Co-Scheduling on Fused CPU-GPU Architectures With Shared Last Level Caches

Fused CPU-GPU architectures integrate a CPU and general-purpose GPU on a single die. Recent fused architectures even share the last level cache (LLC) between CPU and GPU. This enables hardware-supported byte-level coherency. Thus, CPU and GPU can execute computational kernels collaboratively, but novel methods to co-schedule work are required. This paper contributes three dynamic co-scheduling methods. Two of our methods implement workers that autonomously acquire work from a common set of independent work items (similar to bag-of-tasks scheduling). The third method, host-side profiling, uses a fraction of the total work of a kernel to determine a ratio of how to distribute work to CPU and GPU based on profiling. The resulting ratio is used for the following executions of the same kernel. Our methods are realized using OpenCL 2.0, which introduces fine-grained shared virtual memory (SVM) to allocate coherent memory between CPU and GPU. We port the Rodinia Benchmark Suite, a standard suite for heterogeneous computing, to fine-grained SVM and fused CPU-GPU architectures (Rodinia-SVM). We evaluate the overhead of fine-grained SVM and analyze the suitability of OpenCL 2.0's new features for co-scheduling. Our host-side profiling method performs competitively to the optimal choice of executing kernels either on CPU or GPU (hypothetical xor-Oracle). On average, it achieves 97% of xor-Oracle's performance and a 1.43x speedup over using the GPU alone (standard in Rodinia). We show, however, that in most cases it is not beneficial to split the work of a kernel between CPU and GPU compared to exclusively running it on the most suitable single compute device. For a fixed amount of work per device, cache-related stalls can increase by up to 1.75x when both devices are used in parallel instead of exclusively while cache misses remain the same. Thus, not the cost of cache conflicts, but inefficient cache coherence is a major performance bottleneck for current fused CPU-GPU Intel architectures with shared LLC.