CharmPy: A Python Parallel Programming Model

Parallel programming can be extremely challenging. Programming models have been proposed to simplify this task, but wide acceptance of these remains elusive for many reasons, including the demand for greater accessibility and productivity. In this paper, we introduce a parallel programming model and framework called CharmPy, based on the Python language. CharmPy builds on Charm++, and runs on top of its C++ runtime. It presents several unique features in the form of a simplified model and API, increased flexibility, and the ability to write everything in Python. CharmPy is a high-level model based on the paradigm of distributed migratable objects. It retains the benefits of the Charm++ runtime, including dynamic load balancing, asynchronous execution model with automatic overlap of communication and computation, high performance, and scalability from laptops to supercomputers. By being Python-based, CharmPy also benefits from modern language features, access to popular scientific computing and data science software, and interoperability with existing technologies like C, Fortran and OpenMP. To illustrate the simplicity of the model, we will show how to implement a distributed parallel map function based on the Master-Worker pattern using CharmPy, with support for asynchronous concurrent jobs. We also present performance results running stencil code and molecular dynamics mini-apps fully written in Python, on Blue Waters and Cori supercomputers. For stencil3d, we show performance similar to an equivalent MPI-based program, and significantly improved performance for imbalanced computations. Using Numba to JIT-compile the critical parts of the code, we show performance for both mini-apps similar to the equivalent C++ code.