PEEP: Exploiting predictability of memory dependences in SMT processors

Simultaneous multithreading (SMT) attempts to keep a dynamically scheduled processorpsilas resources busy with work from multiple independent threads. Threads with long-latency stalls, however, can lead to a reduction in overall throughput because they occupy many of the critical processor resources. In this work, we first study the interaction between stalls caused by ambiguous memory dependences and SMT processing. We then propose the technique of proactive exclusion (PE) where the SMT fetch unit stops fetching from a thread when a memory dependence is predicted to exist. However, after the dependence has been resolved, the thread is delayed waiting for new instructions to be fetched and delivered down the front-end pipeline. So we introduce an early parole (EP) mechanism that exploits the predictability of dependence-resolution delays to restart fetch of an excluded thread so that the instructions reach the execution core just as the original dependence resolves. We show that combining these two techniques (PEEP) yields a 16.9% throughput improvement on a 4-way SMT processor that supports speculative memory disambiguation. These strong results indicate that a fetch policy that is cognizant of future stalls considerably improves the throughput of an SMT machine.