The log defer interval (LGDFINT)

The LGDFINT system initialization parameter specifies the log defer interval used by CICS® log manager when determining how long to delay a forced journal write request before starting the MVS™ system logger.

The value is specified in milliseconds. Performance evaluations of typical CICS transaction workloads have shown that a value of 5 milliseconds gives the best balance between response time and central processor cost.

CICS performance can be adversely affected by a change to the log defer interval value. Too high a value delays CICS transaction throughput due to the additional wait before starting the MVS system logger.

An example of a scenario where a reduction in the log defer interval might be beneficial to CICS transaction throughout would be where many forced log writes are being issued, and little concurrent task activity is occurring. Such tasks will spend considerable amounts of their elapsed time waiting for the log defer period to expire. In such a situation, there is limited advantage in delaying a call to the MVS system logger to write out a log buffer, since few other log records are added to the buffer during the delay period.

Although the range of possible values for the log defer interval is from 0 to 65535 milliseconds, the default of 5 milliseconds is considered to be the correct interval when setting the parameter in most cases.

A log defer interval value of less than 5 milliseconds reduces the delay in CICS log manager before starting the IXGWRITE macro. This might improve the transaction response time, but increases processor cost for the system because CICS has fewer journal requests into a given call to the MVS system logger, and so must start the IXGWRITE macro more often.

Conversely, increasing the log defer interval value to greater than 5 milliseconds increases the transaction response time, because CICS increases the delay period before starting the IXGWRITE macro. However, more transactions can write their own log data in to the same log buffer before it is written to the MVS system logger, and hence the total processor cost of driving IXGWRITE calls is reduced.

The log defer interval is determined by the LGDFINT system initialization parameter. LGDFINT can be altered with the CEMT SET SYSTEM[LOGDEFER(value)] command while CICS is running.

The CICS log stream global statistics capture information about the log defer interval. See Logstream reports for more information.