5137ch02.fm
Draft Document for Review October 14, 2014 10:19 am
46
IBM Power Systems E870 and E880 Technical Overview and Introduction
2.2.9 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 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 is raised to the maximum frequency possible to provide the best
performance. If the processors are lightly used, the frequency is lowered to the minimum
frequency, which is potentially far below the nominal shipped frequency, to save energy. The
top frequency that is achieved is based on system type and is affected by environmental
conditions. Also, when running at the maximum frequency, 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 usage to maintain the workload throughput
without using more energy than required to do so. At high processor usage levels, the
frequency is raised above nominal, as in the Favor Performance mode. Likewise, at low
processor usage levels, the frequency is 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 Dynamic Power Saver: Favor Performance and Dynamic Power Saver: Favor Power
modes are tuned to provide both energy savings and performance increases. However, there
might 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. Modifying these
parameters should be done only by advanced users. If you must address any issues
concerning the Tunable Parameters, IBM support personal 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 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 usage 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 might
affect overall performance. The delay times and usage levels for entering and exiting this
mode can be adjusted to allow for more or less aggressive energy savings.
The controls for all of these modes are available on the Advanced System Management
Interface (ASMI) and are described in more detail in a white paper that is found at the
following link:
http://public.dhe.ibm.com/common/ssi/ecm/en/pow03039usen/POW03039USEN.PDF
For more information, see 2.13, “Energy management” on page 105.
