Abstract:
One embodiment of the present invention provides a system that facilitates keeping track of memory usage of tasks in a shared heap without performing a full garbage-collection operation. The system operates by determining a memory usage by each task in a young generation of the shared heap. Once the memory usage for each task has been determined, the system then adds the memory usage for each task in the young generation of the shared heap to a global memory usage for each task (obtained during a preceding full garbage-collection operation) to produce an actual memory usage for each task.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to virtual machines within computer systems. More specifically, the present invention relates to a method and an apparatus for keeping track of memory usage for tasks in a shared heap without performing a full garbage-collection operation.  
         [0003]     2. Related Art  
         [0004]     The Java 2 Platform, Micro Edition (J2ME™), has become very popular in the wireless device space. Motorola, Nokia, NTT DoCoMo, RIM, Siemens, and many other key players in the wireless device industry have recently started shipping J2ME-enabled devices in very high volume. It has been estimated that over 200 million J2ME-enabled mobile phones were shipped in 2003 alone.  
         [0005]     In order to effectively run applications on the new memory-constrained computing devices, a number of techniques have been developed to conserve memory. One promising technique involves providing a shared heap wherein objects from different tasks can be stored. This can greatly save memory space in comparison to the conventional technique of providing a separate heap for each thread. However, this shared heap technique complicates both garbage-collection operations and memory accounting operations.  
         [0006]     In traditional non-memory-constrained computing systems, additional data structures, such as lookup tables, can be used to keep track of memory allocation by individual tasks. However, in memory-constrained computing devices, memory is extremely scarce. It is consequently impractical to use additional data structures to keep track of memory allocation, because the additional space used by the data structures would largely eliminate the space savings gained by using a shared heap.  
         [0007]     Hence, what is needed is a method and an apparatus for keeping track of memory usage for tasks in a shared heap without the problems listed above.  
       SUMMARY  
       [0008]     One embodiment of the present invention provides a system that facilitates keeping track of memory usage of tasks in a shared heap without performing a full garbage-collection operation. The system operates by determining a memory usage by each task in a young generation of the shared heap. Once the memory usage for each task has been determined, the system adds the memory usage for each task in the young generation of the shared heap to a global memory usage for each task (obtained during a preceding full garbage-collection operation) to produce an actual memory usage for each task.  
         [0009]     In a variation on this embodiment, determining the memory usage for each task in the young generation of the shared heap involves performing a young generation garbage-collection operation on the young generation of the shared heap.  
         [0010]     In a further variation, performing the young generation garbage-collection operation involves performing a marking operation to mark live objects in the young generation of the shared heap. For each object in the young generation of the shared heap, the system identifies a task that owns the object. If the object has been marked by the preceding marking operation, the system promotes the object to an old generation of the shared heap, and adds the memory usage for the object to the global memory usage for the corresponding task.  
         [0011]     In a further variation, identifying the task that owns the object involves examining an object that is a boundary marker, wherein a boundary marker acts as a delimiter separating objects belonging to different tasks, and wherein each object is owned by the task that is identified by the last boundary marker preceding the object.  
         [0012]     In a variation on this embodiment, identifying the task that owns the object involves examining a header of the object for an identifier that identifies the task.  
         [0013]     In a further variation, the identifier is only present in objects in the young generation of the shared heap, and is removed when the object is promoted to the old generation of the shared heap.  
         [0014]     In a variation on this embodiment, determining the task that owns the object involves examining a card table, wherein each task has a separate card table.  
         [0015]     In a variation on this embodiment, the shared heap is located within a memory-constrained computing device.  
         [0016]     In a variation on this embodiment, the shared heap is located within a platform-independent virtual machine.  
         [0017]     In a further variation, the platform-independent virtual machine is a JAVA Virtual Machine. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0018]      FIG. 1  illustrates a memory-constrained computing device with a shared heap in accordance with an embodiment of the present invention.  
         [0019]      FIG. 2  illustrates a generational shared heap in accordance with an embodiment of the present invention.  
         [0020]      FIG. 3  illustrates boundary objects in accordance with an embodiment of the present invention.  
         [0021]      FIG. 4  presents a flowchart illustrating the process of object allocation in the young generation in accordance with an embodiment of the present invention.  
         [0022]      FIG. 5  presents a flowchart illustrating a full garbage collection in accordance with an embodiment of the present invention.  
         [0023]      FIG. 6  presents a flowchart illustrating the process of a young generation garbage collection in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.  
         [0025]     The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet.  
         [0000]     Memory-Constrained Computing Device With A Shared Heap  
         [0026]      FIG. 1  illustrates a memory-constrained computing device  102  with a shared heap in accordance with an embodiment of the present invention. Memory-constrained computing device  102  can include any type of computing device, including, but not limited to, a cell phone, a computer system based on a microprocessor, a digital signal processor, a portable computing device, a personal organizer, a device controller, and a computational engine within an appliance. Memory-constrained computing device  102  contains Java Virtual Machine (JVM)  104 , which in turn contains shared heap  106 . (The terms JAVA, JVM and JAVA VIRTUAL MACHINE are trademarks of SUN Microsystems, Inc. of Santa Clara, Calif.) Note that JVM  104  could be any type of platform-independent virtual machine, and is not meant to be limited to a Java Virtual Machine.  
         [0027]     Shared heap  106  includes all of the tasks running on JVM  104 , as well as the associated threads and objects. For example, in the illustrated embodiment, shared heap  106  includes task  110 , and associated threads  120 - 124 . Likewise, shared heap  106  also includes task  112 , and associated threads  126 - 128 . In addition, shared heap  106  includes objects  130 - 136  that are associated with the various threads.  
         [0000]     Generational Shared Heap  
         [0028]      FIG. 2  illustrates a generational shared heap  200  in accordance with an embodiment of the present invention. Note that generational shared heap  200  is partitioned into young generation  201  and old generation  221 . In the example illustrated in  FIG. 2 , generational shared heap  200  is partitioned into two generations. However, generational shared heap  200  may be partitioned into any number of generations.  
         [0029]     As objects are allocated to generational shared heap  200 , they are allocated in young generation  201 . During subsequent garbage-collection operations, all live objects in the young generation  201 , such as objects  202 - 210 , are promoted to old generation  221 , and young generation  201  is available for subsequent allocation operations. Additionally, pointers that point to the objects that are being promoted need to be updated, such as object  224 . Occasionally, it becomes necessary to perform a full garbage-collection operation, including old generation  221 , to free space occupied by objects that are no longer live. Note that various methods for generational garbage collection exist, and virtually any of these can be used with the present invention.  
         [0000]     Boundary Objects  
         [0030]      FIG. 3  illustrates boundary objects in accordance with an embodiment of the present invention. In order to keep track of which objects are owned by which tasks, one embodiment of the present invention utilizes boundary objects. In this example, objects are allocated to young generation  201  consecutively from the beginning of young generation  201 . Note that the present invention is not meant to be limited to starting from the beginning of young generation  201  or moving from the beginning to the end of young generation  201 . Various methods exist that could start at virtually any place in young generation  201  and move through young generation  201  in any defined pattern.  
         [0031]     As objects are allocated to young generation  201 , the first object allocated is a boundary object, such as boundary object  302 . Boundary objects are typically the minimum size possible that can accommodate an identifier for the task that owns the objects that follow the boundary object. Once the boundary object is allocated, objects are allocated for the task in consecutive memory locations following the boundary object. For example, objects  304 - 308  are owned by the task identified by boundary object  302 , object  314  is owned by the task identified by boundary object  312 , and objects  324 - 328  are owned by the task identified by boundary object  322 . Note that the system may optionally allocate a termination boundary object, such as boundary object  332 , at the end of the allocated space, wherein no additional objects are allocated after the termination boundary object.  
         [0032]     If the system needs to allocate additional objects for a current task that the system has already allocated objects for, and the system has most recently allocated objects for other tasks, the system can allocate an additional boundary object identifying the current task, and can then allocate the additional object following the additional boundary object for the current task. For example, objects  304 - 308  may belong to task number one as identified by boundary object  302 . Additionally, objects  324 - 328  may also belong to task number one as identified by boundary object  322 , even though they are separated from objects  304 - 308  by intervening objects. In this way, the system does not have to allocate extra space for the current task in anticipation of subsequent requirements for the current task, as is necessary in most current systems.  
         [0033]     In another embodiment of the present invention, the system attaches an identifier indicating the task that owns the object to the object&#39;s header. In this embodiment, the system does not use boundary objects to identify object ownership because each object would contain identification information for its owner.  
         [0034]     In yet another embodiment of the present invention, the system may optionally maintain a separate card table for each task that facilitates identification of the objects that are owned by each task. When a young generation garbage-collection operation is performed, the system starts from global roots plus references from old generation  221  to young generation  201 . Note that the system can easily separate global roots for different tasks, but cannot separate references from old generation  221  to young generation  201 . In this embodiment, a write barrier intercepts assignments of pointers from young generation  201  to the fields of objects in old generation  221  and marks cards for these fields. At the moment of assignment, it is known to which task the reference belongs. Unless the write barrier uses separate card tables for each task, this task ownership knowledge will be lost after a task switch. Hence, when the system scans references from old generation  221  to young generation  201 , it would not be possible to identify the ownership of the reference to young generation  201 , and all objects reachable from this reference without the use of separate card tables. However, this embodiment requires memory to accommodate the card tables, and may not be practical in some situations.  
         [0000]     Object Allocation  
         [0035]      FIG. 4  presents a flowchart illustrating the process of object allocation in young generation  201  in accordance with an embodiment of the present invention. The system starts by receiving a request from JVM  104  to allocate an object for a current task (step  401 ). Next, the system checks to see if the current task is the same as a previous task for the last object allocation (step  402 ). If so, the system skips to step  408  and allocates the object.  
         [0036]     However, if the current task is not the same as the previous task, the system allocates a boundary object with an identifier for the current task (step  404 ). The system also sets the previous task indicator to match the current task to facilitate future object allocations (step  406 ). The system then allocates the object (step  408 ).  
         [0000]     Full Garbage-Collection Operation  
         [0037]      FIG. 5  presents a flowchart illustrating a full garbage-collection operation in accordance with an embodiment of the present invention. During a full garbage-collection operation, the system resets the memory usage for a task to 0 (step  502 ). Next, the system traces the task roots to identify live objects in generational shared heap  200  (step  503 ). As the live objects are identified, the system marks the live objects (step  504 ). In addition, for each live object, the system increments the memory usage for the task that owns the live object by the size of the live object to facilitate accounting of the total memory in use by each task (step  506 ). Note that the ownership of the object is determined by the task roots being traced. Also note that the steps  502 - 506  are repeated for each task so that all live objects for all tasks in generational shared heap  200  are accounted for. Once the marking operation has completed and the memory usage has been determined, the system continues with the normal full garbage-collection operation (step  508 ). Note that the illustrated embodiment in  FIG. 5  uses a marking garbage-collection operation. However, any type of garbage-collection operation may be used.  
         [0000]     Young Generation Garbage-Collection Operation  
         [0038]      FIG. 6  presents a flowchart illustrating the process of a young generation garbage collection in accordance with an embodiment of the present invention. From time to time it may be necessary to perform a garbage-collection operation on just the young generation  201  to free up space or to get an accurate accounting of the memory usage for each task. Note that whenever possible, young generation garbage-collection operations are preferred to old generation garbage-collection operations because they take less time.  
         [0039]     The system starts by marking all live objects for every task in the young generation  201  (step  602 ). The system then scans consecutively through young generation  201  for objects from the beginning of young generation  201  (step  604 ). Upon finding an object, the system determines if the object is a boundary marker object (step  606 ). If so, the system sets the current task indicator to the task indicated by the boundary marker object (step  608 ), and continues to scan the young generation  201  for objects.  
         [0040]     If the object is not a boundary marker object, the system determines if the current object is marked (step  610 ). If not, the system disregards the object and continues to scan the young generation  201  for objects. However, if the current object is marked, the system increments the memory usage for the current task by the size of the object (step  612 ) and promotes the object to the old generation  221  (step  614 ).  
         [0041]     Note that as the objects are promoted from young generation  201  to old generation  221 , the boundary marker objects identifying the ownership of the objects being promoted are dropped. Boundary marker objects are not referenced, so they are considered to be dead. Moreover, boundary marker objects are not promoted to old generation  221 , and so they do not occupy any space in old generation  221 . Additionally, boundary marker objects are not used in full garbage-collection operations, so they do no affect the time required to perform a full garbage-collection operation.  
         [0042]     The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.