Dispatch

Enhanced Definition

In the context of IBM z/OS, **dispatching** is the fundamental operating system process by which the z/OS dispatcher component selects the next ready unit of work (e.g., a TCB or SRB) to be given control of a CPU. It is the core mechanism for allocating CPU time among competing workloads and ensuring efficient utilization of processor resources. In z/OS, **dispatching** is the fundamental process by which the operating system's **dispatcher** selects the next ready unit of work (a `TCB` or `SRB`) to be given control of a CPU. It is crucial for z/OS's ability to manage and execute multiple concurrent workloads by allocating CPU time among competing tasks based on their priorities.

Key Characteristics

- Priority-Driven: The dispatcher uses a sophisticated priority scheme, heavily influenced by the Workload Manager (WLM) goals and importance, to determine which unit of work gets CPU control next.
- Context Switching: When a new unit of work is dispatched, the CPU's execution context (e.g., general-purpose registers, Program Status Word) is saved for the outgoing work and loaded for the incoming work.
- Preemption: Higher-priority work can preempt lower-priority work currently executing on a CPU, ensuring that critical tasks receive timely processing.
- Unit of Work Management: The dispatcher manages and allocates CPU time to Task Control Blocks (TCBs) for user tasks and Service Request Blocks (SRBs) for system-level functions.
- Multiprocessor Awareness: It efficiently manages CPU allocation across multiple processors (CPs, zIIPs, zAAPs) in a multiprocessor environment, optimizing parallel execution.
- Time Slicing: For certain types of work (e.g., TSO users), the dispatcher can implement time slicing, giving each unit of work a small quantum of CPU time before potentially switching to another.

Use Cases

- Batch Job Execution: The dispatcher allocates CPU time to individual job steps and programs within a JCL batch job, allowing them to process data and complete tasks.
- Online Transaction Processing (CICS/IMS): Ensures that CICS transactions or IMS messages are processed efficiently by dispatching the associated TCBs or SRBs that handle the transaction logic.
- System Services and Utilities: Provides CPU time to critical z/OS system components, such as the I/O supervisor, storage manager, and various system utilities, to perform their functions.
- Interactive TSO Sessions: Allocates CPU time slices to users logged into TSO, enabling them to execute commands, edit datasets, and run interactive programs.
- DB2 Query and Transaction Processing: Dispatches the threads (represented by TCBs or SRBs) associated with DB2 applications to execute SQL statements and manage database operations.

Related Concepts

Dispatching is inextricably linked to the Workload Manager (WLM), which sets performance goals and importance for workloads; the dispatcher then uses these WLM-assigned priorities to make real-time CPU allocation decisions. It operates on fundamental z/OS control blocks like the Task Control Block (TCB) and Service Request Block (SRB), which represent the units of work vying for CPU time. Dispatching is the operational mechanism that enables multiprogramming and multiprocessing by allowing multiple programs to concurrently share and utilize the available CPU resources.

Best Practices:

Optimize WLM Policies: Configure WLM policies effectively to assign appropriate importance and goals to critical workloads, directly influencing their dispatching priority and ensuring service level agreements (SLAs) are met.
Monitor CPU Utilization and Dispatching Queues: Regularly use tools like RMF (Resource Measurement Facility) or SMF (System Management Facilities) to monitor CPU utilization, dispatching queue lengths, and individual workload CPU consumption to identify bottlenecks or inefficient dispatching.
Prioritize Critical System Work: Ensure that system-critical tasks (e.g., WLM, SRM, I/O supervisors, security managers) have sufficient dispatching priority to maintain system stability and responsiveness.
Tune Application Performance: Identify and tune applications that consume excessive CPU cycles or enter tight loops, as they can negatively impact the dispatching of other workloads and overall system throughput.
Understand CPU Affinity: While largely managed by z/OS, be aware of concepts like CPU affinity (soft or hard) which can influence where a unit of work is dispatched, potentially impacting cache efficiency, especially in complex multi-LPAR environments.