Improving performance of simultaneous multithreading CPUs using autonomous control of speculative traces

Abstract

Simultaneous Multithreading (SMT) allows for a processor to concurrently execute multiple independent threads while sharing certain data path components to optimize resource waste. Speculative execution allows for these processors to take advantage of Instruction-Level Parallelism but the penalty for a miss speculation includes the wasting of resources amongst these shared resources where clock cycles are wasted at a time. In this paper we show that an average of 13 % of instructions are flushed as a result of incorrect predictions. These flushed out instructions could have potentially taken up shared resources which other non-speculative threads could have used. This paper proposes a technique that can dynamically adjust how many speculative instructions a thread can rename and decode aiming to diminish the waste of the shared resources. Our simulation results show, with the proposed technique, that the average flushed out instruction rate is reduced by 23 % and average throughput is improved by 13 %.