Specifications

Chapter 2. Architecture and technical overview 33
Draft Document for Review May 12, 2014 12:46 pm 5102ch02.fm
functions using a hardware accelerator, with a simplified programming model and efficient
communication with the processor and memory resources.
2.1.9 Power management and system performance
The POWER8 processor has power saving and performance enhancing features that can be
used to lower overall energy usage, while yielding higher performance when needed. The
following modes can be enabled and modified in order to use these features.
Dynamic Power Saver: Favor Performance
This mode is intended to provide the best performance. If the processor is being used even
moderately, the frequency will be raised to the maximum frequency possible to provide the
best performance. If the processors are very lightly used, the frequency will be lowered to the
minimum frequency, which is potentially far below the nominal shipped frequency, to save
energy. Note that the top frequency achieved is based on system type and is affected by
environmental conditions. Also note that when running at the maximum frequency,
significantly more energy is being consumed, which means this mode can potentially cause
an increase in overall energy consumption.
Dynamic Power Saver: Favor Power
This mode is intended to provide the best performance per watt consumed. The processor
frequency is adjusted based on the processor utilization to maintain the workload throughput
without using more energy than required to do so. At very high processor utilization levels, the
frequency will be raised above nominal, just as in the favor performance mode above.
Likewise, at very low processor utilization levels, the frequency will be lowered to the
minimum frequency. The frequency ranges are the same for the two Dynamic Power Saver
modes, but the algorithm that determines which frequency to set is different.
Dynamic Power Saver: Tunable Parameters
The modes just discussed (“Dynamic Power Saver: Favor Performance” and “Dynamic Power
Saver: Favor Power”) are tuned to provide both energy savings and performance increases.
However, there may be situations where only top performance is of concern, or, conversely,
where peak power consumption is an issue. The tunable parameters can be used to modify
the setting of the processor frequency in these modes to meet these various objectives. Note
that modifying these parameters should be done only by advanced users. We suggest that, if
there are issues that need to be addressed by the Tunable Parameters, IBM should be
directly involved in the parameter value selection.
Idle Power Saver
This mode is intended to save the maximum amount of energy when the system is nearly
completely idle. When the processors are found to be nearly idle, the frequency of all
processors is lowered to the minimum. Additionally, workloads are dispatched onto a smaller
number of processor cores so that the other processor cores can be put into a low energy
usage state. When processor utilization increases, the process is reversed: The processor
frequency is raised back up to nominal, and the workloads are spread out once again over all
of the processor cores. There is no performance boosting aspect in this mode, but entering or
exiting this mode may affect overall performance. The delay times and utilization levels for
entering and exiting this mode can be adjusted to allow for more or less aggressive energy
savings.
The controls for all modes listed above are available on the Advanced System Management
Interface and are described in more detail in a white paper available at:
http://public.dhe.ibm.com/common/ssi/ecm/en/pow03039usen/POW03039USEN.PDF