Eliminating squashes through learning cross-thread violations in speculative parallelization for multiprocessors

Abstract

With speculative thread-level parallelization, codes that cannot be fully compiler-analyzed are aggressively executed in parallel. If the hardware detects a cross-thread dependence violation, it squashes offending threads and resumes execution. Unfortunately, frequent squashing cripples performance. This paper proposes a new framework of hardware mechanisms to eliminate most squashes due to data dependences in multiprocessors. The framework works by learning and predicting violations, and applying delayed-disambiguation, value prediction, and stall and release. The framework is suited for directory-based multiprocessors that track memory accesses at the system level with the coarse granularity of memory lines. Simulations of a 16-processor machine show that the framework is very effective. By adding our framework to a speculative CC-NUMA with 64-byte memory lines, we speed-up applications by an average of 4.3 times. Moreover, the resulting system is even 23% faster than a machine that tracks memory accesses at the fine granularity of words-a sophisticated system that is not compatible with mainstream cache coherence protocols.