FlexBulk: Intelligently forming atomic blocks in blocked-execution multiprocessors to minimize squashes

Blocked-execution multiprocessor architectures continuously run atomic blocks of instructions - also called Chunks. Such architectures can boost both performance and software productivity, and enable unique compiler optimization opportunities. Unfortunately, they are handicapped in that, if they use large chunks to minimize chunk-commit overhead and to enable more compiler optimization, inter-thread data conflicts may lead to frequent chunk squashes. In this paper, we present automatic techniques to form chunks in these architectures to minimize the cycles lost to squashes. We start by characterizing the operations that frequently cause squashes. We call them Squash Hazards. We then propose squash-removing algorithms tailored to these Squash Hazards. We also describe a software framework called FlexBulk that profiles the code and transforms it following these algorithms. We evaluate FlexBulk on 16-threaded PARSEC and SPLASH-2 codes running on a simulated machine. The results show that, with 17,000-instruction chunks, FlexBulk eliminates, on average, over 90% of the squash cycles in the applications. As a result, compared to a baseline execution with 2,000-instruction chunks as in previous work, the applications run on average 1.43x faster.