No abstract available.

Workstation clusters are becoming an interesting alter-native to dedicated multiprocessors. In this environment, the probability of a failure, during an application’s exeeution, increases with the execution time and the number of work-stations used. If no provision is made for handling failures, it is unlikely that long running applications will terminate successfully. One solution to this problem is process check-pointing. This paper presents a checkpoint protocol for a multi-threaded distributed shared memory system based on the en-try consistency memory model. The protocol allows trans-parent recovery from single node failures and, in some cases, from multiple node failures. A simple mechanism is used to determine if the system can be brought to a consistent state in the event of multiple machine crashes. The protocol keeps a distributed log of shared data ac-cessesin the volatile memory of the processes, taking advan-tage of the independent failure characteristics of workstation clusters. Periodically, or whenever the log reaches a high-water mark, each process checkpoints its state, independently from the others. The protocol needs no extra messagesdur-ing the failure-fke period, since atl checkpoint control in-formation is piggybacked on the memory coherence protocol messages. 1

Comandos is a project within the European Strategic Programme for Research on Information Technology - ESPRIT and it stems from the identified need of providing simpler and more integrated environments for application development in large distributed systems.

Abstract This article analyses the structure of the distributed operating system of the SMD project— DIOS. The description of the basic structure is followed by a global view of the services and the supporting kernel of the system. Each service is analysed according to its functionality and implementation. DIOS was implemented on bare machines and on top of commercial operating systems. The port of the system to hetereogeneous environments is discussed and some implementation details are described. In parallel with the research on DIOS a multiprocessor station has been developed. These two projects had clear influences on each other; to illustrate some of the major interdependencies, the performance of DIOS in their particular architecture is analysed. The paper ends with information on the current status of the project and on some planned directions of work.