Abstract not found

Replication is an area of interest to both distributed systems and databases. The solutions developed from these two perspectives are conceptually similar but differ in many aspects: model, assumptions, mechanisms, guarantees provided, and implementation. In this paper, we provide an abstract and “neutral ” framework to compare replication techniques from both communities. The framework has been designed to emphasize the role played by different mechanisms and to facilitate comparisons. The paper describes the replication techniques used in both communities, compares them, and points out ways in which they can be integrated to arrive to better, more robust replication protocols. 1.

The abstraction of a shared memory is of growing importance in distributed computing systems. Traditional memory consistency ensures that all processes agree on a common order of all operations on memory. Unfortunately, providing these guarantees entails access latencies that prevent scaling to large systems. This paper weakens such guarantees by defining causal memory, an abstraction that ensures that processes in a system agree on the relative ordering of operations that are causally related. Because causal memory is weakly consistent, it admits more executions, and hence more concurrency, than either atomic or sequentially consistent memories. This paper provides a formal definition of causal memory and gives an implementation for message-passing systems. In addition, it describes a practical class of programs that, if developed for a strongly consistent memory, run correctly with causal memory. College of Computing Georgia Institute of Technology Atlanta, Georgia 30332-0280 This ...

We present a new specification for distributed data services that trade-off immediate consistency guarantees for improved system availability and efficiency, while ensuring the long-term consistency of the data. An eventualIy-serializable data service maintains the operations requested in a partial order that gravitates over time towards a total order. It provides clear and unambiguous guarantees about the immediate and long-term behavior of the system. To demonstrate its utility, we present an algorithm, based on one of Ladin, Liskov, Shrira, and Ghemawat [12], that implements this specification. Our algorithm provides the interface of the abstract service, and generalizes their algorithm by allowing general operations and greater flexibility in specifying consistency requirements. We also describe how to use this specification as a building block for applications such as directory services.

This paper compares the throughput and latency of four protocols that provide total ordering. Two of these protocols are measured with and without message packing. We used a technique that bu ers application messages for a short period of time before sending them, so more messages are packed together. The main conclusion of this comparison is that message packing in uences the performance of total ordering protocols under high load overwhelmingly more than any other optimization that was checked in this paper, both in terms of throughput and latency. This improved performance is attributed to the fact that packing messages reduces the header overhead for messages, the contention on the network, and the load on the receiving CPUs. This work was supported by ARPA/ONR grant N00014-92-J-1866 0

Abstract. Sequential consistency is the most widely used correctness condition for multiprocessor memory systems. This paper studies the problem of testing shared-memory multiprocessors to determine if they are indeed providing a sequentially consistent memory. It presents the first formal study of this problem, which has applications to testing new memory system designs and realizations, providing run-time fault tolerance, and detecting bugs in parallel programs. A series of results are presented for testing an execution of a shared memory under various scenarios, comparing sequential consistency with linearizability, another well-known correctness condition. Linearizability imposes additional restrictions on the shared memory, beyond that of sequential consistency; these restrictions are shown to be useful in testing such memories.

We survey results from distributed computing that show tasks to be impossible, either outright or within given resource bounds, in various models. The parameters of the models considered include synchrony, fault-tolerance, different communication media, and randomization. The resource bounds refer to time, space and message complexity. These results are useful in understanding the inherent difficulty of individual problems and in studying the power of different models of distributed computing.

: A shared memory built on top of a distributed system constitutes a Distributed Shared Memory (DSM). If a lot of protocols implementing DSMs in various contexts have been proposed, no set of homogeneous definitions has been given for the many semantics offered by these implementations. This paper provides a suite of such definitions for atomic, sequential, causal, PRAM and a few others consistency criteria. These definitions are based on a unique framework: a parallel computation is defined as a partial order on the set of read and write operations invoked by processes, and a consistency criterion is defined as a constraint on this partial order. Such an approach provides a simple classification of consistency criteria, from the more to the less constrained one. This paper can also be considered as a survey on consistency criteria for DSMs. Key-words: shared memory, distributed system, consistency criteria, partial order. (R'esum'e : tsvp) IRISA, Campus de Beaulieu, 35042 Rennes-C'...

This paper presents and proves correct a distributed algorithm that implements a sequentially consistent collection of shared read/update objects. This algorithm is a generalization of one used in the Orca shared object system. The algorithm caches objects in the local memory of processors according to application needs; each read operation accesses a single copy of the object, while each update accesses all copies. The algorithm uses broadcast communication when it sends messages to replicated copies of an object, and it uses point-to-point communication when a message is sent to a single copy, and when a reply is returned. Copies of all the objects are kept consistent using a strategy based on sequence numbers for broadcasts. The algorithm is presented in two layers. The lower layer uses the given broadcast and point-to-point communication services, plus sequence numbers, to provide a new communication service called a context multicast channel. The higher layer uses a context multi...

Sequential consistency and causal consistency constitute two of the main consistency criteria used to define the semantics of accesses in the shared memory model. An execution is sequentially consistent if all processes can agree on a same legal sequential history of all the accesses; if processes perceive distinct legal sequential histories of all the accesses, the execution is only causally consistent (legality means that a read does not get an overwritten value). This paper studies synchronization constraints that, when obeyed by operations of a given causally consistent execution, make it sequentially consistent. More precisely, the paper introduces the MSC synchronization (mixed synchronization constraint) which generalizes (1) the known DRF (data race free) and CWF (concurrent write free) synchronizations and (2) a new one called CRF (concurrent read free). The MSC synchronization allows for concurrent conflicting operations on a same object, while ensuring sequential consiste...