Base Priority

Enhanced Definition

In z/OS, Base Priority refers to the initial dispatching priority assigned to a work unit (e.g., a task, SRB, or address space) by the Workload Manager (WLM) or the operating system. It represents the fundamental importance or urgency of the work before any dynamic adjustments are made by the dispatcher.

Key Characteristics

- WLM-Driven: For WLM-managed work, the base priority is primarily derived from the service class definition, specifically the importance level and the defined goal.
- Static Component: It is a relatively stable component of a work unit's overall dispatching priority, serving as the starting point for dynamic priority adjustments.
- Internal Representation: z/OS uses an internal numeric scale (e.g., 0-255 for dispatching priority) where higher numbers typically indicate higher priority.
- Influences Dispatching: Work units with higher base priorities are generally favored by the z/OS dispatcher when competing for CPU resources, affecting their access to the processor.
- SRB and TCB Application: Both Service Request Blocks (SRBs) and Task Control Blocks (TCBs) have associated base priorities that determine their initial standing in the dispatching queue.

Use Cases

- Batch Job Prioritization: Assigning higher base priorities to critical batch jobs (e.g., end-of-month financial processing) through WLM service classes to ensure timely completion.
- Online Transaction Systems: Configuring CICS or IMS regions within WLM service classes with appropriate base priorities to guarantee responsive online transaction processing.
- System Services: Core z/OS system tasks and address spaces (e.g., JES2, VTAM, RMF) are assigned very high base priorities to maintain system stability and responsiveness.
- Development vs. Production: Differentiating between development and production workloads by assigning lower base priorities to non-critical development environments to prevent them from impacting critical production systems.

Related Concepts

Base Priority is a fundamental component of the overall dispatching priority, which also includes dynamic priority adjustments made by the z/OS dispatcher based on factors like CPU consumption, I/O wait, and WLM recommendations. It works in conjunction with WLM *goals* and *importance levels* to ensure that system resources are allocated according to business objectives. While base priority sets the initial standing, WLM's *resource groups* and *service classes* provide the framework for defining and managing these priorities across the system, ultimately influencing how the z/OS *dispatcher* allocates CPU cycles.

Best Practices:

WLM-Centric Prioritization: Leverage WLM service classes and importance levels to define base priorities, avoiding the use of manual, fixed priority assignments (e.g., DISP=H in JCL) for most user workloads.
Avoid Priority Inversion: Carefully design WLM policies to prevent situations where high-priority work waits for low-priority work, which can lead to performance degradation and missed service level objectives.
Monitor with RMF/SMF: Regularly monitor dispatching priority and resource utilization using RMF (Resource Measurement Facility) and SMF (System Management Facilities) data to validate WLM policy effectiveness.
Tiered Approach: Implement a tiered approach to base priorities, assigning the highest priorities to critical system services, followed by production online systems, critical batch, and then less critical workloads.
Periodic Review: Periodically review and adjust WLM service definitions and associated base priorities to align with evolving business requirements and workload characteristics.