computation. An Obliq computation may involve multiple threads of control within an address space, multiple address spaces on a machine, heterogeneous machines over a local network, and multiple networks over the Internet. Obliq objects have state and are local to a site. Obliq computations can roam over the network, while maintaining network connections.

ly, the remote procedure call problem, which an RPC protocol undertakes to solve, consists of emulating LPC using message passing. LPC has a number of "properties" -- a single procedure invocation results in exactly one execution of the procedure body, the result returned is reliably delivered to the invoker, and exceptions are raised if (and only if) an error occurs. Given a completely reliable communication environment, which never loses, duplicates, or reorders messages, and given client and server processes that never fail, RPC would be trivial to solve. The sender would merely package the invocation into one or more messages, and transmit these to the server. The server would unpack the data into local variables, perform the desired operation, and send back the result (or an indication of any exception that occurred) in a reply message. The challenge, then, is created by failures. Were it not for the possibility of process and machine crashes, an RPC protocol capable of overcomi...

This paper describes Shasta, a system that supports a shared address space in software on clusters of computers with physically distributed memory. A unique aspect of Shasta compared to most other software distributed shared memory systems is that shared data can be kept coherent at a fine granularity. In addition, the system allows the coherence granularity to vary across different shared data structures in a single application. Shasta implements the shared address space by transparently rewriting the application executable to intercept loads and stores. For each shared load or store, the inserted code checks to see if the data is available locally and communicates with other processors if necessary. The system uses numerous techniques to reduce the run-time overhead of these checks. Since Shasta is implemented entirely in software, it also provides tremendous flexibility in supporting different types of cache coherence protocols. We have implemented an efficient cache co...

This paper introduces the C Region Library (CRL), a new all-software distributed shared memory (DSM) system. CRL requires no special compiler, hardware, or operating system support beyond the ability to send and receive messages. It provides a simple, portable shared address space programming model that is capable of delivering good performance on a wide range of multiprocessor and distributed system architectures. We have developed CRL implementations for two platforms: the CM-5, a commercial multicomputer, and the MIT Alewife machine, an experimental multiprocessor offering efficient support for both message passing and shared memory. We present results for up to 128 processors on the CM-5 and up to 32 processors on Alewife. In a set of controlled experiments, we demonstrate that CRL is the first all-software DSM system capable of delivering performance competitive with hardware DSMs. CRL achieves speedups within 30% of those provided by Alewife's native support for shared memory, eve...

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This paper presents a methodology for designing extensible languages for distributed computing.

We introduce River, a data-flow programming environment and I/O substrate for clusters of computers. River is designed to provide maximum performance in the common case --- even in the face of nonuniformities in hardware, software, and workload. River is based on two simple design features: a high-performance distributed queue, and a storage redundancy mechanism called graduated declustering. We have implemented a number of data-intensive applications on River, which validate our design with near-ideal performance in a variety of non-uniform performance scenarios.

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A software distributed shared memory (DSM) system allows shared memory parallel programs to execute on networks of workstations. This thesis presents a new class of protocols that has lower communication requirements than previous DSM protocols, and can consequently achieve higher performance. The lazy release consistent protocols achieve this reduction in communication by piggybacking consistency information on top of existing synchronization transfers. Some of the protocols also improve performance by speculatively moving data. We evaluate the impact of these features by comparing the performance of a software DSM using lazy protocols with that of a DSM using previous eager protocols. We found that seven of our eight applications performed better on the lazy system, and four of the applications showed performance speedups of at least 18%. As part of this comparison, we show that the cost of executing the slightly more complex code of the lazy protocols is far less important than the ...

Distributed shared memory (DSM) is a software abstraction of shared memory on a distributed memory machine. One of the key problems in building an e cient DSM system is to reduce the amount ofcommunication needed to keep the distributed memories consistent. In this paper we presentfour techniques for doing so: (1) software release consistency � (2) multiple consistency protocols � (3) write-shared protocols � and (4) an update-with-timeout mechanism. These techniques have been implemented in the Munin DSM system. We compare the performance of seven application programs on Munin, rst to their performance when implemented using message passing and then to their performance when running on a conventional DSM system that does not embody the above techniques. On a 16-processor cluster of workstations, Munin's performance is within 5 % of message passing for four out of the seven applications. For the other three, performance is within 29 % to 33%. Detailed analysis of two of these three applications indicates that the addition of a function shipping capability would bring their performance within 7 % of the message passing performance. Compared to a conventional DSM system, Munin achieves performance improvements ranging from a few to several hundred percent, depending on the application.