To improve the individual possibilities of control - depending on the respective user requirements - the tasks are subdivided into categories. There are 16 categories.
Four of these are always provided and are given the following default category names:
SYS: for system tasks only
TP: Transaction tasks
DIALOG: Dialog tasks
BATCH: Batch tasks
Further categories can be set up by systems administration under freely selectable names. Each task is assigned to a category.
In addition, there are four task attributes whose names are the same as the default category names SYS, TP, DIALOG and BATCH. The task attributes are assigned special execution parameters important to task scheduling.
The task attribute TP is different from the other task attributes in that it includes optimized main memory management tailored to fit the requirements of transaction mode (small number of functionally related user tasks, usually with relatively large working set requirements, serving a large number of terminals).
The task attribute TP can be provided by defining it in the job class or by calling the TINF macro (in the latter case the required authorization must have been entered in the user catalog).
The most important attribute of each category is the multiprogramming level (MPL). This refers to the number of tasks per category which are authorized to use main memory, i.e. are in the active state.
Systems support uses the category attributes MIN MPL, MAX MPL and WEIGHT (/MODIFY-TASK-CATEGORIES) to specify the relative importance of the categories with respect to activation (i.e. assignment of authorization to use main memory).
Whether or not a task actually becomes active or is preempted depends on the category attributes, as well as on the system workload and the priority assigned to the task in question (see section “Task priorities”).
Specifying MIN MPL guarantees a certain amount of minimum support for a category. The system sets out to reach the specified MIN-MPL value.
Specifying MAX MPL does not signify a fixed limit, i.e. activation will take place beyond the MPL value as long as there are no resource bottlenecks.
By means of the WEIGHT parameter, the activation or deactivation sequence is controlled. In addition, this parameter has a slight influence on which CPU is allocated.
Modifying the assignment of a task to a category
Systems support can use /MOVE-TASK-TO-CATEGORY to modify the assignment of a task to a category if, for example, different (better) support of this task or a reduction of the load on a category is required (with or without the use of PCS).
Effects of category control
Owing to the above-named attributes, category control has a negligible effect in the lower or normal workload range.
In the full workload range or overload range (i.e. in the event of resource bottlenecks) category control has a major influence and is therefore used for load limitation. Less significant categories are pushed into the background.