A concurrent object is a data object shared by concurrent processes. Linearizability is a correctness condition for concurrent objects that exploits the semantics of abstract data types. It permits a high degree of concurrency, yet it permits programmers to specify and reason about concurrent objects using known techniques from the sequential domain. Linearizability provides the illusion that each operation applied by concurrent processes takes effect instantaneously at some point between its invocation and its response, implying that the meaning of a concurrent object’s operations can be given by pre- and post-conditions. This paper defines linearizability, compares it to other correctness conditions, presents and demonstrates a method for proving the correctness of implementations, and shows how to reason about concurrent objects, given they are linearizable.

A wait-free implementation of a concurrent data object is one that guarantees that any process can complete any operation in a finite number of steps, regardless of the execution speeds of the other processes. The problem of constructing a wait-free implementation of one data object from another lies at the heart of much recent work in concurrent algorithms, concurrent data structures, and multiprocessor architectures. In the first part of this paper, we introduce a simple and general technique, based on reduction to a consensus protocol, for proving statements of the form "there is no wait-free implementation of X by Y ." We derive a hierarchy of objects such that no object at one level has a wait-free implementation in terms of objects at lower levels. In particular, we show that atomic read/write registers, which have been the focus of much recent attention, are at the bottom of the hierarchy: they cannot be used to construct wait-free implementations of many simple and familiar da...

A con.curren.t object is a data structure shared by concurrent processes. Conventional techniques for implementing concurrent objects typically rely on criticaI sections: ensuring that only one process at a time can operate on the object. Nevertheless, critical sections are poorly suited for asynchronous systems: if one process is halted or delayed in a critical section, other, non-faulty processes will be unable to progress. By contrast, a concurrent object implementation is non-blocking if it always guarantees that some process will complete an operation in a finite number of steps, and it is wait-free if it guarantees that each process will complete an operation in a finite number of steps. This paper proposes a new methodology for constructing non-blocking aud wait-free implementations of concurrent objects. The object’s representation and operations are written as st,ylized sequential programs, with no explicit synchronization. Each sequential operation is automatically transformed into a non-blocking or wait-free operation usiug novel synchronization and memory management algorithms. These algorithms are presented for a multiple instruction/multiple data (MIM D) architecture in which n processes communicate by applying read, write, and comparekYswa,p operations to a shared memory. 1

This is the nineteenth edition of a (usually) quarterly column that covers new developments in the theory of NP-completeness. The presentation is modeled on that used by M. R. Garey and myself in our book ‘‘Computers and Intractability: A Guide to the Theory of NP-Completeness,’ ’ W. H. Freeman & Co., New York, 1979 (hereinafter referred to as ‘‘[G&J]’’; previous columns will be referred to by their dates). A background equivalent to that provided by [G&J] is assumed, and, when appropriate, cross-references will be given to that book and the list of problems (NP-complete and harder) presented there. Readers who have results they would like mentioned (NP-hardness, PSPACE-hardness, polynomial-time-solvability, etc.) or open problems they would like publicized, should

A number of recent studies have examined the performance of concurrency control algorithms for database management systems. The results reported to date, rather than being definitive, have tended to be contradictory. In this paper, rather than presenting “yet another algorithm performance study,” we critically investigate the assumptions made in the models used in past studies and their implica-tions. We employ a fairly complete model of a database environment for studying the relative performance of three different approaches to the concurrency control problem under a variety of modeling assumptions. The three approaches studied represent different extremes in how transaction conflicts are dealt with, and the assumptions addressed pertain to the nature of the database system’s resources, how transaction restarts are modeled, and the amount of information available to the concurrency control algorithm about transactions ’ reference strings. We show that differences in the underlying assumptions explain the seemingly contradictory performance results. We also address the question of how realistic the various assumptions are for actual database systems.

In a replicated database, a data item may have copies residing on several sites. A replica control protocol is necessary to ensure that data items with several copies behave as if they consist of a single copy, as far as users can tell. We describe a new replica control protocol that allows the accessing of data in spite of site failures and network partitioning. This protocol provides the database designer with a large degree of flexibility in deciding the degree of data availability, as well as the cost of accessing data.

Transactional memory (TM) is an appealing abstraction for programming multi-core systems. Potential target applications for TM, such as business software and video games, are likely to involve complex data structures and large transactions, requiring specific software solutions (STM). So far, however, STMs have been mainly evaluated and optimized for smaller scale benchmarks. We revisit the main STM design choices from the perspective of complex workloads and propose a new STM, which we call SwissTM. In short, SwissTM is lock- and word-based and uses (1) optimistic (commit-time) conflict detection for read/write conflicts and pessimistic (encounter-time) conflict detection for write/write conflicts, as well as (2) a new two-phase contention manager that ensures the progress of long transactions while inducing no overhead on short ones. SwissTM outperforms state-of-theart STM implementations, namely RSTM, TL2, and TinySTM, in our experiments on STMBench7, STAMP, Lee-TM and red-black tree benchmarks. Beyond SwissTM, we present the most complete evaluation to date of the individual impact of various STM design choices on the ability to support the mixed workloads of large applications.

Concurrency and failures are fundamental problems in distributed computing. One likes to think that the mechanisms needed to address these problems can be separated from the rest of the distributed application: in modern words, these mechanisms could be aspectized. Does this however make sense? This paper relates an experience that conveys our initial and indeed biased intuition that the answer is in general no. Except for simple academic examples, it is hard and even potentially dangerous to separate concurrency control and failure management from the actual application.

An optimistic concurrency control technique is one that allows transactions to execute without synchronization, relying on commit-time validation to ensure serializability. Several new optimistic concurrency control techniques for objects in decentralized distributed systems are described here, their correctness and optimality properties are proved, and the circumstances under which each is likely to be useful are characterized. Unlike many methods that classify operations only as Reads or Writes, these techniques syste-matically exploit type-specific properties of objects to validate more interleavings. Necessary and sufficient validation conditions can be derived directly from an object’s data type specification. These techniques are also modular: they can be applied selectively on a per-object (or even per-operation) basis in conjunction with standard pessimistic techniques such as two-phase locking, permitting optimistic methods to be introduced exactly where they will be most effective. These techniques can be used to reduce the algorithmic complexity of achieving high levels of concurrency, since certain scheduling decisions that are NP-complete for pessimistic schedulers can be validated after the fact in time, independent of the level of concurrency. These techniques can also enhance the availability of replicated data, circumventing certain tradeoffs between concurrency

A partition occurs when functioning sites in a distributed system are unable to communicate. This paper introduces a new method for managing replicated data objects in the presence of partitions. Each operation provided by a replicated object has a set. of quorums, which are sets of sites whose cooperation suffices to execute the operation. The method permits an object’s quorums to be adjusted dynamically in response to failures and recoveries. A transaction that is unable to progress using one set of quorums may switch to another, more favorable set, and transactions in different. partitions may progress using different sets. This method has three novel aspects: (1) it supports a wider range of quorums than earlier proposals, (2) it, scales up effectively to large systems because quorum adjustments do not require global reconfiguration, and (3) it, systematically exploits the semantics of typed objects to support more flexible quorum adjustment.