Supporting State-sensitive Computation in a Dataaow System Computation Structures Group Memo 294 March 1989 Managing Shared Resources in a Parallel System

One well-known construct for managing shared resources in a parallel system is Hoare's monitorss4], which encapsulate the shared data, operations on it, and synchronization between operations. This paper describes managers, a version of monitors for the declarative language Idd8]. Like monitors, managers provide encapsulation; however, managers make two key improvements critical to parallel execution. First, operations on a shared resource have internal concurrency. This concurrency allows diicult locking issues encountered in monitors, such as nested monitors, recursive monitors, and the precise semantics of wait and signal, to be resolved programmatically without losing abstraction. Second, the implementation uses low overhead, non-busy-waiting locks for mutual exclusion. This low overhead increases the availability of shared resources, and encourages composite manager structures which reduce bottlenecks. The construct is described in detail, and our experience with applications using managers is described. This experience indicates that managers are eeective programming construct for systems with multiple, dynamic threads of control and eecient context switching.