Showing papers by "Samir R. Das published in 1993"

A performance study of the cancelback protocol for Time Warp

Abstract: This work presents results from an experimental evaluation of the space-time tradeoffs in Time Warp augmented with the cancelback protocol for memory management. An implementation of the cancelback protocol on Time Warp is described that executes on a shared memory multiprocessor, a 32 processor Kendall Square Research Machine (KSR1). The implementation supports canceling back more than one object when memory has been exhausted. The limited memory performance of the system is evaluated for three different workloads with varying degrees of symmetry. These workloads provide interesting stress cases for evaluating limited memory behavior. We, however, make certain simplifying assumptions (e.g., uniform memory requirement by all the events in the system) to keep the experiments tractable. The experiments are extensively monitored to determine the extent to which various overheads affect performance. It is observed that (i) depending on the available memory and asymmetry in the workload, canceling back several (called the salvage parameter) events at one time may improve performance significantly, by reducing certain overheads, (ii) a performance nearly equivalent to that with unlimited memory can be achieved with only a modest amount of memory depending on the degree of asymmetry in the workload.

Efficient implementation of event sets in Time Warp

Abstract: The implementation of the pending event set (PES) is crucial to the execution speed of discrete event simulation programs. This paper studies the implementation of the PES in the context of simulations executing on parallel computers using the Time Warp mechanism. We present a scheme for implementing Time Warsp's PES based on well-known data structures for priority queues. This scheme supports efficient management of future and past events, especially for rollback and fossil collection operations. A comparative study of several queue implementations is presented. Experiments with a Time Warp system executing on a Kendall Square Research multiprocessor (KSR1) demonstrate that the implementation of the input queue can have a dramatic impact on performance, as large as an order of magnitude, that is much greater than what can be accounted for by simply the reduced execution time to access the data structure. In particular, it is demonstrated that an efficient input queue implementation can also significantly reduce the number of rollbacks, and the efficiency of memory management policies such as Jefferson's cancelback protocol. In the context of this work we also present an improved version of the skew heap that allows dequeueing of arbitrary elements at low cost. In particular, the possibility of dequeueing arbitrary elements will improve memory utilization. This ability is also important in applications where frequent rescheduling may occur, as in ready queues used to select the next logical process to execute.