The activity keypoint frequency (AKPFREQ)

The activity keypoint frequency value, AKPFREQ, specifies the number of write requests to the CICS system log stream output buffer required before CICS writes an activity keypoint. A keypoint is a snapshot of inflight tasks in the system at that time.

During emergency restart, CICS® needs to read back for records for only those tasks that are identified in a keypoint. CICS reads the system log backward until the first activity keypoint is encountered (which is the last activity keypoint taken).

Taking a keypoint imposes an overhead on the running system:
  • If you set AKPFREQ too high, such that the keypoint frequency is too low, writing keypoints slows the system for only a short time.
  • If you set AKPFREQ too low, such that the keypoint frequency is too high, the emergency restart time might be short, but you also incur increased processing, because more activity keypoints are processed.

It is advisable to set AKPFREQ to the default value of 4000. With an optimum setting of AKPFREQ, the whole of the system log can remain in the coupling facility.

Increasing the AKPFREQ value increases the amount of primary storage required for the system log. Decreasing the AKPFREQ value has the following effects:
  • Restart time might be reduced.
  • The amount of primary storage required for the system log decreases.
  • Task wait time and processor cycles tend to increase.
  • Paging might increase.
The last two effects can affect system performance, but not significantly.

If you set the AKPFREQ value to zero, emergency restart takes longer. In this situation, CICS cannot perform log tail deletion until shutdown, by which time the system log spills to auxiliary storage. Because there are no activity keypoints, CICS needs to read the whole of the system log, so it needs to retrieve the spilled system log from DASD offload data sets.

Activity keypoint frequency is determined by the AKPFREQ system initialization parameter. You can alter AKPFREQ while CICS is running by using the CEMT SET SYSTEM AKP(value) command.

The CICS log stream global statistics include information about the activity keypoint frequency. See Logstream statistics for more information.

A message, DFHRM0205, is written to the CSMT transient data destination each time that a keypoint is taken.