PowerVM

In addition to the four modes mentioned, there is the option of turning on Idle Power Saver.  With Idle Power Saver enabled, the frequency will be reduced after long periods of complete system idleness (minutes).  There are settings for idle power delay time and idle usage thresholds.  Whether Idle Power Saver is on or off, the Dynamic Performance mode will still drop to the minimum frequency when there is little or no workload for milliseconds.  When in Maximum Performance mode or the nominal mode, the frequency will only drop if Idle Power Saver is turned on.  Further information on Idle Power Saver can be found at this web site: https://www.ibm.com/support/knowledgecenter/5148-22L/p8hby/ideal_power.htm. 

Measuring the frequency from AIX
The recommended method for measuring core frequency in AIX is to use mpstat -E 1 1 or lparstat -E 1 1.  The command mpstat -E 1 1 provides individual core frequencies, while lparstat -E 1 1 averages frequencies across all the cores in the lpar.

The lparstat –E 1 1 command will show the current power saver mode and the current frequency:

https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/com.ibm.aix.cmds3/lparstat.htm
https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/com.ibm.aix.cmds3/mpstat.htm
Note: The pmcycles command in AIX should NOT be used for reading the current processor frequency. The recommended approach is to use lparstat - E 1 1 and mpstat -E 1 1 commands as discussed above.

Measuring the frequency from Linux
For Linux there are several ways to look at CPU frequencies depending on the flavor of Linux being used.  The command, lscpu will show the running frequency on some flavors of Linux, but not all.  The command dmesg (dmesg | grep freq) will also provide frequency information.  The file /proc/cpuinfo contains frequency information

On some flavors of Linux, the commands below can be used.
Nominal frequency range:
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies

Energy Scale Frequency range:
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_boost_frequencies

Current running frequency of any core:
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
 
Note that in a purely Linux environment (with OPAL), the Linux operating system sets the frequency on a per core basis.  In this case, the frequency is capped by the POWER On Chip Controller (OCC).

Measuring the frequency from IBM i
For the IBM i operating system, the following methods can be used to collect performance information.

IBM iDoctor for IBM i displays the CPU rate for the IBM i partition over time on the Collection Overview graph.  The CPU rate for the partition is the ratio of scaled to unscaled processor utilized time, expressed as a percentage.  There are two hardware registers that provide energy scaling information; the PURR and the SPURR.  The Processor Utilization Register (PURR) is incremented monotonously as work is performed on a processor.  The Scaled Processor Utilization register (PURR) is scaled in relation to the current frequency.  If the processor is running at nominal frequency, the PURR and the SPURR will accumulate cycles at the same frequency.  If the frequency is running higher than the nominal frequency, the ratio of the SPURR to the PURR will correspond to the increase in frequency.  The processor utilized time reported by IBM i is the accumulation of non-idle virtual processor SPURR and PURR over each time interval.

The Work with System Activity (WRKSYSACT) command displays the Average CPU rate.  The Average CPU rate for the partition is the ratio of scaled to unscaled processor utilized time, expressed as a percentage.  The processor utilized time is the accumulation of non-idle virtual processor SPURR and PURR for the interval since the last refresh.