Progress thread placement for overlapping MPI non-blocking collectives using simultaneous multi-threading

Abstract

Non-blocking collectives have been proposed so as to allow communications to be overlapped with computation in order to amortize the cost of MPI collective operations. To obtain a good overlap ratio, communications and computation have to run in parallel. To achieve this, different hardware and software techniques exists. Dedicated some cores to run progress threads is one of them. However, some CPUs provide Simultaneous Multi-Threading, which is the ability for a core to have multiple hardware threads running simultaneously, sharing the same arithmetic units. Our idea is to use them to run progress threads to avoid dedicated cores allocation. We have run benchmarks on Haswell processors, using its Hyper-Threading capability, and get good results for both performance and overlap only when inter-node communications are used by MPI processes. However, we also show that enabling Simultaneous Multi-Threading for intra-communications leads to bad performances due to cache effects.