Assigning priorities

By assigning a priority higher than the default priority of 255, it is possible to give preference to individual tasks with regard to activation and, especially, to the utilization of the resource CPU.

As a rule, assigning a variable priority of medium size is sufficient.

Variable priority

Priority should be varied in accordance with the workload in units of 5 or 10 until the desired performance target is attained. Larger priority increments are required when

• many tasks are active

• the active tasks are predominantly I/O-intensive.

Depending on the control target, the following values are recommended for the initial setting:

Main application: TP

Background application: dialog, batch

TP category

If a DB/DC application with several tasks is involved, database systems which operate in multithread mode (SESAM/SQL, UDS) should be assigned a lower priority than the associated DC tasks (UTM, DCAM, ...). This allows a considerable reduction in the necessary communication between the DB and DC tasks for each database call.

If there are two or more TP applications of different weight, the priorities may be assigned incrementally.

Recommended priority range: 130 - 180

DIALOG category

The dialog application is to run in the background. As a rule, no preference is given to individual dialog tasks in this category.

Recommended priority: 210

BATCH category

For pure background batch tasks, the default priority is sufficient. To emphasize tasks within the category, it is sufficient to raise the priority to 240 (maximum).

Recommended priority range: 240 - 255

Main application: Dialog

Background application: TP, batch

DIALOG category

In general, dialog tasks are treated equally as far as priority is concerned.

Recommended priority: 180

TP category

Unlike the setting of category parameters, where an attempt is made to avoid deactivation of TP tasks even when an overload occurs, the background application TP should receive lower priority with regard to the use of the resource CPU.

If, in the case of dialog tasks, loads similar to the batch category are found (compilations, programming sequences within a response time) processing should continue as for the main application TP.

Recommended priority: 210

BATCH category

When operating solely in the background, the default priority is sufficient.

Recommended priority range: 230 - 255

Under normal circumstances, batch tasks are run in the background, with the purpose of taking advantage of free resources. All tasks which are background batch tasks should be executed with the lowest priority (255), whereas all other tasks should have a priority of 210 or better. Regardless of whether the TP or DIALOG category is the main application, batch tasks which execute in the background must consist of compute-intensive programs, so that the system can regulate the use of available resources. These tasks are then made to handle their I/O operations via channels and disk drives which are not being used for foreground tasks.

Fixed priorities

Fixed priorities are provided for special applications with extremely high real-time requirements. The following points should be borne in mind in this connection:

Fixed priorities severely restrict the system's freedom to make decisions. Tasks awaiting activation or initiation cause other tasks to be immediately suppressed when resources are being used to full capacity.

Therefore, to ensure improved system behavior, fixed priorities should be chosen only after a thorough analysis of the workload and the allocation of resources has been made and in conjunction with steps to limit the workload (MAX MPL).

Tasks given a fixed priority should not be compute-intensive, neither in part nor as a whole, since they are subject to stringent demands with regard to freedom from errors.