Apparatus and method for removing elements from a linked list

Methods, apparatus and computer program products for removal of elements from a linked list while other elements of the linked list are allowed to be accessed during the removal operation. In one embodiment, the method, apparatus and computer program product include identifying an add/remove area of a linked list and a static area of the linked list. Elements may only be added or removed from the linked list in the add/remove area or by a garbage collector that performs garbage collection only on elements in the static area of the linked list. The garbage collector identifies an element after the last element in the add/remove area and performs garbage collection beginning with that element and moving through the static area. In an alternative embodiment, a “next element” pointer in a previous list element is set to point to the element being deleted's “next element” pointer. Any global references to the element being deleted must be modified. A message may then be issued to the processors of a multiprocessor system at a same interrupt priority as a reading process priority. Once the processors respond to the message, garbage collection may be performed on the element to be deleted.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention generally relates to removal of elements from a linked list. In particular, the present invention provides apparatus and methods for removing elements from a linked list while allowing access to other elements of the linked list during the removal.

2. Description of Related Art

In data management, a linked list is a group of items, each of which contains a pointer to the next item. A linked list allows for the organization of a set of data in noncontiguous storage locations. Linked lists are used in many types of computing environments and are used for the management of various types of data.

One computing environment in which linked lists may be utilized is a multiprocessor system. In a multiprocessor system a plurality of processors may attempt accesses and/or modifications to elements in the linked list at substantially the same time. Thus, there is a system of locks utilized to make sure that only a single processor may access and/or modify a particular element in the linked list at one time. If such a mechanism were not used, two or more different processors may perform conflicting actions on the linked list element or the action of one processor on the linked list element may be negatively impacted by the actions performed by the other processors.

This problem is further exacerbated when elements need to be removed from the linked list, e.g., elements that are no longer being used by any of the processors (garbage collection). It is important, however, during such removal of elements of a linked list in a multiprocessor system, that elements are not modified or removed that are or may be utilized by one or more of the processors during the removal process.

In known mechanisms for removal of elements in a linked list, the list is protected from any changes at all times during the removal process. Such an approach has the disadvantage that it will synchronize all accesses to the list, making multiprocessor use of the list prohibitively slow. Another common way of handling such removal of elements is to have a second linked list that maintains elements to be freed, i.e. deferred freeing of the elements, and free the entire second list at once. Unfortunately, this implementation requires the element be removed from the first list by, again, locking the linked list and adjusting pointers in the linked list elements to remove the element. Thus, in all known mechanisms for removing elements from a linked list, a lock of the linked list is required and serialization of accesses to the linked list results.

Therefore, it would be beneficial to have an apparatus and method for removing elements from a linked list that allows access to elements of the linked list during removal of other elements of the linked list. In this way, serialization of accesses to the linked list are avoided.

SUMMARY OF THE INVENTION

The present invention provides a method, apparatus and computer program product for removal of elements from a linked list while other elements of the linked list are allowed to be accessed during the removal operation. In one embodiment, the method, apparatus and computer program product include identifying an add/remove area of a linked list and a static area of the linked list. Elements may only be added or removed from the linked list in the add/remove area or by a garbage collector that performs garbage collection only on elements in the static area of the linked list. The garbage collector identifies an element after the last element in the add/remove area and performs garbage collection beginning with that element and moving through the static area.

The identification of the element in the static area with which to begin garbage collection may be performed using a pointer to the head of the linked list and an offset into the linked list determined based on a known size of the add/remove area. Alternatively, the identification of the element in the static area with which to being garbage collection may be performed by filling the add/remove area with dummy elements and identifying and maintaining a pointer to a first dummy element added to the add/remove area. The elements that were present in the linked list before adding the first dummy element may be identified as the static area of the linked list and garbage collection may be performed on those elements.

In an alternative embodiment, to remove an element from a linked list, a “next element” pointer in a previous list element is set to point to the element being deleted's “next element” pointer. As a result, any process following the linked list will not find the element being deleted. Any global references to the element being deleted must be removed.

A message may then be issued to the processors of a multiprocessor system at a same interrupt priority as a reading process priority. When the processors handle the message, the processing of the message indicates that the processor does not hold any local variables that reference the linked list. Once all the processors respond to the message, garbage collection may be performed on the element to be deleted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides methods and apparatus for removing elements from a linked list in a multiprocessor system such that processors may continue to access elements of the linked list during removal of other elements of the linked list. The present invention may be implemented on any multiprocessor system, whether such processors are located in the same or different computing devices. Thus, the present invention may be used in a stand alone computing device in which multiple processors are present, or in a distributed data processing system in which one or more processors are present in remotely located devices. For ease of explanation, the present invention will be described in terms of a single computing device in which multiple processors are present. However, no limitation to the environment in which the present invention may be implemented is intended or implied by the selection of this illustration of the present invention.

FIG. 1is an exemplary diagram illustrating a multiprocessor system in which the present invention may be implemented. As shown inFIG. 1, data processing system100may be a symmetric multiprocessor (SMP) system including a plurality of processors102and104that are connected to system bus106. Also connected to system bus106is memory controller/cache108, which provides an interface to local memory109. I/O bus bridge110is connected to system bus106and provides an interface to I/O bus112. Memory controller/cache108and I/O bus bridge110may be integrated as depicted.

Peripheral component interconnect (PCI) bus bridge114connected to I/O bus112provides an interface to PCI local bus116. A number of modems may be connected to PCI local bus116. Typical PCI bus implementations will support four PCI expansion slots or add-in connectors. Communications links to clients may be provided through modem118and network adapter120connected to PCI local bus116through add-in boards.

Additional PCI bus bridges122and124provide interfaces for additional PCI local buses126and128, from which additional modems or network adapters may be supported. In this manner, data processing system100allows connections to multiple network computers. A memory-mapped graphics adapter130and hard disk132may also be connected to I/O bus112as depicted, either directly or indirectly.

The data processing system depicted inFIG. 1may be, for example, an IBM eServer pSeries system, a product of International Business Machines Corporation in Armonk, N.Y., running the Advanced Interactive Executive (AIX) operating system or LINUX operating system. However, those of ordinary skill in the art will appreciate that the hardware depicted inFIG. 1may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention.

FIG. 2is an exemplary diagram illustrating a linked list according to the present invention. As shown inFIG. 2, the linked list200is comprised of one or more list elements210. The list elements210may simply be pointers to data, may include the data itself, or may be more complex data structures having pointers, data, and other information appropriate to the particular implementation.

In the depicted example, the list elements210include a pointer data structure220that points to a next element in the linked list. The list elements210further include a garbage collection flag data structure230which is used to mark list elements for garbage collection, as discussed hereafter. The list elements210may include other data structures not explicitly shown inFIG. 2. It should be appreciated that whileFIG. 2illustrates the linked list200as a top-down linked list, the opposite configuration, a bottom-up linked list, may be utilized without departing from the spirit and scope of the present invention.

In order to remove list elements210from the linked list200while allowing for simultaneous addition and/or removal of other list elements, i.e. performing removal of linked list elements210without obtaining a lock on the linked list200, it is important to distinguish an area of the linked list200that is changing, i.e. an add/remove area where linked list elements are added and/or removed, and an area of the linked list200that is not changing, i.e. a static area250.

In order to identify an add/remove area240and a static area250of a linked list200, the present invention ensures that elements are only added to the linked list200within a certain region, e.g., the first three elements of the linked list200. With the present invention, atomic operations are used to add elements from a linked list200in only a predefined area, e.g., the first element of the linked list200(either at the head or tail of the linked list), the first three elements of the linked list, or the like. An atomic operation is an operation that must be performed entirely or not at all. For example, if machine failure prevents an atomic operation to be processed to completion, the system will be rolled back to the start of the atomic operation.

In addition, the present invention ensures that elements are never removed from the linked list200except by a garbage collector or if they are known to be within the add/remove area240during the entire garbage collection process. The ensuring of removal only by the garbage collector or in the add/remove area240may be performed in a number of different ways as detailed hereafter.

If elements are only added to the linked list200in the add/remove area240and elements are only removed from the linked list200by either the garbage collector or if they are only removed from the add/remove area240, then the linked list200structure will be static everywhere except within the add/remove area240. Therefore, by ensuring the limitations on addition and removal of elements from the linked list200according to the present invention, an add/remove area240may be defined and the remainder of the linked list200may be considered a static area250.

The actual removal of elements from the static area250of the linked list200is performed by a garbage collector. A garbage collector is a software routine or method that searches the linked list200for linked list elements210that are no longer being used by any processor in the multiprocessor system and reclaims those linked list200elements for reuse. The garbage collector of the present invention may be asynchronous with regard to the adding and removing of elements in the add/remove area240.

Garbage collection and garbage collectors are generally known in the art. The present invention uses a modified version of known garbage collectors in that the garbage collector of the present invention begins operation at a particular point—an element of the static area250, and does not perform garbage collection on the add/remove area240.

The garbage collector of the present invention only considers those elements flagged for garbage collection, i.e. have the garbage collection flag data structure230set, which are in the static area250of the linked list200in order to ensure that the elements being removed are not subject to change by one or more of the processors in the multiprocessor system. There a number of different ways in which only the elements in the static area250are considered rather than the entire linked list200.

A first exemplary mechanism for garbage collection of only those elements in the static area250is shown inFIG. 3. In this first exemplary mechanism, the identification of the element in the static area250with which to begin garbage collection is performed using a pointer310, which may be stored in a data structure associated with the linked list200, to the head of the linked list200and an offset320into the linked list200determined based on a known size of the add/remove area240, which may also be stored in the data structure associated with the linked list200. Using this pointer310and offset320, an element330in the static area250may be identified.

The garbage collector340of the present invention may then access this pointer310and offset320to identify the element330. Once this starting element330is identified, the garbage collector340may traverse the elements in the static area250of the linked list200until a last element350in the linked list200is encountered. Those elements having their garbage collection flag230set will be garbage collected by the garbage collector340as it traverses the linked list200.

In the meantime, while garbage collection is being performed on the static area250of the linked list200, elements may be added and/or removed from the add/remove area240of the linked list since a lock on the linked list200has not been acquired. In this way, processors may continue to access the linked list200without having their accesses synchronized. As a result, the performance losses due to synchronization experienced in prior art mechanisms for removing elements from a linked list are avoided.

FIG. 4shows an alternative mechanism for identifying a starting point with which to begin garbage collection according to the present invention. As shown inFIG. 4, dummy elements410-430are added to the linked list200in the add/remove area240. The garbage collector440maintains a pointer450to the first dummy element410added to the linked list and continues to add dummy elements up to a known size of the add/remove area.

While these dummy elements410-430, are added, additional linked list elements may be added in the add/remove area240, such as element460. The addition of these linked list elements may push dummy elements, such as dummy element410, into the static area250. As a result, the garbage collection may not begin with the first element in the static area250and may actually begin with an element further down the list in the static area250.

Once the dummy elements410-430are added, the garbage collector440starts garbage collection at the element470after the first dummy element410added (or before the first dummy element if a bottom-up linked list is utilized). The garbage collection may then continue through the list of elements in the static area250of the linked list200.

Other mechanisms for identifying a starting element from which to begin garbage collection may be utilized without departing from the spirit and scope of the present invention. The key concept of the present invention is that such garbage collection may be performed virtually simultaneously with addition and/or removal of other elements in the linked list. As a result, a lock on the entire linked list is not required to perform garbage collection.

FIG. 5is a flowchart outlining an exemplary operation of an embodiment of the present invention. As shown inFIG. 5, the operation starts with marking of elements for removal from the linked list (step510). Such marking may involve, for example, setting a garbage collection flag associated with the linked list elements that are to be removed.

Thereafter, a static area of the linked list is identified and thus, an element from which to start garbage collection is identified (step520). This may be done in any of a number of different ways including using a pointer and offset or a pointer to a first added dummy entry, as described previously.

Finally, the elements in the static area of the linked list are stepped through by the garbage collector and those marked for removal are removed (step530) and the operation ends. WhileFIG. 5illustrates these steps510-530as one contiguous method, in actuality the steps510-530may be performed as part of different stages of processing and may be performed at different times. For example, step510may be performed synchronously and may not be part of the actual garbage collection stage. Similarly, steps520and530may be performed at a different time from when the elements in the list are marked for removal and may be performed repeatedly.

An alternative mechanism may be used for removal of elements from a linked list as depicted inFIG. 6. As shown inFIG. 6, when an element610is to be removed from a linked list600, the “next element” pointer of the element620preceding the element610to be removed (or after the element to be removed if a bottom-up linked list is utilized) is set to the “next element” pointer of the element610to be removed. As a result, the element620now points to the element630after the element610to be removed (or before the element to be removed if a bottom-up linked list is utilized). As a result, any process traversing the linked list after this modification will not encounter the element610.

After having modified the pointers of the elements in this manner, the garbage collector then removes any global references to the element being removed (generally by replacing them with a reference to the successor in the linked list). Next the garbage collector broadcasts a message to all of the processors in the multiprocessor system. The message is preferably an interrupt message at an interrupt priority level which is the same as read processes for reading elements of the linked list. Thus, the processors can only receive and handle the message when the processor is at a less favored interrupt priority.

Each message broadcast has a message handler, i.e. code that runs when the message is received, associated with it. When the message is received by each processor they call this code associated with the message and then return a response to the processor that sent the message. For purposes of the present invention, the message handler need only return a response to the broadcast message.

Once the message is sent, the garbage collector then awaits a response from each of the processors. Once each of the processors responds, it is known that there are not local variables on the processors that hold a pointer to the element being removed. It is known that no local variable holds a pointer to the element being removed because the only place in which local variables get pointers to elements of the list are at some favored priority. The ability to get to the item being removed via the list has been removed since the pointers in the list elements have been modified so that the element being removed is not accessed. In addition, global pointers to the element have been removed. Furthermore, because each processor responded to the message broadcast, each processor was at least temporarily at an interrupt priority less favored than that used to examine the list. Thus, after responding to the message, each processor then has no way of getting a pointer to the element being removed. Since there is no local variable that holds a pointer to the element being removed, the garbage collector may remove the element610.

FIG. 7is a flowchart outlining an exemplary operation of the present invention according to this alternative embodiment. As shown inFIG. 7, the operation starts with modifying a pointer of a previous element to have a value of a next pointer of the element being removed (step710). Global pointers to the element being removed are modified to point to another element of the list (generally the next element) (step715).

A message is then broadcast to the processors of the multiprocessor system (step720) and the operation waits to receive a response from each of the processors (step730). A determination is made as to whether each processor has responded to the broadcast message (step740). If all processors have not responded then the operation returns to step730and continues to wait for responses from all processors. Once all processors have responded, the element is recycled (step760). The operation then ends.

Thus, the present invention provides apparatus, methods and computer program products for removal of elements from a linked list which allow for accessing of other elements in the list at substantially the same time as the removal of the elements. In this way, the performance penalties due to having to lock or otherwise protect the linked list from alteration during the removal process are avoided.

While the present invention has been described in terms of a top-down linked list or a bottom-up linked list, the present invention is not limited to such. Rather, any manner of providing a linked list is intended to be within the spirit and scope of the present invention. Thus, linked lists in which elements are added to a middle portion of the linked list may be included in the scope of the present invention. Other ways in which linked lists are implemented are also included.

Moreover, while the present invention has been described in terms of alternative approaches to performing removal of elements in a linked list, the invention is not limited to such.