Central Processor

CP

Enhanced Definition

The Central Processor (CP), often referred to as a General Purpose Processor (GPP) in the z/OS context, is the fundamental hardware component responsible for fetching, decoding, and executing instructions. It performs arithmetic, logical, and control operations, serving as the core engine for running the z/OS operating system and all associated workloads.

Key Characteristics

- Instruction Execution: CPs are designed to execute the IBM z/Architecture instruction set, processing both operating system code and application programs.
- Multiprocessing: Modern IBM Z mainframes are highly parallel systems, featuring multiple CPs that can concurrently execute instructions, enabling high throughput and responsiveness.
- Resource for LPARs: CPs are assigned to Logical Partitions (LPARs), providing the processing power for the operating systems and applications running within each LPAR.
- Registers: Each CP contains a set of general-purpose registers, control registers, and floating-point registers used for temporary data storage and managing program execution state.
- Interaction with Central Storage: CPs directly access central storage (main memory) to retrieve instructions and data, and to store results of computations.
- Distinction from Specialty Engines: While CPs handle all general workloads, IBM Z also offers specialty engines like zIIPs, zAAPs, and IFLs to offload specific types of work, thereby reducing GPP utilization and potentially software licensing costs.

Use Cases

- Application Execution: Running core business applications developed in COBOL, PL/I, Assembler, or C/C++ under CICS, IMS TM, or batch environments.
- Operating System Functions: Processing z/OS kernel services, managing system resources, handling interrupts, and dispatching tasks.
- Database Operations: Executing DB2 SQL queries, IMS DB calls, and managing data access for transactional and batch workloads.
- Batch Job Processing: Providing the computational power for JCL-driven batch jobs, including data processing, reporting, and utility functions.
- Middleware Processing: Supporting MQ message processing, Java applications running in JVMs, and other middleware components.

Related Concepts

CPs are the foundational processing units that Logical Partitions (LPARs) consume; PR/SM manages their allocation to LPARs. The Workload Manager (WLM) dynamically manages the dispatching priority of tasks across available CPs within an LPAR to meet defined service goals. CPs constantly interact with central storage for instructions and data, and with the I/O subsystem to initiate and complete data transfers. They are distinct from specialty engines (like zIIPs and zAAPs), which are designed to handle specific workloads more efficiently, freeing up GPPs for general-purpose computing.

Best Practices:

Monitor CP Utilization: Regularly monitor CP utilization using tools like RMF and SMF to identify bottlenecks, forecast capacity needs, and ensure optimal performance.
Optimize WLM Goals: Configure Workload Manager (WLM) service goals effectively to prioritize critical workloads and ensure CPs are allocated efficiently across competing tasks and LPARs.
Leverage Specialty Engines: Identify and configure eligible workloads (e.g., DB2 threads for zIIP, Java for zAAP, Linux for IFL) to run on specialty engines to reduce GPP consumption and potentially lower software costs.
Capacity Planning: Conduct thorough capacity planning to ensure sufficient CP resources are available to handle peak workloads and future growth, preventing performance degradation.
Understand Processor Affinity: Be aware of processor affinity considerations