Abstract: Transactional programming promises to substantially simplify the development and maintenance of correct, scalable, and efficient concurrent programs. Designs for supporting transactional programming using transactional memory implemented in hardware, software, and a mixture of the two have emerged recently. Unfortunately, conventional debugging programs are often inadequate when employed in relation to code that employs transactional memory and new or modified techniques are needed. We describe techniques whereby certain facilities of a transactional memory implementation can be leveraged to provide replay debugging. With replay debugging, the user can examine a partial or complete execution of an atomic block after it has happened—for example, right before the execution commits. Moreover, in some cases the user can modify the replayed execution, and decide to commit the new modified execution instead of the original replayed one.

Abstract: Transactional programming promises to substantially simplify the development and maintenance of correct, scalable, and efficient concurrent programs. Designs for supporting transactional programming using transactional memory implemented in hardware, software, and mixtures of the two have emerged recently. However, conventional debuggers do not provide features and capabilities that could be desirable for debugging programs executed using transactional memory. For example, conventional debuggers are not adapted to provide a coherent or consistent view of variables in a system that employs transactional memory. Because transactional memory implementations provide the “illusion” of multiple memory locations changing value atomically, while in fact they do not, there are challenges involved with integrating debuggers with such programs to provide the user with a coherent view of program execution.

Abstract: Solutions to a value recycling problem that we define herein facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations of the techniques described herein allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). A class of general solutions to value recycling is described in the context of an illustration we call the Repeat Offender Problem (ROP), including illustrative Application Program Interfaces (APIs) defined in terms of the ROP terminology. Furthermore, specific solutions, implementations and algorithm, including a Pass-The-Buck (PTB) implementation are also described. Solutions to the proposed value recycling problem have a variety of uses.

Abstract: The design of nonblocking linked data structures using single-location synchronization primitives such as compare-and-swap (CAS) is a complex affair that often requires severe restrictions on the way pointers are used. One way to address this problem is to provide stronger synchronization operations, for example, ones that atomically modify one memory location while simultaneously verifying the contents of others. We provide a simple and highly efficient nonblocking implementation of such an operation: an atomic k-word-compare single-swap operation (KCSS). Our implementation is obstruction-free. As a result, it is highly efficient in the uncontended case and relies on contention management mechanisms in the contended cases. It allows linked data structure manipulation without the complexity and restrictions of other solutions.

Abstract: Solutions to a value recycling problem that we define herein facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations of the techniques described herein allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). A variety of solutions to the proposed value recycling problem may be implemented. A class of general solutions to value recycling is described in the context of an illustration we call the Repeat Offender Problem (ROP), including illustrative Application Program Interfaces (APIs) defined in terms of the ROP terminology. Furthermore, specific solutions, implementations and algorithm, including a Pass-The-Buck (PTB) implementation are also described. Solutions to the value recycling problem can be applied in a variety of ways to implement dynamic-sized data structures.

Abstract: One embodiment of the present invention provides a system for releasing a memory location from transactional program execution. The system operates by executing a sequence of instructions during transactional program execution, wherein memory locations involved in the transactional program execution are monitored to detect interfering accesses from other threads, and wherein changes made during transactional execution are not committed until transactional execution completes without encountering an interfering data access from another thread. Upon encountering a release instruction for a memory location during the transactional program execution, the system modifies state information within the processor to release the memory location from monitoring.

Abstract: Solutions to a value recycling problem that we define herein facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations of the techniques described herein allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). Indeed, we present several exemplary realizations of dynamic-sized, non-blocking shared data structures that are not prevented from future memory reclamation by thread failures and which depend (in some implementations) only on widely-available hardware support for synchronization. Some exploitations of the techniques described herein allow non-blocking, indeed even lock-free or wait-free, implementations of dynamic storage allocation for shared data structures.

Abstract: A computer system stores a dynamically sized array as a base array that contains references to subarrays in which the (composite) array's data elements reside. Each of the base-array elements that thus refers to a respective subarray is associated with a respective subarray size. Each base-array index is thereby at least implicitly associated with a cumulative base value equal to the sum of all preceding base indexes' associated subarray sizes. In response to a request for access to the element associated with a given (composite-array) index, the array-access system identifies the base index associated with the highest cumulative base value not greater than the composite-array index and performs the access to the subarray identified by the element associated with that base index. Composite-array expansion can be performed in a multi-threaded environment without locking, simply by employing a compare-and-swap or similar atomic operation.

Abstract: One embodiment of the present invention provides a system that facilitates selectively unmarking load-marked cache lines during transactional program execution, wherein load-marked cache lines are monitored during transactional execution to detect interfering accesses from other threads. During operation, the system encounters a release instruction during transactional execution of a block of instructions. In response to the release instruction, the system modifies the state of cache lines, which are specially load-marked to indicate they can be released from monitoring, to account for the release instruction being encountered. In doing so, the system can potentially cause the specially load-marked cache lines to become unmarked. In a variation on this embodiment, upon encountering a commit-and-start-new-transaction instruction, the system modifies load-marked cache lines to account for the commit-and-start-new-transaction instruction being encountered.

Abstract: The Hat Trick deque requires only a single DCAS for most pushes and pops. The left and right ends do not interfere with each other until there is one or fewer items in the queue, and then a DCAS adjudicates between competing pops. By choosing a granularity greater than a single node, the user can amortize the costs of adding additional storage over multiple push (and pop) operations that employ the added storage. A suitable removal strategy can provide similar amortization advantages. The technique of leaving spare nodes linked in the structure allows an indefinite number of pushes and pops at a given deque end to proceed without the need to invoke memory allocation or reclamation so long as the difference between the number of pushes and the number of pops remains within given bounds. Both garbage collection dependent and explicit reclamation implementations are described.

Abstract: We present a methodology for transforming concurrent data structure implementations that depend on garbage collection to equivalent implementations that do not. Assuming the existence of garbage collection makes it easier to design implementations of concurrent data structures, particularly because it eliminates the well-known ABA problem. However, this assumption limits their applicability. Our results demonstrate that, for a significant class of data structures, designers can first tackle the easier problem of an implementation that does depend on garbage collection, and then apply our methodology to achieve a garbage-collection-independent implementation. Our methodology is based on the well-known reference counting technique, and employs the double compare-and-swap operation.

Abstract: A novel linked-list-based concurrent shared object implementation has been developed that provides non-blocking and linearizable access to the concurrent shared object. In an application of the underlying techniques to a deque, non-blocking completion of access operations is achieved without restricting concurrency in accessing the deque's two ends. In various realizations in accordance with the present invention, the set of values that may be pushed onto a shared object is not constrained by use of distinguishing values. In addition, an explicit reclamation embodiment facilitates use in environments or applications where automatic reclamation of storage is unavailable or impractical.

Abstract: One embodiment of the present invention provides a system that facilitates selectively unmarking load-marked cache lines during transactional program execution, wherein load-marked cache lines are monitored during transactional execution to detect interfering accesses from other threads. During operation, the system encounters a release instruction during transactional execution of a block of instructions. In response to the release instruction, the system modifies the state of cache lines, which are specially load-marked to indicate they can be released from monitoring, to account for the release instruction being encountered. In doing so, the system can potentially cause the specially load-marked cache lines to become unmarked. In a variation on this embodiment, upon encountering a commit-and-start-new-transaction instruction, the system modifies load-marked cache lines to account for the commit-and-start-new-transaction instruction being encountered.

Abstract: We explore techniques for designing nonblocking algorithms that do not require advance knowledge of the number of processes that participate, whose time complexity and space consumption both adapt to various measures, rather than being based on predefined worst-case scenarios, and that cannot be prevented from future memory reclamation by process failures. These techniques can be implemented using widely available hardware synchronization primitives. We present our techniques in the context of solutions to the well-known Collect problem. We also explain how our techniques can be exploited to achieve other results with similar properties; these include long-lived renaming and dynamic memory management for nonblocking data structures.

Abstract: We present a technique for implementing obstruction-free atomic multi-target transactions that target special “transactionable” locations in shared memory. A programming interface for using operations based on these transactions can be structured in several ways, including as n-word compare-and-swap (NCAS) operations or as atomic sequences of single-word loads and stores (e.g., as transactional memory).

Abstract: We propose a new form of software transactional memory (STM) designed to support dynamic-sized data structures, and we describe a novel non-blocking implementation. The non-blocking property we consider is obstruction-freedom. Obstruction-freedom is weaker than lock-freedom; as a result, it admits substantially simpler and more efficient implementations. An interesting feature of our obstruction-free STM implementation is its ability to use of modular contention managers to ensure progress in practice.

Abstract: We introduce obstruction-freedom—a new non-blocking condition for shared data structures that weakens the progress requirements of traditional nonblocking conditions, and as a result admits solutions that are significantly simpler and more efficient in the typical case of low contention. We demonstrate the merits of obstruction-freedom by showing how to implement an obstruction-free double-ended queue that has better properties than any previous nonblocking deque implementation of which we are aware. The beauty of obstruction-freedom is that we can modify and experiment with the contention management mechanisms without needing to modify (and therefore reverify) the underlying non-blocking algorithm. In contrast, work on different mechanisms for guaranteeing progress in the context of lock-free and wait-free algorithms has been hampered by the fact that modifications to the “helping” mechanisms has generally required the proofs for the entire algorithm to be done again.

Abstract: Solutions to a value recycling problem that we define herein facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations of the techniques described herein allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). A variety of solutions to the proposed value recycling problem may be implemented. A class of general solutions to value recycling is described in the context of an illustration we call the Repeat Offender Problem (ROP), including illustrative Application Program Interfaces (APIs) defined in terms of the ROP terminology. Furthermore, specific solutions, implementations and algorithm, including a Pass-The-Buck (PTB) implementation are also described. Solutions to the value recycling problem can be applied in a variety of ways to implement dynamic-sized data structures.

Abstract: Solutions to a value recycling problem facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). Some exploitations allow non-blocking, indeed even lock-free or wait-free, implementations of dynamic storage allocation for shared data structures. In some exploitations, our techniques provide a way to manage dynamically allocated memory in a non-blocking manner without depending on garbage collection. While exploitations of solutions to the value recycling problem that we propose include management of dynamic storage allocation wherein values managed and recycled tend to include values that encode pointers, they are not limited thereto.

Abstract: Solutions to a value recycling problem that we define herein facilitate implementations of computer programs that may execute as multithreaded computations in multiprocessor computers, as well as implementations of related shared data structures. Some exploitations of the techniques described herein allow non-blocking, shared data structures to be implemented using standard dynamic allocation mechanisms (such as malloc and free). Indeed, we present several exemplary realizations of dynamic-sized, non-blocking shared data structures that are not prevented from future memory reclamation by thread failures and which depend (in some implementations) only on widely-available hardware support for synchronization. Some exploitations of the techniques described herein allow non-blocking, indeed even lock-free or wait-free, implementations of dynamic storage allocation for shared data structures.