increased. This priority may be increased until any locks obtained by garbage collection threads  312  are released. Once entered garbage collection  322  has occurred, or a lock has been released by a thread within threads  308 , the priority of threads  308  may be reduced. In this manner, threads  308  do not contend with garbage collection threads  312  for processor resources. The priorities may be restored after the garbage collection state ends. 
     In these depicted examples, operating system  314  may change the priority of threads  308  and garbage collection threads  312  by sending priority change  326  to scheduler  328 . Scheduler  328  schedules the execution of threads such as threads  308  and garbage collection threads  312 . 
     Additionally, operating system  314  also may perform other operations such as, for example, paging out non-garbage collection threads and paging in garbage collection threads and including expected data area accesses in this paging process. As another example, data areas previously or expected to be used by garbage collection threads may be paged in for use. A processor in a number of processors in a multi-processor data processing system may be assigned to perform the garbage collection. 
     In an alternative embodiment, the support for garbage collection processing may be performed using profiler  330 . Virtual machine  302  may send notification  332  to profiler  330  when a garbage collection state occurs. In this example, virtual machine  302  is used to identify when a garbage collection process occurs as opposed to using operating system  314  as described above. When profiler  330  receives notification  332 , profiler  330  may use garbage collection interface  316  to change the priority for garbage collection threads  312 . In other examples, profiler  330  may use data collected during previous garbage collection processing to adjust thread priorities and to touch data areas to preload processor caches with heap data. 
     In these examples, the steps performed by operating system  314  to perform actions to increase the performance of garbage collection may be performed using an operating system process, such as, for example, a device driver or other operating system process within operating system  314 . 
     With this type of embodiment, profiler  330  may notify a device driver such as, for example, device driver  206  in  FIG. 2 , to obtain thread identification information when garbage collection occurs. This information, collected using the process described in  FIG. 2 , may be used to obtain an identification of threads that are active during garbage collection as well as the data areas that are active during garbage collection. Although device driver  206  is used in this example, other software components may be used to obtain thread identification information. The application software or virtual machine may designate threads that are garbage collection threads and send this information directly to the device driver or make calls to the operating system directly. These calls may be, for example, increasing or decreasing thread priorities. 
     In this manner, previously collected information may be used to adjust thread priorities and pre-fetch data in heap data areas. In particular, the priorities for threads  308  may be decreased while the priorities for garbage collection threads  312  may be increased while a garbage collection state is present. This thread information may be stored in history  334  for use by profiler  330 . 
     The illustration of the components in  FIG. 2  and garbage collection environment  300  in  FIG. 3  are not meant to imply physical and/or architectural limitations to the manner in which different illustrative embodiments may be implemented. For example, in some illustrative embodiments, the identification of different garbage collection states and the management of garbage collection threads may be performed in response to other events other than interrupts  220  and  222  in  FIG. 2 . Also, in some illustrative embodiments, processes performed by device driver  206  may be implemented in other software components or code depending on the particular implementation. 
     With reference now to  FIG. 4 , a flowchart of a process that may be initiated for other processing when garbage collection is occurring is depicted in accordance with an illustrative embodiment. The process illustrated in  FIG. 4  may be implemented in an operating system such as, for example, operating system  314  in  FIG. 3 . 
     The process begins by identifying a set of garbage collection threads (step  400 ). Thereafter, the priority of the garbage collection threads is increased (step  402 ). The process then identifies a set of non-garbage collection threads (step  404 ). The priority of the set of non-garbage collection threads are decreased (step  406 ), with the process terminating thereafter. 
     Of course, other steps also may be performed. For example, data areas previously or expected to be used by garbage collection threads may be paged into memory for use. A processor within a multi-processor system may be assigned to perform garbage collection. The changing of the priority of threads in these examples may be performed by requesting thread priority changes via operating system interfaces. 
     Of course, various other actions may be performed depending on the condition identified within the operating system. The examples of different conditions and actions that may be initiated are provided for purposes of illustration and not meant to limit the conditions or actions that may be taken. The different illustrative embodiments may monitor for other conditions and perform other actions depending upon the rules within the policy. 
     With reference now to  FIG. 5 , a flowchart of a process for increasing garbage collection performance is depicted in accordance with an illustrative embodiment. The process illustrated in  FIG. 5  may be performed by an operating system such as operating system  314  in  FIG. 3 . This process may be initiated when a garbage collection condition is detected. 
     The process begins by identifying non-garbage collection threads and/or associated data areas located in primary memory (step  500 ). In these examples, the primary memory is a random access memory. The process then pages out the identified non-garbage collection threads and/or associated data areas to a secondary memory (step  502 ). This secondary memory may be, for example, a hard disk. 
     The process then identifies any garbage collection threads and/or associated data areas that are not in the primary memory (step  504 ). The associated data areas may be ones that are expected to be used or touched by the garbage collection threads. The process then pages in the identified garbage collection threads and/or associated data areas into primary memory from the secondary memory (step  506 ) with the process terminating thereafter. 
     The different steps illustrated in  FIGS. 4 and 5  may be implemented in other software components other than an operating system to manage garbage collection. For example, in other illustrative embodiments, the different processes may be implemented in a virtual machine, a device driver, and/or any application performing garbage collection. 
     In this manner, the performance of garbage collection may be improved. This performance may be improved through the placement of garbage collection threads and data areas into the primary memory rather than having those threads being accessed from a secondary memory. In these examples, an operating system may perform other processing such as, for example, the steps described above, to enhance garbage collection processes. 
     Thus, the different illustrative embodiments provide a computer implemented method, apparatus, and computer program code for managing garbage collection. In the different illustrative embodiments, monitoring may be performed for a garbage collection state in a data processing system. If a garbage collection state is detected, the priority of garbage collection threads may be changed. The garbage collection threads may have their priority increased. 
     The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. 
     For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. 
     The invention can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment containing both hardware and software elements. In a preferred embodiment, the invention is implemented in software, which includes, but is not limited to, firmware, resident software, microcode, etc. 
     Furthermore, the invention can take the form of a computer program product accessible from a computer usable or computer readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer usable or computer readable medium can be any tangible apparatus that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk, and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W), and DVD. 
     A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
     Input/output or I/O devices (including, but not limited to, keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. 
     Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modems, and Ethernet cards are just a few of the currently available types of network adapters. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.