MPP scheduling in DB/DC and DCCTL environments

After an MPP region has been initialized, it can execute an application program within its virtual storage. By using the scheduling algorithm, the control program selects a message for processing. Using the transaction code, the appropriate application program is loaded into dependent region storage from the IMS.PGMLIB data set.

The application program is identified by the PSB name that was declared in system definition to be associated with that transaction code. The convention used by IMS TM for MPPs is that the application program name is the same as the PSB name. The first message segment is then made available from the message queue, and control is passed to the application program.

The scheduling algorithm also controls the amount of processing performed by the application program. You can specify a limit to the number of messages processed in the scheduling of one program. When this number is reached, IMS TM does the following:

• If any equal or higher priority transactions are queued, IMS TM terminates the application program. The region becomes available for another program to be scheduled into its storage. IMS uses the scheduling algorithm to choose the program to schedule.
• If no equal or higher priority transactions are queued and messages are still queued for the current application program, the region goes through quick reschedule and returns the next message to the application program.
• If equal priority transactions are enqueued, IMS allows the transaction to quick reschedule if the PROCLIM value has not been reached. If the PROCLIM value has been reached, IMS disallows the quick reschedule and processes the other transactions.
  Quick rescheduling is also affected by the IMS scheduling algorithm, which takes into account the following factors and more:

  • MAXRGN value
  • PARLIM value
  • Current number of regions in which the transaction is scheduled
• If no more messages exist for the scheduled transaction, IMS TM determines if other work for the region is ready to be processed.
• If no additional work is ready to be processed, IMS TM determines if the region can become pseudo wait-for-input (pseudo WFI). This determination causes one of the following actions:
  • If the region is eligible for pseudo WFI, the region remains scheduled for the transaction and waits until another message is entered for the region. If the next message is for the scheduled transaction, the message is passed to the application program. If the next message is for a different transaction, IMS TM terminates the application program and schedules a new application program to process the new message.
  • If the region is not eligible to become a pseudo WFI, IMS TM tells the application program that no more messages exist, and the application program terminates.

The master terminal operator directly schedules batch message programs by the entry of the JCL to start the batch message region. The program and PSB to be used are explicitly given in the EXEC statement.