An Empirical Evaluation of Performance-Memory Trade-Offs in Time Warp

The performance of the Time Warp mechanism is experimentally evaluated when only a limited amount of memory is available to the parallel computation. An implementation of the cancelback protocol is used for memory management on a shared memory architecture, viz., KSR to evaluate the performance vs. memory tradeoff. The implementation of the cancelback protocol supports canceling back more than one memory object when memory has been exhausted (the precise number is referred to as the salvage parameter) and incorporates a non-work-conserving processor scheduling technique to prevent starvation. Several synthetic and benchmark programs are used that provide interesting stress cases for evaluating the limited memory behavior. The experiments are extensively monitored to determine the extent to which various factors may affect performance. Several observations are made by analyzing the behavior of Time Warp under limited memory: 1) Depending on the available memory and asymmetry in the workload, canceling back several memory objects at one time (i.e., a salvage parameter value of more than one) improves performance significantly, by reducing certain overheads. However, performance is relatively insensitive to the salvage parameter except at extreme values. 2) The speedup vs. memory curve for Time Warp programs has a well-defined knee before which speedup increases very rapidly with memory and beyond which there is little performance gain with increased memory. 3) A performance nearly equivalent to that with large amounts of memory can be achieved with only a modest amount of additional memory beyond that required for sequential execution, if memory management overheads are small compared to the event granularity. These results indicate that contrary to the common belief, memory usage by Time Warp can be controlled within reasonable limits without any significant loss of performance.