Out-Of-Order Execution May Not Be Cost-Effective on Processors Featuring Simultaneous Multithreading

To achieve a high performance on a single process, superscalar processors now rely on very complex out-of-order execution. Using more and more speculative execution (e.g. value prediction) will be needed for further improvements. On the other hand, most operating systems now offer time-shared multiprocess environments. For the moment most of the time is spent in a single thread, but this should change as the computer will perform more and more independent tasks. Moreover, desktop applications tend to be multithreaded. Most of the users should then be more concerned with the performance throughput on the workload than with the performance of the processor on a single process. Simultaneous multithreading (SMT) is a promising approach to deliver high throughput from superscalar pipelines. In this paper, we show that when executing 4 threads on an SMT processor, out-of-order execution induces small performance benefits over in-order execution. Then, for application domains where performance throughput is more important than ultimate performance on a single application, SMT combined with in-order execution may be a more cost-effective alternative than ultimate aggressive out-of-order superscalar processors or out-of-order execution SMT