Measuring Thread Timing to Assess the Feasibility of Early-bird Message Delivery

Early-bird communication is a communication/computation overlap technique that combines fine-grained communication with partitioned communication to improve application run-time. Communication is divided among the compute threads such that each individual thread can initiate transmission of its portion of the data as soon as it is complete rather than waiting for all of the threads. However, the benefit of early-bird communication depends on the completion timing of the individual threads. In this paper, we measure and evaluate the potential overlap, the idle time each thread experiences between finishing their computation and the final thread finishing. These measurements help us understand whether a given application could benefit from early-bird communication. We present our technique for gathering this data and evaluate data collected from three proxy applications: MiniFE, MiniMD, and MiniQMC. To characterize the behavior of these workloads, we study the thread timings at both a macro level, i.e., across all threads across all runs of an application, and a micro level, i.e., within a single process of a single run. We observe that these applications exhibit significantly different behavior. While MiniFE and MiniQMC appear to be well-suited for early-bird communication because of their wider thread distribution and more frequent laggard threads, the behavior of MiniMD may limit its ability to leverage early-bird communication.