Which way was the association, more CPUs -> faster CPU-GPU transfer, or slower? By "physical CPUs" I assume you mean cores in a single socket, but in pre-Haswel

Which way was the association, more CPUs -> faster CPU-GPU transfer, or slower? By "physical CPUs" I assume you mean cores in a single socket, but in pre-Haswell multi-socket systems, the uncore clock speed was tied to the core clock speed, so idle CPUs that enter a low power state can actually cause horrible performance because cache coherence snoop signals have to wait for the low-power/slow CPUs to respond. See https://software.intel.com/en-us/forums/intel-moderncode-for-parallel-architectures/topic/379378 https://software.intel.com/en-us/forums/intel-moderncode-for...