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 invoking 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.
Be aware that CICS performance can be adversely affected by a change to the log defer interval value. Too high a value will delay CICS transaction throughput due to the additional wait before invoking 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 will be added to the buffer during the delay period.
Although the range of possible values for the log defer interval is from 0 to 65535ms, the default of 5ms is considered to be the correct interval when setting the parameter in most cases.
A log defer interval value of less than 5ms will reduce the delay in CICS log manager before invoking the IXGWRITE macro. This might improve the transaction response time, but will increase CPU cost for the system since CICS will buffer fewer journal requests into a given call to the MVS system logger, and so have to invoke the IXGWRITE macro more often.
Conversely, increasing the log defer interval value above 5 will increase the transaction response time, because CICS will increase the delay period before invoking the IXGWRITE macro. However, more transactions will be able to write their own log data into the same log buffer before it is written to the MVS system logger, and hence the total CPU cost of driving IXGWRITE calls will be reduced.
The log defer interval is determined by the LGDFINT system initialization parameter. LGDFINT can be altered with the CEMT SET SYSTEM[LOGDEFER(value)] while CICS is running.
The CICS log stream global statistics include information about the log defer interval (see Logstream statistics).