Tuning automatic installation of terminals
During autoinstall processing, CICS® obtains storage from the control subpool in the extended CICS dynamic storage area (ECDSA), to handle each autoinstall request.
The amount of virtual storage obtained is determined by the length of the CINIT request unit, which varies for different LU types. For a typical autoinstall request from an LU 6.2 terminal, the amount of dynamic virtual storage obtained is 120 -250 bytes.
The principal consumer of CICS resource in autoinstall processing is the autoinstall task (CATA) itself. If, for some reason, the autoinstall process is not proceeding at the rate expected during normal operations, there is a risk that the system could be filled with CATA transaction storage.
Maximum concurrent autoinstalls
The AIQMAX system initialization parameter codes the maximum number of devices that can be queued concurrently for autoinstall.
The AIQMAX value does not limit the total number of devices that can be autoinstalled.
The restart delay parameter
The AIRDELAY system initialization parameter specifies whether you want autoinstalled terminal definitions to be retained by CICS across a restart.
The value of the restart delay is specified as hhmmss and
the default is 000700, which is seven minutes. This
delay means that if a terminal does not log on to CICS within seven minutes after an emergency
restart, its terminal entry is scheduled for deletion.
Setting the restart delay to zero means that you do not want CICS to reinstall the autoinstalled terminal entries from the global catalog during emergency restart. In this case, CICS does not write the terminal entries to the catalog while the terminal is being autoinstalled. This setting can have positive performance effects on the following processes:
- Autoinstall
- By eliminating the I/O activity, autoinstall has a shorter pathlength and becomes more processor-intensive. So, in general, the time taken to autoinstall a terminal is reduced. However, the response time of other tasks might increase slightly because CATA has a high priority and does not have to wait for as much I/O activity.
- Emergency and warm restart
- When no autoinstalled terminal entries are cataloged, CICS has to restore fewer entries from the global catalog data set during emergency restart. Thus, if you have many autoinstalled terminals, the restart time can be improved when restart delay is set to zero.
- Normal shutdown
- CICS deletes AI terminal entries from the global catalog data set during normal shutdown unless they were not cataloged (AIRDELAY=0) and the terminal has not been deleted. If the restart delay is set to zero, CICS has not cataloged terminal entries when they were autoinstalled, so they are not deleted. This setting can reduce normal shutdown time.
You must consider the risk of having some terminal users log on again because tracking has not completed, against the benefits introduced by setting the restart delay to zero. Because catchup takes only a few minutes, the chance of such a takeover occurring is typically small.
The delete delay parameter
The AILDELAY system initialization parameter lets you control how long an autoinstalled terminal entry remains available after the terminal has logged off. The default value of zero means that the terminal entry is scheduled for deletion as soon as the terminal is logged off. Otherwise, CICS schedules the deletion of the TCTTE as a timer task.
In general, setting the delete delay to a nonzero value can improve the performance of CICS when many autoinstalled terminals are logging on and off during the day. However, this setting does mean that unused autoinstalled terminal entry storage is not freed for use by other tasks until the delete delay interval has expired. This parameter provides an effective way of defining a terminal whose storage lifetime is somewhere between the lifetime of an autoinstalled terminal and a statically defined terminal.
The effect of setting the delete delay to a nonzero value can have different effects depending on the value of the restart delay:
Nonzero restart delay When the restart delay is nonzero, CICS catalogs autoinstalled terminal entries in the global catalog.
- Deleting the terminal entry in virtual storage
- An I/O to the catalog and recovery log
- Rebuilding the terminal entry when the terminal logs on again.
Zero restart delay When the restart delay is zero, CICS does not catalog autoinstalled terminal entries in the global catalog whatever value is specified for the delete delay.
If the delete delay is nonzero, CICS retains the terminal entry so that it is reused when the terminal logs back on. This delay can save the processing effect of deleting the terminal entry in virtual storage and the rebuilding of the terminal entry when the terminal logs on again.
Effects
- By using the transaction class limit to restrict the number of autoinstall tasks that can exist concurrently (see Using transaction classes (MAXACTIVE) to control transactions).
- By using the CATA and CATD transactions to install and delete autoinstall terminals dynamically. If you have many devices autoinstalled, shutdown can fail due to the MXT system initialization parameter being reached or CICS becoming short on storage. To prevent this possible cause of shutdown failure, consider putting the CATD transaction in a class of its own to limit the number of concurrent CATD transactions.
- By specifying AIQMAX to limit the number
of devices that can be queued for autoinstall. This setting protects
against abnormal consumption of virtual storage by the autoinstall
process, caused as a result of some other abnormal event.
If this limit is reached, the AIQMAX system initialization parameter affects the LOGON and BIND processing by CICS. CICS requests z/OS® Communications Server to stop passing LOGON and BIND requests to CICS. z/OS Communications Server holds such requests until CICS indicates that it can accept further LOGONs and BINDs (occurs when CICS has processed a queued autoinstall request).
Suggestions
If the autoinstall process is noticeably slowed down by the AIQMAX limit, raise it. If the CICS system shows signs of running out of storage, reduce the AIQMAX limit. If possible, set the AIQMAX system initialization parameter to a value higher than the value reached during normal operations.
Settings of (restart delay=0) and (delete delay= hhmmss>0) are the most efficient for processor and DASD utilization. However, this efficiency is gained at a cost of virtual storage, because the TCT entries are not deleted until the delay period expires.
A value of zero for both restart delay and delete delay is the best overall setting for many systems from an overall performance and virtual storage usage point of view.
If restart delay is greater than zero (cataloging active), the performance of autoinstall is affected by the definition of the global catalog (DFHGCD). The default buffer specifications used by VSAM might not be sufficient in a high activity system.
Because a considerable number of messages are sent to transient data during logon and logoff, consider the performance of these output destinations.
Monitoring
Monitor the autoinstall rate during normal operations by inspecting the autoinstall statistics regularly.