SAM: Optimizing Multithreaded Cores for Speculative Parallelism

This work studies the interplay between multithreaded cores and speculative parallelism (e.g., transactional memory or thread-level speculation). These techniques are often used together, yet they have been developed independently. This disconnect causes major performance pathologies: increasing the number of threads per core adds conflicts and wasted work, and puts pressure on speculative execution resources. These pathologies often squander the benefits of multithreading.We present speculation-aware multithreading (SAM), a simple policy that addresses these pathologies. By coordinating instruction dispatch and conflict resolution priorities, SAM focuses execution resources on work that is more likely to commit, avoiding aborts and using speculation resources more efficiently.We design SAM variants for in-order and out-of-order cores. SAM is cheap to implement and makes multithreaded cores much more beneficial on speculative parallel programs. We evaluate SAM on systems with up to 64 SMT cores. With SAM, 8-threaded cores outperform single-threaded cores by 2.33x on average, while a speculation-oblivious policy yields a 1.85x speedup. SAM also reduces wasted work by 52%.