Limiting the number of concurrent logon/logoff requests (OPNDLIM)

The OPNDLIM operand defines the number of concurrent VTAM® logons and logoffs that are to be processed by CICS®. In systems running ACF/VTAM Release 3.2 and later, this operand is not necessary and will be ignored. In all other instances this system initialization parameter limits the number of concurrent logon OPNDST and logoff CLSDST requests. The smaller this value, the smaller the amount of storage that is required during the open and close process.

Each concurrent logon/logoff requires storage in the CICS dynamic storage areas for the duration of that processing.

Effects

Particularly when logons are being done automatically with either the CICS CONNECT=AUTO facility or the VTAM LOGAPPL facility, large numbers of logons can occur at CICS startup or restart times. In systems running ACF/VTAM with a release prior to 3.2 this can require significant amounts of storage, which can be reduced with the OPNDLIM operand. In ACF/VTAM Release 3.2 and later systems, this operand is not necessary and will be ignored.

If an automatic logon facility is required, the LOGAPPL facility offers two advantages. It requires approximately 3500 bytes less storage in VTAM than the CONNECT=AUTO facility, and it logs terminals back on to CICS each time the device is activated to VTAM, rather than only at CICS initialization.

Where useful

The OPNDLIM system initialization parameter can be used in CICS systems that use VTAM as the terminal access method.

The OPNDLIM system initialization parameter can also be useful if there are times when all the user community tends to log on or log off at the same time, for example, during lunch breaks.

Limitations

If too low a value is specified for OPNDLIM, real and virtual storage requirements are reduced within CICS and VTAM buffer requirements may be cut back, but session initializations and terminations take longer.

Recommendations

Use the default value initially and make adjustments if statistics indicate that too much storage is required in your environment or that the startup time (DEFINE TYPETERM AUTOCONNECT attribute in CEDA) is excessive.

OPNDLIM should be set to a value not less than the number of LUs connected to any single VTAM line.

How implemented

OPNDLIM is a system initialization parameter.

How monitored

Logon and logoff activities are not reported directly by CICS or any measurement tools, but can be analyzed using the information given in a VTAM trace or VTAM display command.

Related tasks
Networking and VTAM: improving performance
Setting the size of the terminal input/output area (TYPETERM IOAREALEN or TCT TIOAL)
Setting the size of receive-any input areas (RAMAX)
Setting the size of the receive-any pool (RAPOOL)
Using the MVS high performance option (HPO) with VTAM
Adjusting the number of transmissions in SNA transaction flows (MSGINTEG, and ONEWTE)
Using SNA chaining to segment large messages (TYPETERM RECEIVESIZE, BUILDCHAIN, and SENDSIZE)
Adjusting the terminal scan delay (ICVTSD)
Compressing output terminal data streams
Tuning automatic installation of terminals
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