Limiting the number of concurrent logon and logoff requests
The OPNDLIM system initialization parameter defines the number of concurrent z/OS® Communications Server logon and logoff requests that are to be processed by CICS®. This parameter can be used in CICS systems that use the z/OS Communications Server as the terminal access method.
The OPNDLIM 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.
This 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. For more information about this parameter, see OPNDLIM system initialization parameter.
Each concurrent logon and logoff requires storage in the CICS dynamic storage areas for the duration of that processing.
Effects
When logons occur automatically with either the CICS CONNECT=AUTO facility or the z/OS Communications Server LOGAPPL facility, large numbers of logons can occur at CICS startup or restart times.
The LOGAPPL facility offers two advantages if an automatic logon facility is required: it requires approximately 3500 bytes less storage in the z/OS Communications Server than the CONNECT=AUTO facility, and it logs terminals back on to CICS each time the device is activated to the z/OS Communications Server, rather than only at CICS initialization.
Limitations
If the value specified for OPNDLIM is too low, real and virtual storage requirements are reduced within CICS, and the z/OS Communications Server buffer requirements might be cut back, but session initialization and terminations take longer.
Suggestions
Use the default value initially and adjust if statistics indicate that too much storage is required in your environment or that the startup time is excessive.
Set OPNDLIM to a value not less than the number of logical units (LU) connected to any single z/OS Communications Server line.
Monitoring
Logon and logoff activities are not reported directly by CICS or any measurement tools, but can be analyzed using the information given in a z/OS Communications Server trace or z/OS Communications Server display command.