By means of a priority assignment, the user specifies the urgency with which his/her requests are to be executed. There are 3 priority classes:
Values | Meaning | Priority |
0 to 29 | System priorities | high |
30 to 127 | Fixed priorities | medium |
128 to 255 | Variable priorities | low |
The variable and fixed priorities are available to the user. In this form they are referred to as external priorities.
Priorities can be assigned:
by the user at command level with
/SET-LOGON-PARAMETERS /ENTER-JOB /ENTER-PROCEDURE /CHANGE-TASK-PRIORITY
by the user at program level using the TINF macro
by the system administrator or operator as default value in the job class (RUN-PRIORITY) and with /CHANGE-TASK-PRIORITY
The maximum permissible priority must be entered in the job class or in the user catalog.
Variable priorities
Variable priorities are characterized by the dynamic setting of the internal priority using the HRN algorithm (Highest-Response-ratio Next), based on the ratio Elapsed time / CPU time used and taking into account a “start priority.” The start priority is computed on the basis of the specified external priority, the category weights, the performance power of the system, and the current workload.
This makes for expedient operation, even in the case of tasks with a low start priority.
The variable priority is calculated or upgraded at the following times:
at activation time (see section "Activating/deactivating tasks")
at micro time slice runout (see section "Micro Time Slice" (Initiation/deinitiation of tasks))
periodically (every second).
The period check is performed for all active tasks as well as all inactive tasks ready to execute (i.e. those waiting in the “inactive ready” state for authorization to use main memory).
Fixed priorities
In the case of fixed priorities, the predefined external priority remains unchanged. It is converted without change to the internal priority.
Effects of priority
Since the priority is taken into account during both activation and initiation, issuing a priority which deviates from the standard external priority of 255, i.e. a better start priority, has practically the same effect no matter what the workload is, i.e. high, low or excessive.
When an I/O request is placed in the device queue, the priority is not taken into account. The effects of priority increase in direct proportion to the recourse made to the CPU by competing tasks.
Recommendations and guidelines
PRIOR is used to control the allocation of the resources main memory and CPU to user and system tasks in accordance with the preset category and priority parameters.
In this case priority assignments play an especially important part, since they can be used to give priority to individual tasks, whereas category control can only be used to give priority to entire categories. In the case of TP applications in particular, the priority of the central tasks should be raised. Extreme caution should be exercised when working with fixed priorities. They are not subjected to the HRN algorithm and might therefore monopolize the system. Under no circumstances should priorities better than 80 to 100 be issued.
Note
The parameters for the SYS category cannot be changed and are set to the following values:
MIN MPL=30
MAX MPL=64
WEIGHT=512
System tasks have the following priorities:
Permanent system tasks: 22
Exceptions:
TSN VMM: 16
TSN PGE: 1
TSN RMM: 128 at startup and 50 after SYSTEM READYDynamically generated tasks (e.g. DIAA, DSSM): 60
Exceptions (e.g. BCAM): 30
The user should bear in mind that resource shortages cannot be compensated for by the judicious selection of value settings. Although a user with a great deal of experience may be able to alleviate a CPU bottleneck or insufficient main memory, at least for especially important tasks, albeit to the detriment of others, this is scarcely possible when the bottlenecks occur in peripheral devices.
In the sections below, it is assumed that appropriate measures were taken during the capacity planning stage with regard to sizing the system to suit the problems involved. With the main objective being the optimum utilization of the resources installed.