automatic thread distribution for nested parallelism in openmp
OpenMP is becoming the standard programming model for shared-memory parallel architectures. One of its most interesting features in the language is the support for nested parallelism. Previous research and parallelization experiences have shown the benefits of using nested parallelism as an alternative to combining several programming models such as MPI and OpenMP. However, all these works rely on the manual definition of an appropriate distribution of all the available thread across the different levels of parallelism. Some proposals have been made to extend the OpenMP language to allow the programmers to specify the thread distribution.This paper proposes a mechanism to dynamically compute the most appropriate thread distribution strategy. The mechanism is based on gathering information at runtime to derive the structure of the nested parallelism. This information is used to determine how the overall computation is distributed between the parallel branches in the outermost level of parallelism, which is constant in this work. According to this, threads in the innermost level of parallelism are distributed.The proposed mechanism is evaluated in two different environments: a research environment, the Nanos OpenMP research platform, and a commercial environment, the IBM XL runtime library. The performance numbers obtained validate the mechanism in both environments and they show the importance of selecting the proper amount of parallelism in the outer level.
nested parallelism openmp thread clustering