Investigating Average versus Worst-Case Timing Behavior of Data Caches and Data Scratchpads

This paper shows that a program using a time-predictable memory system for data storage can achieve a similar worst-case execution time (WCET) to the average-case execution time (ACET) using a conventional heuristic-based memory system including a data cache. This result is useful within any embedded system where time-predictability and performance are both important, particularly hard real-time systems carrying out intensive data processing activities. It is a counter-example to the conventional wisdom that time-predictable means “slow” in comparison to ACET-focused heuristics. To carry out the investigation, 36 “memory access models” are derived from benchmark programs and assumed to be representative of typical code. The models generate LOAD/STORE instructions to exercise a data cache or scratchpad memory management unit (SMMU). The ACET is determined for the data cache and the WCET is determined for the SMMU. After improvements are applied, results show that the SMMU WCET is within 5% of the data cache ACET for 34 models. In 16 of 36 cases, the SMMU WCET is better than the data cache ACET.