Why is Total DB2 CPU time greater than Total DB2 Elapsed time in the Thread Response History screen?

Document ID : KB000003954
Last Modified Date : 14/02/2018
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Issue:

When client is reviewing a thread's historical information on the Thread Response History screen, they see that the thread's Total DB2 CPU time is greater than the Total DB2 Elapsed time. This condition can exist when the thread used a ZIIP processor during its execution, and the ZIIP processor is faster than the general processor.

Resolution:

The Total DB2 CPU time can be greater than the Total DB2 Elapsed time for those threads where a ZIIP engine was in use. The ZIIP CPU times are added to the overall total of DB2 CPU time. However, the ZIIP CPU times in the DB2 trace record have been normalized to the speed of the general processor.

As a result, the DB2 CPU time reported can be greater than the DB2 elapsed time when the ZIIP processor is faster than the general processor.

The ZIIP CPU time is normalized to the general purpose processor time using the following formula:

zIIP_CPU_Time * zIIP_Normalization_Factor / 256

The ZIIP normalization factor is in field SVT_SUP_NORMALIZATION in the SVT system control block.

The customer can confirm this by reversing the process using the following formula for all ZIIP CPU times, recalculating the total CPU used and seeing if the result is less than the reported elapsed time.

Insight_Reported_zIIP_CPU_Time / (zIIP_Normalization_Factor / 256)