Tightfit: Adaptive parallelization with foresight

Irregular applications often exhibit data-dependent parallelism: Different inputs, and sometimes also different execution phases, enable different levels of parallelism. These changes in available parallelism have motivated work on adaptive concurrency control mechanisms. Existing adaptation techniques mostly learn about available parallelism indirectly, through runtime monitors that detect pathologies (e.g. excessive retries in speculation or high lock contention in mutual exclusion). We present a novel approach to adaptive parallelization, whereby the effective level of parallelism is predicted directly based on input features, rather than through circumstantial indicators over the execution environment (such as retry rate). This enables adaptation with foresight, based on the input data and not the run prefix. For this, the user specifies input features, which our system then correlates with the amount of available parallelism through offline learning. The resulting prediction rule serves in deployment runs to foresee the available parallelism for a given workload and tune the parallelization system accordingly. We have implemented our approach in TIGHTFIT, a general framework for input-centric offline adaptation. Our experimental evaluation of TIGHTFIT over two adaptive runtime systems and eight benchmarks provides positive evidence regarding TIGHTFIT's efficacy and accuracy.