Abstract:
The present invention provides a technique for skipping a locking process for an object in memory when a thread accesses an object that only it will access in order to reduce the load imposed on a system and to improve the overall system performance. A program executing in a computer system has multiple threads that share and access objects stored in memory. The objects have thread locality flags associated therewith that indicate the presence or absence of thread localities. The threads examine the thread locality flags for the objects they attempt to access to determine whether the corresponding objects, which are to be accessed, have localities for the threads. If, so the, threads skip the locking process and access objects immediately. If not, the object is locked prior to being accessed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a computer system that provides a data processing environment in which a program is executed as multiple threads. More particularly, the present invention relates to a system in which the threads share and access data that is stored in a memory device.  
           [0003]    2. Description of the Related Art  
           [0004]    In a data processing environment, a program may be executed as multiple threads, and the individual threads access shared data. In this type of processing environment, when a specific thread accesses specific data, a locking process is performed that inhibits the use of the data by other threads. Therefore, when a first thread needs to access data that is being employed by a second thread, the data is locked, and the first thread must wait for the data to be unlocked.  
           [0005]    In this processing environment, data may also be present that is accessed only by a specific thread. However, even when the specific thread accesses such data, the locking process is conventionally performed relative to that data. That is, even when the probability is practically nil that other threads will seek to access the data in question, the locking process will still be performed, even though there is no need.  
           [0006]    In addition, in such a processing environment, data that will no longer be accessed by any thread is retained in a storage area on a data storage device. Thus, the available storage area space may be considerably reduced. Therefore, as needed, to increase the available storage area space, garbage collection is performed to collectively remove data that are no longer necessary.  
           [0007]    Conventional methods exists that are employed for garbage collection for programs executed as threads. One method temporarily halts the execution of a program while data that are no longer being accessed are searched for and removed from the storage area. Another method performs parallel processing during the execution of a program to determine which data are being accessed in order to select and remove data that are no longer necessary.  
           [0008]    An example data processing environment is the operating environment for a program written in the Java language (hereinafter referred to as the Java operating environment). Java is a trademark of Sun Microsystems, Inc. In the Java language, when the threads of a program access objects that are created in a designated object pool in the storage area of a memory device, all the processes are performed. Therefore, in the Java operating environment, locking processes and garbage collection are frequently performed.  
           [0009]    Thus, a demand exists for a high speed locking process and for a technique for reducing the required garbage collection time.  
           [0010]    As is described above, in a data processing environment wherein multiple threads access shared data (objects), there are data that can be accessed only by one specific thread. Even when a thread accesses data for which the probability is practically nil that it will be accessed by other threads, an unnecessary locking process is performed, uselessly. As a result, the overall performance of the system is deteriorated.  
           [0011]    According to one of the garbage collection methods, a program being executed as threads is temporarily halted to remove unnecessary data, and all the other processes of the system must be halted. In this case the usability for a user is very unsatisfactory.  
           [0012]    According to another prior art technique, unnecessary data are removed while a program being executed as threads continues to run, and the data are searched for and removed during the operation. The result is that, for a user, the overall performance of the system is degraded.  
         SUMMARY OF THE INVENTION  
         [0013]    It is, therefore, one object of the present invention to skip the locking process for data that is to be accessed by one specific thread and for which the probability is practically nil that it will be accessed by other threads, so that the load imposed on the system can be reduced and the overall performance of the system improved.  
           [0014]    Another object of the present invention is to enhance the efficiency of the garbage collection process performed in parallel to normal processing, so that the deterioration of the overall performance of the system is reduced.  
           [0015]    To achieve the above objects, according to the present invention, a computer system is provided having a data processing environment in which a program is executed as multiple threads, and in which the threads share and access data that is stored in a memory device, wherein the data stored in the memory device have flag data indicating a locality for specific data that will be accessed only by a specific thread, wherein when the flag data indicate the locality for the specific data, a locking process, to reject access attempts by other threads, is not performed by the specific thread before accessing the specific data, and wherein, when the flag data do not indicate the locality for the specific data, the specific thread performs the locking process before accessing the data.  
           [0016]    Since the locking process is not required when data is accessed for which a locality is indicated for a specific thread, the load imposed on the specific thread can be reduced, and the overall performance of the system can be improved.  
           [0017]    The specific thread detects data, included in the data stored in the memory device, indicating that the flag data has a locality but the specific thread does not have a reference pointer to the data, and thereafter releases the area occupied by the data to provide storage space that is freely available.  
           [0018]    This arrangement is preferable because a predetermined thread can locally erase data that it does not refer to and release the data area to provide additional storage space, and thus normal processing can be performed in parallel by the other threads, without their being halted.  
           [0019]    According to the present invention, a computer system is provided having a data processing environment in which multiple threads share and access objects, wherein flag data is provided for an object indicating the existence of a locality specifying the object is to be accessed only by a specific thread; wherein, when the flag data for the object indicates the locality for the specific thread, the specific thread does not perform a locking process, to reject access attempts by other threads or other objects, before accessing the specific data, and wherein, when the flag data does not indicate the locality for the specific thread, the specific thread performs the locking process before accessing the object.  
           [0020]    When the object is created by a thread, the flag data of the object is set to indicate a locality exists for the thread. And before the object is changed so that it can be accessed by another thread or another object, the locality indicated by the flag data is canceled. Specifically, initially each object represents the existence of a locality for one of the threads, but when a predetermined condition is present, the locality is canceled in accordance with a state change concerning access by the pertinent thread. It should be noted that when an object&#39;s reference is substituted into another object, or into global data, the thread can detect the occurrence of a condition whereby the state of object should be changed. Further, at this time the locality may be canceled by performing a complete examination to determine whether a locality exists in object into which the substitution is made.  
           [0021]    The specific thread detects an object for which the flag data indicates the existence of a locality for the specific thread but the specific thread does not have a reference pointer to the object, and thereafter releases the object to provide in a memory device storage space that is freely available.  
           [0022]    According to the present invention, a memory management method for a data processing environment in which a program is executed as multiple threads, and in which the threads share and access objects that are stored in a memory device, comprises the steps of setting flag data indicating the existence of a locality for a specific object that is created by a specific thread and that is to be accessed only by the specific thread; canceling the locality indicated by the flag data before the specific object is changed so that the specific object can be accessed by another thread; not performing a locking process, to reject access attempts by other threads or objects, before accessing the specific object when the flag data for the specific object indicates the existence of a locality for the specific thread; and performing the locking process before accessing the specific object when the flag data indicates the absence of a locality for the specific thread. Since the locking process is not required when accessing an object for which a locality exists for a specific thread, the overall performance of the system can be improved.  
           [0023]    The step of canceling the locality indicated by the flag data for the specific object includes a step of: performing the locking process, when the specific object has a locality for a predetermined thread, that was skipped at the time the specific object was accessed by the predetermined thread. This arrangement is necessary because when the locality is canceled for a predetermined object, the locking status must be made consistent in advance. The fact that the locking process was skipped must be verified in order for it to be performed later. For this verification, the execution stack for the thread or a count that is kept of the number of times the locking process was skipped can be referred to.  
           [0024]    According to the present invention, a memory management method for a data processing environment in which a program is executed as multiple threads, and in which the threads share and access objects that are stored in a memory device, comprises the steps of: setting flag data indicating the existence of a locality indicating that a specific object that is created by a specific thread is to be accessed only by the specific thread; permitting the specific thread to detect an object for which flag data indicates the existence of a locality for the specific thread and the specific thread does not have a reference pointer to the object; and unlocking the detected object to provide in a memory device storage space that is freely used. This arrangement is preferable because since garbage collection can be locally performed by a predetermined thread, it is not necessary to halt parallel, normal processing that is being performed by other threads.  
           [0025]    According to the present invention, provided is a storage medium on which input means for a computer stores a computer-readable program, which permits the computer to perform: a process for setting flag data indicating the existence of a locality for a specific object that is created by a specific thread and that is to be accessed only by the specific thread; a process for canceling the locality indicated by the flag data before the specific object is changed so that the specific object can be accessed by another thread; a process for not performing a locking process, to reject access attempts by other threads or objects, before accessing the specific object when the flag data for the specific object indicates the existence of a locality for the specific thread; and a process for performing the locking process before accessing the specific object when the flag data indicates the absence of a locality for the specific thread. With this arrangement, for all computers that install this program, a locking process is not required when an object is accessed for which a locality exists for a specific thread, and thus, the overall performance of the system can be improved.  
           [0026]    According to the present invention, provided is a storage medium on which input means for a computer stores a computer-readable program, which permits the computer to perform: a process for setting flag data indicating the existence of a locality indicating that a specific object that is created by a specific thread is to be accessed only by the specific thread; a process for permitting the specific thread to detect an object for which flag data indicates the existence of a locality for the specific thread and the specific thread does not have a reference pointer to the object; and a process for unlocking the detected object to provide in a memory device storage space that is freely used. With this arrangement, for all computers that install this program, garbage collection can be locally performed by a predetermined thread, and it is not necessary to halt parallel, normal processing that is being performed by other threads.  
           [0027]    According to the present invention, a program transmission means comprises storage means for storing a program that permits a computer to perform a process for setting flag data indicating the existence of a locality for a specific object that is created by a specific thread and that is to be accessed only by the specific thread, a process for canceling the locality indicated by the flag data before the specific object is changed so that the specific object can be accessed by another thread, a process for not performing a locking process, to reject access attempts by other threads or objects, before accessing the specific object when the flag data for the specific object indicates the existence of a locality for the specific thread, and a process for performing the locking process before accessing the specific object when the flag data indicates the absence of a locality for the specific thread; and transmission means for reading the program from the storage means and for transmitting the program. With this arrangement, for all computers that download and execute this program, the locking process is not required when an object is accessed for which a locality exists for a specific thread, and the overall performance of the system can be improved.  
           [0028]    According to the present invention, a program transmission means comprises storage means for storing a program that permits a computer to perform a process for setting flag data indicating the existence of a locality indicating that a specific object that is created by a specific thread is to be accessed only by the specific thread, a process for permitting the specific thread to detect an object for which flag data indicates the existence of a locality for the specific thread and the specific thread does not have a reference pointer to the object, and a process for unlocking the detected object to provide in a memory device storage space that is freely used; and transmission means for reading the program from the storage means and for transmitting the program. With this arrangement, for all computers that download and execute this program, garbage collection can be locally performed by a predetermined thread, and it is not necessary to halt parallel, normal processing that is being performed by other threads.  
           [0029]    The present invention will now be described in detail while referring to the preferred embodiment shown in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a diagram for explaining the general arrangement of a computer system that carries out memory management in accordance with one embodiment of the present invention;  
         [0031]    [0031]FIG. 2 is a flowchart for explaining the processing performed in accordance with the embodiment when a thread creates an object;  
         [0032]    [0032]FIG. 3 is a flowchart for explaining the processing related to the locking process performed in accordance with the embodiment when the thread refers to the object;  
         [0033]    [0033]FIG. 4 is a flowchart for explaining the processing related to the unlocking process performed in accordance with the embodiment when the thread has referred to the object;  
         [0034]    [0034]FIG. 5 is a flowchart for explaining the processing in accordance with the embodiment for canceling the thread locality for a specific thread that exists in an object; and  
         [0035]    [0035]FIG. 6 is a flowchart for explaining the garbage collection processing using the thread locality flag in accordance with the embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0036]    [0036]FIG. 1 is a diagram showing the general arrangement of a computer system that carries out memory management in accordance with the embodiment. The computer system in this embodiment has a data processing environment wherein a program is executed as multiple threads, and the threads share and access objects stored in a memory device. In the following explanation, a computer system is employed that has as an operating environment a program written in the Java language (hereinafter referred to as a Java operating environment).  
         [0037]    In FIG. 1, an object pool  110  is a storage area in a memory device in which objects  121  to  124  are stored. A reference pointer  141  is from a thread  131  to object  121 ; a reference pointer  142  is from a thread  132  to object thread  122 ; a reference pointer  143  is from a thread  133  to object  123 ; a reference pointer  144  is from object  121  to object  122 ; and a reference pointer  145  is from object  121  to object  124 . Thread locality flags  151  to  154  are respectively provided for objects  121  to  124 .  
         [0038]    In a normal operation, as needed, threads  131  to  133  create and employ objects in the object pool  110 . In FIG. 1, thread  131  refers to object  121 , and also refers to objects  122  and  124  via object  121 . Similarly, thread  132  refers to object  122 , and thread  133  refers to object  123 . When this state is viewed from the objects, object  121  is directly referred to only by thread  131 . Object  122  is directly referred to by thread  132 , and by thread  131  via object  121 . Object  123  is referred to only by thread  133  directly, and object  124  is referred to only by thread  131  via object  121 .  
         [0039]    In the following explanation, the state wherein the object is referred to directly by only one thread is called an existence of a thread locality.  
         [0040]    Specifically, in FIG. 1, thread localities exist in object  121  that is directly referred to only by thread  131  and in object  123  that is directly referred to only by thread  133 . Although object  124  is referred to only by thread  131  via object  121 , the embodiment consider that a thread locality does not exist in object  124 . A thread locality does not exist in an object, such as object  124 , that is referred to via a specific object, because a complicated process is required to determine whether the pertinent object is referred to actually only by one thread, or by multiple threads via other objects.  
         [0041]    Objects  121 ,  122 ,  123  and  124  are respectively provided with thread locality flags  151 ,  152 ,  153  and  154  indicating each object has a thread locality. In this embodiment, when objects  121 ,  122 ,  123  and  124  have thread localities, the thread locality flags  151 ,  152 ,  153  and  154  are set ON, while, when objects  121 ,  122 ,  123  and  124  do not have thread localities, the thread locality flags  151 ,  152 ,  153  and  154  are set OFF.  
         [0042]    When threads  131 ,  132  and  133  create objects, they set the thread localities of the objects ON. Before the created objects are changed by threads  131 ,  132  and  133  so that they can be referred to by other threads or other objects, the thread localities of these objects are set OFF. Various methods can be employed by threads  131 ,  132  and  133  to detect the occurrence of a condition where an object having a thread locality for a local thread should be changed so that it can be referred to by other threads or other objects. For example, when a reference to a specific object is substituted into another object or global data, the state can be changed. At this time, the locality flag may be released by performing a complete examination to determine whether an object into which the specific object is substituted is local.  
         [0043]    Once a thread locality flag is set OFF, it is not set ON again. Actually, there are some cases where an object having a thread locality for a specific thread is temporarily referred to by another thread, and the reference pointer is then erased and the thread locality is again recovered. However, if this case is also taken into account, the process performed to determine a thread locality becomes complicated, and thus the thread locality flag is maintained OFF.  
         [0044]    [0044]FIG. 2 is a flowchart for explaining the processing performed during which threads  131 ,  132  and  133  create objects  121 ,  122  and  123 . In FIG. 2, first, threads  131 ,  132  and  133  obtain areas in the object pool  110  as object areas, and initialize these areas (Step  201 ). Then, the thread locality flags  151 ,  152  and  153  are allocated to the object areas and are set ON (Step  202 ). Following this, reference pointers to objects  121 ,  122  and  123  are stored in threads  131 ,  132  and  133  (Step  203 ).  
         [0045]    When objects  121 ,  122  and  123  are created, all the thread locality flags  151 ,  152  and  153  are ON. In FIG. 1, object  122  thereafter loses the thread locality, and the thread locality flag  152  is set OFF. It should be noted that in the example in FIG. 1, object  122  can be referred to by object  121 . Further, in FIG. 1, object  124 , which is created by one of threads  131 ,  132  and  133  loses thread locality, and the thread locality flag  154  is set OFF. Thereafter, object  124  is changed so that it can be referred to by object  121 , and then the other reference pointer is erased, so that object  124  can now to be referred to only by object  121 .  
         [0046]    In a data processing environment, such as Java, where multiple threads share and access objects, when a specific thread is referring to a specific object, other threads should be inhibited from referring to this object. Thus, when each thread refers to the object, it performs a locking process to reject attempts by other threads to access this object. And when the locking process is employed, a thread whose access attempt was rejected must wait until the specific object is unlocked to refer to it.  
         [0047]    However, in this embodiment the thread that refers to the object having the thread locality is limited to only one thread. Therefore, when a thread refers to an object having the thread locality for the pertinent thread, the locking process need not be performed.  
         [0048]    [0048]FIG. 3 is a flowchart for explaining the operation concerning the locking process that is performed when threads  131 ,  132  and  133  refer to objects  121 ,  122 ,  123  and  124 . In FIG. 3, first, threads  131 ,  132  and  133  determine whether the thread locality flags  151 ,  152 ,  153  and  154  of objects  121 ,  122 ,  123  and  124  are ON (Step  301 ). When the thread locality flags  151  to  154  are ON, threads  131  to  133  refer to objects  121  to  124  without performing the following process. At this time, the processing accompanied by the skipping of the locking process is performed as needed (Step  305 ). This case corresponds to thread  131  referring to object  121  and thread  133  referring to object  123 . The processing that accompanies the skipping of the locking process will be described later.  
         [0049]    When the thread locality flags  151  to  154  are OFF, the locking process is performed normally. That is, thread  131 ,  132  or  133 , which refers to object  121 ,  122 ,  123  or  124 , determines whether the pertinent object has been locked by another thread  131 ,  132  or  133  (Step  302 ). When object  121 ,  122 ,  123  or  124  has been locked, thread  131 ,  132  or  133  waits until this object is unlocked (Step  304 ). When object  121 ,  122 ,  123  or  124  has not been locked by another thread  131 ,  132  or  133  (including a case where it has been unlocked), the locking process is performed for the pertinent object (Step  303 ), and the object is referred to. This case corresponds to when thread  132  refers to object  122 , and when thread  131  refers to object  122  or  124  via object  121 .  
         [0050]    In this embodiment, the thread does not perform the locking process when it refers to the object. The unlocking process that is to be performed after the object is referred to is also not required.  
         [0051]    [0051]FIG. 4 is a flowchart for explaining the operation concerning the unlocking process to be performed when threads  131  to  133  have referred to objects  121  to  124 . In FIG. 4, first thread  131 ,  132  or  133  determines whether the setting for the thread locality flag  151 ,  152 ,  153  or  154  of object  121 ,  122 ,  123  or  124  that has been referred to is ON (Step  401 ). When the setting for the thread locality flag  151 ,  152 ,  153  or  154  is ON, the reference to the pertinent object is terminated without performing the following processing. At this time, the accompanying processing is performed, as needed, that is performed when the unlocking process is not required (Step  403 ). In the same manner as when the locking process is skipped, this case corresponds to thread  131  referring to object  121  and to thread  133  referring to object  123 .  
         [0052]    When the thread locality flag  151 ,  152 ,  153  or  154  is OFF, the unlocking process is performed normally (Step  402 ).  
         [0053]    An explanation will now be given for an operation wherein an object, such as object  121  or  123 , whose thread locality flag is ON and which that has the thread locality for a specific thread, is changed so that it can be referred to by another thread or another object. FIG. 5 is a flowchart for explaining this operation.  
         [0054]    Assume that thread  131 ,  132  or  133  detects the occurrence of a condition wherein object  121 ,  122 ,  123  or  124 , which has a thread locality for a pertinent thread, should be changed so that it can be referred to by another thread or another object. As is described above, various methods can be employed to detect such a condition. In this embodiment, when a reference to a pertinent object is substituted into another object or global data, it is ascertained that a condition has occurred wherein the state of the object should be changed.  
         [0055]    In this case, first, thread  131 ,  132  or  133  determines whether the setting for the thread locality flag  151 ,  152 ,  153  or  154  of object  121 ,  122 ,  123  or  124  that is to be accessed is ON (Step  501 ). If the thread locality flag  151 ,  152 ,  153  or  154  is OFF, no special process is performed, and the reference pointer of object  121 ,  122 ,  123  or  124  is substituted into another object or data (Step  504 ). This case corresponds to substitution of object  122  or  124  into another object or global data.  
         [0056]    When the setting for the thread locality flag  151 ,  152 ,  153  or  154  is ON, thread  131 ,  132  or  133  instructs object  121 ,  122 ,  123  or  124  to set the thread locality flag  151 ,  152 ,  153  or  154  to OFF. Upon the receipt of the instruction, object  121 ,  122 ,  123  or  124  sets the corresponding thread locality flag  151 ,  152 ,  153  or  154  to OFF (Step  502 ). Thus, the thread locality for object  121 ,  122 ,  123  or  124  is lost, and the following accompanying processing must be performed.  
         [0057]    Assume that the thread locality of object  123  for thread  133  is to be canceled. When the thread locality of object  123  is canceled, thread  132  and other objects must lock object  123  if they access it. And if object  123  is locked by thread  133 , thread  132  must wait until it has been unlocked. However, since object  123  will maintain the thread locality for thread  133  until thread  132  refers to it, the locking process and the unlocking process were skipped when thread  133  referred to object  123 . Therefore, before thread  132  can refer to object  123 , the locking process and the unlocking process that were skipped must be performed to provide matching processes.  
         [0058]    Therefore, when setting the thread locality flag  151 ,  152 ,  153  or  154  OFF, thread  131 ,  132  or  133 , for which object  121 ,  122 ,  123  or  124  has the thread locality, performs the locking process and the unlocking process that were not performed for object  121 ,  122 ,  123  or  124  (Step  503 ). After the locking status becomes consistent, the reference pointer of object  121 ,  122 ,  123  or  124  is substituted into another object (Step  504 ). Since, before object  121 ,  122 ,  123  or  124  loses the thread locality for thread  131 ,  132  or  133 , the processing for obtaining consistency is performed by thread  131 ,  132  or  133 , a problem, such as Racing, does not occur.  
         [0059]    To perform the locking process and the unlocking process that are not performed at Step  503 , it is necessary for the locking process and the unlocking process that were skipped to be identified when a request for a new access is issued. An arbitrary method for performing the identification can be employed. Two methods will be explained as examples.  
         [0060]    The first method is a method that provides for the scanning of the execution stack of a thread for which a thread locality exists in order to identify a locking process and an unlocking process that were skipped. In this case, each time an object is referred to, the reference history is automatically stored in the execution stack of a thread. Therefore, there is no substantial process present for the processing (Step  305 ) in the flowchart in FIG. 3 that is accompanied by the skipping of the locking process, and the processing (Step  403 ) in the flowchart in FIG. 4 that is accompanied by the skipping of the unlocking process. However, when the thread locality of an object is lost, overhead is increased due to the need to scan the execution stack of the thread.  
         [0061]    The second method is a method that provides for the counting of the number of times the locking process and the unlocking process were skipped, and for performing the skipped processes based on the count available at the time the thread locality of the object is lost. In this case, the process counts the number of times the process skipping was performed during the processing (Step  305 ) in the flowchart in FIG. 3, which is accompanied by the skipping of the locking process, and the processing (Step  403 ) in the flowchart in FIG. 4, which is accompanied by the skipping of the unlocking process. According to the second method, when the thread locality of an object is lost, there is no increase in overhead due to the scanning of the execution stack of the object. Since the load imposed by the counting process at Steps  305  and  403  is much lighter than that imposed by the locking process and the unlocking process that are skipped, the actual load imposed on the system can be reduced considerably.  
         [0062]    As is described above, in a data processing environment wherein multiple threads share and access objects, there may be objects in the object pool that are not referred to by any threads. That is, threads create objects to execute various processes, and when the objects need not be referred to any longer and the reference pointers have been erased, these objects remain left in the object pool, even though they are unrelated to the program execution process. Therefore, it is important for the garbage collection process to be efficiently performed and for all unneeded objects to be removed so that the maximum usable space is available in the object pool.  
         [0063]    Before collecting an unneeded object from the object pool, confirmation must be obtained that the pertinent object is not being referred to by any thread. Therefore, generally, whether the object in question is being referred to or not must be determined by tracing the reference pointers of all the threads.  
         [0064]    In this embodiment, however, since a thread locality flag, indicating that a thread locality exists for a specific thread, is added to an object, if the thread locality flag of the object is ON, it is guaranteed that only the thread for which the thread locality exists can refer to the pertinent object. Therefore, to determine whether an object can be collected, the only examination that need be performed is one to ascertain whether a reference pointer to a pertinent object remains in a specific thread.  
         [0065]    [0065]FIG. 6 is a flowchart for explaining the garbage collection processing using a thread locality flag. In FIG. 6, a thread that is to execute the garbage collection process selects an object that has a thread locality for a specific thread (Step  601 ). The thread then determines whether a reference pointer to the object is present in the specific thread (Step  603 ). When no reference pointer is found, the object is collected (Step  604 ). This process is repeated until the thread has examined all the objects that have thread localities for the specific thread. The processing is thereafter terminated (Step  602 ).  
         [0066]    Through this processing, an object that is not being referred to by a specific thread (i.e., not being referred to by any of the threads) can be collected from among the objects whose thread localities are ON. Since in this process the stack of a thread need only be scanned at one step to determine whether a predetermined object can be collected, and since the tracing of a reference pointer to the object is not required, garbage collection can be performed at an extremely high speed. Further, since a predetermined thread locally performs the garbage collection process, the normal processing performed by other threads need not be halted, and can be performed in parallel to the garbage collection.  
         [0067]    In this embodiment, even when garbage collection using the thread locality flags is performed sequentially for the individual threads, an object whose thread locality flag is cleared and that is not being referred to by any of the threads can&#39;t be collected. To collect such an object, the conventional garbage collection method must be employed whereby the processing being performed by all the threads is temporarily halted, and the object pool is examined to find an object that is not referred to by any thread. That is, garbage collection for which the thread locality flag of this embodiment is employed can be used with the conventional garbage collection process.  
         [0068]    As is described above, according to the present invention, the locking process is skipped when data is accessed that can be accessed by only one thread, and for which the probability is practically nil that an access request will be issued by other threads. Thus, the load imposed on the system is reduced and the overall system performance is improved. And since garbage collection can be performed in parallel to the normal processing, the efficiency of the garbage collection process is increased, and the deterioration of the overall system performance can be reduced.