Controlling the length of the terminal input/output area (SESSIONS IOAREALEN) for MRO sessions

For MRO function shipping, the SESSIONS definition attribute, IOAREALEN, is used. This attribute regulates the length of the terminal input/output area (TIOA) to be used for processing messages transmitted on the MRO link. These TIOAs are located above the 16 MB line.

The IOAREALEN value controls the length of the TIOA that is used to build a message transmitted to the other CICS® system (that is, an outgoing message). You can specify two values (value1 and value2). Value1 specifies the initial size of the TIOA to be used in each session that is defined for the MRO connection. If the size of the message exceeds value1, CICS acquires a larger TIOA to accommodate the message. Only one value is required. However, if value2 is specified, CICS uses value2 whenever the size of the message exceeds value1.

A value of zero causes CICS to get a storage area exactly the size of the outgoing message, plus 600 bytes for CICS requirements. If the IOAREALEN value is not specified, it defaults to 4 KB.

Where useful

The IOAREALEN attribute can be used in the definition of sessions for either MRO transaction routing or function shipping. For MRO transaction routing, the value determines the initial size of the TIOA, whereas in the MRO function shipping environment, the value presents some tuning opportunities.

Limitations

If the IOAREALEN value is too large for most messages transmitted on your MRO link, real and virtual storage might be wasted. If IOAREALEN is smaller than most messages, or zero, excessive FREEMAIN and GETMAIN requests might occur, resulting in additional processor requirements.

Recommendations

For optimum storage and processor utilization, make IOAREALEN slightly larger than the length of the most commonly encountered formatted application data transmitted across the MRO link for which the sessions are defined.

For efficient operating system paging, add 600 bytes for CICS requirements and round up the total to a multiple of 64 bytes. A multiple of 64 bytes (or less) minus 600 bytes for CICS requirements ensures a good use of operating system pages.

How implemented

The TIOA size can be specified in the IOAREALEN attribute of the SESSIONS definition.