MIMD synchronization on SIMT architectures

Abstract

In the single-instruction multiple-threads (SIMT) execution model, small groups of scalar threads operate in lockstep. Within each group, current SIMT hardware implementations serialize the execution of threads that follow different paths, and to ensure efficiency, revert to lockstep execution as soon as possible. These constraints must be considered when adapting algorithms that employ synchronization. A deadlock-free program on a multiple-instruction multiple-data (MIMD) architecture may deadlock on a SIMT machine. To avoid this, programmers need to restructure control flow with SIMT scheduling constraints in mind. This requires programmers to be familiar with the underlying SIMT hardware. In this paper, we propose a static analysis technique that detects SIMT deadlocks by inspecting the application control flow graph (CFG). We further propose a CFG transformation that avoids SIMT deadlocks when synchronization is local to a function. Both the analysis and the transformation algorithms are implemented as LLVM compiler passes. Finally, we propose an adaptive hardware reconvergence mechanism that supports MIMD synchronization without changing the application CFG, but which can leverage our compiler analysis to gain efficiency. The static detection has a false detection rate of only 4%-5%. The automated transformation has an average performance overhead of 8.2%-10.9% compared to manual transformation. Our hardware approach performs on par with the compiler transformation, however, it avoids synchronization scope limitations, static instruction and register overheads, and debuggability challenges that are present in the compiler only solution.