Message queue pool tuning
The message queue pool acts as a buffer to the message queue data sets. Optimize the message queue pool to reduce unnecessary I/O activity.
If message queue I/O occurs during transaction processing, the message queue data sets can be detected as IWAIT items on the IMS Monitor Region IWAIT report. If the LRECL sizes of the short and long message queue buffers are not in correct proportion to each other, I/O can occur, because the mix of transactions uses up one or the other section of the buffer even though the size of the pool seems large enough.
The number of buffers is specified in the MSGQUEUE macro. You can override the number at execution time with the message queue buffer (QBUF) parameter in the EXEC statement. One tuning technique is to decrease the number of buffers until the monitor data shows increasing IWAIT instances marked as message queue I/O. Then you can use the QBUF parameter value plus one page-size unit.
You might want to identify unusual demands on message queue handling. Such demands can include:
- Large multi-segment input and output messages
- Extended processing between segment inserts
- Remote print output
- Terminal operator paging
- Multi-terminal output
- Excessive numbers of program-to-program switches
- Zero-priority messages for BMPs
- SPA sizes for conversational transactions
SPA sizes can adversely affect the performance of the message queues, detracting from the efficiency of single-segment messages that are used as input to well-designed application program processing.
If the security-data section of the OTMA message prefix contains network security credentials, usage of the message queue pool is increased.
The message queue LRECL size must allow for the SPA size, because the SPA is placed on the queue as the first segment of the message. The SPA segment can be split into multiple logical records when it is written to the IMS.LGMSG data set, or a larger LRECL size can be specified. This choice depends on the relative number of transactions requiring large SPA sizes.
You can use the Log Transaction Analysis utility to monitor the I/O queue times. The IMS Statistical Analysis utility produces a Line and Terminal report showing the average size sent and received, but does not show separate long and short message statistics. Average message lengths and counts for each transaction and terminal are part of the information in the system log type X'40' checkpoint records.