A Validation of DRAM RAPL Power Measurements

Abstract

Recent Intel processors support the Running Average Power Level (RAPL) interface, which among other things provides estimated energy measurements for the CPUs, integrated GPU, and DRAM. These measurements are easily accessible by the user, and can be gathered by a wide variety of tools, including the Linux perf_event interface. This allows unprecedented easy access to energy information when designing and optimizing energy-aware code. While greatly useful, on most systems these RAPL measurements are estimated values, generated on the fly by an on-chip energy model. The values are not documented well, and the results (especially the DRAM results) have undergone only limited validation. We validate the DRAM RAPL results on both desktop and server Haswell machines, with multiple types of DDR3 and DDR4 memory. We instrument the hardware to gather actual power measurements and compare them to the RAPL values returned via Linux perf_event. We describe the many challenges encountered when instrumenting systems for detailed power measurement. We find that the RAPL results match overall energy and power trends, usually by a constant power offset. The results match best when the DRAM is being heavily utilized, but do not match as well in cases where the system is idle, or when an integrated GPU is using the memory. We also verify that Haswell server machines produce more accurate results, as they include actual power measurements gathered through the integrated voltage regulator.