Steps

I am getting this reported regularly.

The techie usually confirms the server is in Max-Performance mode - Which would be my first question back to the person asking the question.

Typically, don't say how they are monitoring the GHz.

• I am 99% sure they are using older commands and thus listing the nominal frequency and than never changes and mot the current frequency which bounces up to the highest GHz and stays there. 

If the POWER9 is very large config = maximum CPU cores + maximum memory size + loads of high speed adapter (that can generate heat too) + if the model has them, all the disk slots filled (HDD are hot and block air in take space) AND you thrash every LPAR (VM) to "within a inch of its life" the machine might get warm and the GHz can be throttled back a little to reduce heat.

So what AIX command can be used to clarify what is going on . . .

Try using on AIX: lparstat -E 1 3

  Example  

  $ lparstat -E 1 3    System configuration: type=Shared mode=Uncapped smt=8 lcpu=8 mem=8192MB ent=1.00 Power=Dynamic-Performance    Physical Processor Utilisation:     --------Actual--------              ------Normalised------   user   sys  wait  idle      freq    user   sys  wait  idle   ----  ----  ----  ----   ---------  ----  ----  ----  ----  0.796 0.001 0.000 0.203 3.9GHz[119%] 0.950 0.001 0.000 0.049  0.883 0.001 0.000 0.116 3.9GHz[119%] 1.054 0.001 0.000 0.000  0.796 0.001 0.000 0.204 3.9GHz[119%] 0.949 0.001 0.000 0.050  $

• Note the spelling mistake in the CPU mode above:  Dynamic-Performance - this should spelt Max-Performance. Already reported and may get fixed at the next AIX TL
• The 3.9 GHz is the max frequency on my S924 and it hints we are running at 19% overclocking.  The Austin designers don't like the "overclocking" word and would call this 19% extra optimisation because the server is well inside the safe heat envelope. 

nmon - oops!

Even nmon gets it wrong :-(

• The nmon code has been fixed up but awaits the next AIX TL (November 2019) to be released in only the new AIX 7.2 TL4 onwards levels.  If you AIX is older than this you can't get the current CPU GHz.  If viewing nmon online click "r" and look for Current Speed in MHz.  If saving nmon data to a file add to the command line -y dfreq=on and then you will find new lines of data starting with "CPUMHZ" these will be graphed with updated version of the nmon Analyser and nmonchart.

njmon - cool!

Alternative is trying my new njmon tool that reports the current and nominal Frequency in MHz

• The main point of njmon is that it reports the data in JSON format that can be real-time injected in to modern time-series databases and tools for live flexible graphing like InfluxDB+Grafana, Splunk, ELK etc. 
• It gathers many more stats on AIX, VIOS and Linux
• Very little weight data collection and the data is directly transported to you sysadmin server ssh or hostname+port options

Find the YouTube videos on njmon for all the details

Here is a sample graph from my POWER9 S924 - the popup showing live Current GHz and Nominal GHz.

• Time-Series Database = InfluxDB 1.7
• Graphing Engine = Grafana 5.3
• Both open source but you can take the Enterprise version for advanced features and full support or use a Cloud offering.