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i Can … Understand Scaled CPU Time

March 29, 2010

In the 6.1 release, if you study the Collection Services database files carefully, you will discover a new field called “Scaled CPU time.”  This field in the QAPMJOBMI file is described as follows:

“Scaled CPU time used by thread in microseconds. Ratio of JBSCPU to JBCPU shows the current processor speed in relation to nominal processor speed.” 

There's also

“Total scaled CPU time," which is described as being “Total scaled CPU time used by job in microseconds. Total scaled CPU time used by all threads of a multi-threaded job....”

In the QAPMSYSTEM file you will find:

“Scaled CPU time used (milliseconds). On some system models, the processors may operate at different speeds at different times, depending on power consumption or operating temperature. Ratio of scaled CPU time to CPU time shows the current processor speed in relation to nominal processor speed.”

So just what the heck is this “Scaled CPU time,” anyway?

With POWER6 and POWER7 processors, you can manage the power consumption of these systems by using Active Energy Manager (AEM). AEM is a plug-in to IBM Systems Director that measures, monitors, and manages the power components built into IBM systems. Using AEM, you can understand your power trends, and you can also implement power-capping or power-saving modes. When you use power-capping or power-savings modes and these technologies are in effect, the frequency of the processor may change.  It's under these conditions that the scaled CPU time becomes important.

Scaled CPU time is based on nominal speed, so it shows what the utilization would be if the processor had been running at it's normal speed. CPU time is based upon the current processor speed, whether it's running at the normal speed, or faster or slower.

If the processor is slowed down in order to save energy, that's something that can be important to be aware of if you're concerned about performance. The scaled CPU time allows you to know whether the energy-management features have taken effect on that partition, without manually going into AEM to check the settings or see the trending information.

If you happen to have one of these particular system models that's configured to take advantage of the power-management features, you will also discover a new field on the Work with System Activity (WRKSYSACT) display--“Average CPU Rate,” which displays the ratio of scaled CPU time to CPU time.  You will want to make sure you have PTF SI36329 (or its supercedes) loaded.

Here are some example screen captures that show what WRKSYSACT will display under various conditions; note the change in the Average CPU rate that occurs.

This first example is POWER7 running in normal mode at 100-percent utilization:

Ican 3.29.10 Fig 1
This second example is POWER7 in Static Power Saver mode at 100-percent utilization:

Ican3.29.10 Fig2
This third example is POWER7 with Dynamic Power Optimizer w/Maximum Performance at 100-percent utilization:

Ican 3.29.10 Fig3
As the example screen captures demonstrate, if the processor is running at it's normal speed, the ratio of scaled CPU time to CPU time will be 1. However, if the processor is running at a reduced speed, you will see this ratio be less than 1.  If the processor is running with maximum performance, you may see this ratio be greater than one.

With the October 2009 enhancements to the Performance Data Investigator, you will discover some of the charts include scaled CPU time as well. For example, “CPU Utilization Overview” now includes a chart that displays the ratio of Scaled CPU time with CPU time.  (Note - this chart came from an IBM test system for example purposes and is not representative of what you will see in a normal environment.)

Ican 3.29.10 Fig4
Using the metric finder (the “Search” button on the main Investigate Data task), we can find all the charts within the Performance Data Investigator that show “Scaled CPU Time:"

Ican 3.29.10 Fig5

If you want to know more about the IBM EnergyScale technologies, refer to these whitepapers:

IBM EnergyScale for POWER6 Processor-Based Systems 

IBM Energyscale for POWER7 Processor-Based Systems

Posted March 29, 2010| Permalink