3. Efficiency, Energy Efficiency and Programming of Accelerated HPC Servers: Highlights of PRACE Studies

During the last few years the convergence in architecture for High-Performance Computing systems that took place for over a decade has been replaced by a divergence. The divergence is driven by the quest for performance, cost-performance and in the last few years also energy consumption that during the life-time of a system have come to exceed the HPC system cost in many cases. Mass market, specialized processors, such as the Cell Broadband Engine (CBE) and Graphics Processors, have received particular attention, the latter especially after hardware support for double-precision floating-point arithmetic was introduced about three years ago. The recent support of Error Correcting Code (ECC) for memory and significantly enhanced performance for double-precision arithmetic in the current generation of Graphic Processing Units (GPUs) have further solidified the interest in GPUs for HPC. In order to assess the issues involved in potentially deploying clusters with nodes consisting of commodity microprocessors with some type of specialized processor for enhanced performance or enhanced energy efficiency or both for science and engineering workloads, PRACE, the Partnership for Advanced Computing in Europe, undertook a study that included three types of accelerators, the CBE, GPUs and ClearSpeed, and tools for their programming. The study focused on assessing performance, efficiency, power efficiency for double-precision arithmetic and programmer productivity. Four kernels, matrix multiplication, sparse matrix-vector multiplication, FFT, random number generation were used for the assessment together with High-Performance Linpack (HPL) and a few application codes. We report here on the results from the kernels and HPL for GPU and ClearSpeed accelerated systems. The GPU performed surprisingly significantly better than the CPU on the sparse matrix-vector multiplication on which the ClearSpeed performed surprisingly poorly. For matrix-multiplication, HPL and FFT the ClearSpeed accelerator was by far the most energy efficient device.