  In High Performance Computing, systems are evaluated based on their
computational throughput. However, performance in contemporary server
processors is primarily limited by power and thermal constraints. Ensuring
operation within a given power envelope requires a wide range of sophisticated
control mechanisms. While some of these are handled transparently by hardware
control loops, others are controlled by the operating system. A lack of
publicly disclosed implementation details further complicates this topic.
However, understanding these mechanisms is a prerequisite for any effort to
exploit the full computing capability and to minimize the energy consumption of
today's server systems. This paper highlights the various energy efficiency
aspects of the AMD Zen 2 microarchitecture to facilitate system understanding
and optimization. Key findings include qualitative and quantitative
descriptions regarding core frequency transition delays, workload-based
frequency limitations, effects of I/O die P-states on memory performance as
well as discussion on the built-in power monitoring capabilities and its
limitations. Moreover, we present specifics and caveats of idle states, wakeup
times as well as the impact of idling and inactive hardware threads and cores
on the performance of active resources such as other cores.
