Investigating Db2 not accounted time

When you encounter high Db2 class 2 not accounted times, you can focus your investigations on certain resources and activities.

About this task

Db2 class 2 not accounted time represents time that Db2 cannot account for. It is Db2 accounting class 2 elapsed time that is not recorded as class 2 CPU time or class 3 suspensions.
The following formula defines Db2 Class 2 Not Accounted Time when no parallelism is involved:
Db2 Class 2 Not Accounted Time = Db2 Class 2 Elapsed time -
(Db2 Class 2 CPU time + Db2 Class 3 suspension time)

Significant increases in class 2 not accounted time might be the result of any of the following conditions:

  • Too much detailed online tracing by monitor programs that use OPn or OPX buffer destinations. This situation is usually the primary cause of high not-accounted-for time on systems that are not CPU-constrained.
  • Running in a very high CPU utilization environment and waiting for CPU cycles, if Db2 work WLM service class goals are not set properly.
  • Running in a high z/OS® paging environment and waiting for storage allocation.
  • If the specialty engines are highly utilized and the SYS1.PARMLIB(IEAOPTxx) member has IIPHONORPRIORITY=NO and IFAHONORPRIORITY=NO setting.
  • Frequent gathering of data set I/O statistics (SMF record type 42 subtype 6)
  • DD consolidation (z/OS parm DDCONS=YES DETAIL) overhead - APAR II07124
  • CF Lock Structure system managed DUPLEXing because Db2 is not informed about related suspension waits.
  • Delays because of asynchronous processing of lock requests when the CF Lock structure is in a remote CF
  • In very I/O intensive environments, the Media Manager might be running out of request blocks.
  • Time spent waiting for parallel tasks to complete (when query parallelism is used for the query). This problem is often a result when the value of the PARAMDEG subsystem parameter is too high.
  • HSM (Hierarchical Storage Manager) data set recall.
  • Waiting for requests to be returned from SNA Db2 Server.
  • Data set open contention related to PCLOSET being too small.
  • Db2 internal suspend and resume looping when several threads are waiting for the same resource.

Procedure

To investigate high Db2 class 2 not accounted times, complete the following investigations:

  • Check for the following types of activities and wait times:
    • Paging activity, by using RMF reports.
    • Processor wait time, by using RMF reports.
    • Return wait time for requests to be returned from VTAM® or TCP/IP, by using RMF reports.
    • The use of online performance monitors. You can sometimes reduce eliminate or significantly reduce high not account times by turning off or reducing the intensity of performance traces that are used by online monitors.
    • Wait time for completion of parallel tasks. A high not accounted time is acceptable if it is caused by wait time for completion of parallel tasks.
  • Check the SER.TASK SWTCH field in the Class 3 Suspensions section of the IBM® OMEGAMON® AI for Db2 accounting reports.