Supporting OpenMP on Cell

Abstract

The Cell processor is a heterogeneous multi-core processor with one Power Processing Engine (PPE) core and eight Synergistic Processing Engine (SPE) cores. Each SPE has a directly accessible small local memory (256K), and it can access the system memory through DMA operations. Cell programming is complicated both by the need to explicitly manage DMA data transfers for SPE computation, as well as the multiple layers of parallelism provided in the architecture, including heterogeneous cores, multiple SPE cores, multithreading, SIMD units, and multiple instruction issue. There is a significant amount of ongoing research in programming models and tools that attempts to make it easy to exploit the computation power of the Cell architecture. In our work, we explore supporting OpenMP on the Cell processor. OpenMP is a widely used API for parallel programming. It is attractive to support OpenMP because programmers can continue using their familiar programming model, and existing code can be re-used. We base our work on IBM's XL compiler, which already has OpenMP support for AIX multi-processor systems built with Power processors. We developed new components in the XL compiler and a new runtime library for Cell OpenMP that utilizes the Cell SDK libraries to target specific features of the new hardware platform. To describe the design of our Cell OpenMP implementation, we focus on three major issues in our system: 1) how to use the heterogeneous cores and synchronization support in the Cell to optimize OpenMP threads; 2) how to generate thread code targeting the different instruction sets of the PPE and SPE from within a compiler that takes single-source input; 3) how to implement the OpenMP memory model on the Cell memory system. We present experimental results for some SPEC OMP 2001 and NAS benchmarks to demonstrate the effectiveness of this approach. Also, we can observe detailed runtime event sequences using the visualization tool Paraver, and we use the insight into actual thread and synchronization behaviors to direct further optimizations.