Specifying the number of SAPs IMS allocates for OTMA input messages

To maintain performance and limit the usage of ECSA storage, you can adjust the number save area prefixes (SAPs) that IMS pre-allocates for each OTMA client and the maximum number of SAPs that IMS can allocate dynamically during run time by specifying the DSAP and DSAPMAX parameters in the OTMA DFSYDTx member of the IMS PROCLIB data set.

IMS uses the SAPs with the OTMA input message (OIM) task control block (TCBs) of an OTMA client to process OTMA input messages. IMS requires one SAP for each input message. After IMS finishes processing a message, IMS reuses the SAP for another message.

By default, IMS pre-allocates 18 SAPs for an OTMA client when the client connects. The pre-allocated SAPs remain allocated to the OTMA client until IMS is restarted.

If all SAPs are in use when new input messages are received from an OTMA client, IMS dynamically allocates more SAPs as needed until the maximum number of SAPs is reached. By default, the maximum number of SAPs that can be dynamically allocated to an OTMA client is 500. After dynamically allocated SAPs are no longer needed, IMS gradually cleans them up, until only the pre-allocated SAPs remain.

If IMS cannot dynamically allocate more SAPs quickly enough or the maximum number of SAPs are already allocated, IMS uses selective dispatching to prioritize processing so that higher priority work is processed before lower priority work.

During IMS system checkpoints, if IMS used selective dispatching for OTMA input messages for any length of time since the last system checkpoint, IMS issues message DFS0769I, OIM SELECTIVE DISPATCHING - SAPS.

In most cases, selective dispatching is active only momentarily until IMS allocates more SAPs and does not indicate a problem nor significantly impact performance. However, if IMS uses selective dispatching frequently or you expect regular high volumes of critical work, you might consider adjusting the number of SAPs that IMS pre-allocates to ensure that there are enough to process all incoming OTMA messages. You might also consider increasing the maximum number of SAPs that IMS can allocate to OTMA clients.

Statistics for the usage of SAPs by OIM TCBs are written at an IMS level in the x'450F' dispatcher statistics log record. They are written at an individual OTMA member level in the X'4518' individual TCB dispatcher statistics log record. Examining the statistics in these log records can provide useful information about the occurrences of selective dispatching.

IMS stores SAPs in the extended common storage area (ECSA). Allocating more SAPs uses more ECSA storage.

To specify the number of SAPs that IMS pre-allocates for an OTMA client, specify the DSAP parameter in the OTMA client descriptor in the DFSYDTx member of the IMS PROCLIB data set.
To specify the maximum number of SAPs that IMS can allocate dynamically for an OTMA client, specify the DSAPMAX parameter in the OTMA client descriptor in the DFSYDTx member of the IMS PROCLIB data set.
To specify the OTMA system default value for the number of pre-allocated SAPs, specify the DSAP parameter in the DFSOTMA descriptor in the DFSYDTx member of the IMS PROCLIB data set.
To specify the OTMA system default value for the maximum number of SAPs that IMS can allocate dynamically, specify the DSAPMAX parameter in the DFSOTMA descriptor in the DFSYDTx member of the IMS PROCLIB data set.