Hierarchical local storage: Exploiting flexible user-data sharing between MPI tasks

With the advent of the multicore era, the number of cores per computational node is increasing faster than the amount of memory. This diminishing memory to core ratio sometimes even prevents pure MPI applications to benefit from all cores available on each node. A possible solution is to add a shared memory programming model like Open MP inside the application to share variables between Open MP threads that would otherwise be duplicated for each MPI task. Going to hybrid can thus improve the overall memory consumption, but may be a tedious task on large applications. To allow this data sharing without the overhead of mixing multiple programming models, we propose an MPI extension called Hierarchical Local Storage (HLS) that allows application developers to share common variables between MPI tasks on the same node. HLS is designed as a set of directives that preserve the original parallel semantics of the code and are compatible with C, C++ and Fortran languages and the Open MP programming model. This new mechanism is implemented inside a state-of-the-art MPI 1.3 compliant runtime called MPC. Experiments show that the HLS mechanism can effectively reduce memory consumption of HPC applications. Moreover, by reducing data duplication in the shared cache of modern multicores, the HLS mechanism can also improve performances of memory intensive applications.