Predictable Thread Coarsening

Abstract

Thread coarsening on GPUs combines the work of several threads into one. We show how thread coarsening can be implemented as a fully automated compile-time optimisation that estimates the optimal coarsening factor based on a low-cost, approximate static analysis of cache line re-use and an occupancy prediction model. We evaluate two coarsening strategies on three different NVidia GPU architectures. For NVidia reduction kernels we achieve a maximum speedup of 5.08x, and for the Rodinia benchmarks we achieve a mean speedup of 1.30x over 8 of 19 kernels that were determined safe to coarsen.