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 16MB line.

Effects

The IOAREALEN value controls the length of the TIOA which is used to build a message transmitted to the other CICS® system (that is, an outgoing message).

Two values (value1 and value2) can be specified. Value1 specifies the initial size of the TIOA to be used in each session 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 will use value2 whenever the message cannot be accommodated by the value1.

A value of zero causes CICS to get a storage area exactly the size of the outgoing message, plus 24 bytes for CICS requirements.

If the IOAREALEN value is not specified, it defaults to 4KB.

Where useful

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

Limitations

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

Recommendations

For optimum storage and processor utilization, IOAREALEN should be made 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 24 bytes for CICS requirements and round the total up to a multiple of 64 bytes. A multiple of 64 bytes (or less) minus 24 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.

Related tasks
MRO and ISC: performance considerations
CICS intercommunication facilities and performance: overview
Managing queues for intersystems sessions
Using transaction classes DFHTCLSX and DFHTCLQ2 to control storage use
Batching requests (MROBTCH)
Extending the life of mirror transactions (MROLRM and MROFSE)
Controlling the deletion of shipped terminal definitions (DSHIPINT and DSHIPIDL)
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