Dispatcher

Enhanced Definition

The Dispatcher is a fundamental component of the z/OS operating system kernel responsible for allocating CPU resources to ready units of work, primarily Task Control Blocks (TCBs) and Service Request Blocks (SRBs). It determines which unit of work will execute next on an available processor based on its dispatching priority and current state, thereby enabling multiprogramming and multitasking within the mainframe environment. The z/OS component that allocates CPU time to ready tasks based on priority, scheduling algorithms, and workload management policies. The dispatcher is central to the multitasking capabilities of z/OS. The Dispatcher is a core component of the z/OS Supervisor responsible for allocating CPU time to the highest-priority ready work unit on an available processor. It manages the execution of tasks and service requests, ensuring efficient utilization of system resources in a multiprogramming environment. In z/OS, the Dispatcher is a core component of the operating system's supervisor responsible for allocating CPU resources to ready tasks (work units). It selects the next eligible task to be executed on a processor based on its dispatching priority and availability, ensuring efficient utilization of the system's processing power.

Key Characteristics

- CPU Allocation: The Dispatcher is the ultimate arbiter of which ready unit of work (TCB or SRB) gains control of an available CPU at any given moment.
- Priority-Driven: It operates based on dispatching priorities, where work units with higher dispatching priorities are given preference for CPU time over lower-priority ones.
- Preemptive: The z/OS Dispatcher is preemptive, meaning it can interrupt a currently running lower-priority task to allow a higher-priority task that has become ready to execute.
- Manages Work Units: It primarily manages the execution of TCBs (representing user tasks, batch jobs, or system processes) and SRBs (representing system-level services, asynchronous events, or critical system functions).
- State Management: It monitors the state of work units (e.g., ready, running, waiting) and selects from the queue of ready work units to assign to a processor.
- Time-Slicing (for TCBs): For TCBs of the same dispatching priority, the Dispatcher may employ time-slicing to ensure fair CPU distribution among them, although SRBs are generally non-preemptable by TCBs.

Use Cases

- Scheduling Batch Jobs: The Dispatcher allocates CPU time to JES (Job Entry Subsystem) initiators and the batch jobs they run, ensuring they progress according to their WLM defined importance.
- Managing Online Transactions: In environments like CICS or IMS TM, the Dispatcher ensures that transaction-processing tasks receive CPU cycles to handle user requests efficiently, often giving them higher dispatching priorities.
- Executing System Services: Critical system functions, such as I/O completion routines, SRM processing, or VSAM services, are often executed via SRBs which are dispatched with very high priority to maintain system responsiveness.
- Handling Interrupts: After an interrupt (e.g., I/O completion, timer expiration), the Dispatcher is invoked to re-evaluate the ready queue and decide which work unit should run next, ensuring system responsiveness.

Related Concepts

The Dispatcher is tightly integrated with the System Resources Manager (SRM), which sets the dispatching priorities for work units based on WLM policies and system goals. While WLM defines *what* the system should achieve in terms of performance, the Dispatcher is the low-level mechanism that implements *how* CPU resources are allocated to achieve those goals. It operates on TCBs and SRBs, which are fundamental data structures representing units of work in z/OS, and is crucial for the operating system's ability to perform multiprogramming and multitasking.

Best Practices:

Optimize WLM Policies: Carefully define WLM service classes, goals, and importance levels to ensure that critical applications and system functions receive appropriate dispatching priority, allowing the Dispatcher to prioritize