Improving Tool Support for Nested Parallel Regions with Introspection Consistency.

The OpenMP 5 standard defines OMPT-an application programming interface for tools that includes a set of introspection routines. At any point in time, a sampling-based performance tool may invoke these introspection routines from a signal handler to inquire about the nesting of parallel and task regions. Unfortunately, the OpenMP 5 standard doesn't precisely specify what one may observe with these routines when monitoring a program as it executes nested parallel regions. To address this shortcoming, we propose that the OpenMP standard require that an OpenMP implementation supports introspection consistency. This paper defines introspection consistency, describes why tools need it, and explains a novel strategy for implementing it using wait-free coordination between an OpenMP implementation and its OMPT introspection routines. We describe an implementation of this technique in the LLVM OpenMP runtime and evaluate the runtime overhead of supporting introspection consistency in LLVM OpenMP using a microbenchmark for nested parallel regions and SPEC OMP2012.