  Dependable power measurements are the backbone of energy-efficient computing
systems. The IBM PowerNV platform offers such power measurements through an
embedded PowerPC 405 processor: The On-Chip Controller (OCC). Among other
system-control tasks, the OCC provides power measurements for several domains,
such as system, CPU, and GPU. This paper provides a detailed description and an
in-depth evaluation of these OCC-provided power measurements. For that, we
describe the provided interfaces themselves and experimentally verify their
overhead (3.6 us to 10.8 us per access) and readout rate (24.95 Sa/s). We also
study the consistency of the reported sensor readouts across the measurement
domains and compare it to externally measured data. Furthermore, we estimate
the internal sampling rate (1996 Sa/s) by provoking aliasing errors with
artificial workloads, and quantify the errors that such aliasing could
introduce in practice (for power consumption of processors 12% in our
experimental worst-case scenario). Given these insights, practitioners using
the IBM PowerNV platform can assess the quality of the embedded measurements,
permitting sought-after energy efficiency improvements.
