Concurrency - Simultaneous execution

Enhanced Definition

Concurrency in the mainframe z/OS environment refers to the operating system's and applications' ability to manage and execute multiple independent tasks, transactions, or programs seemingly at the same time. It allows multiple units of work to make progress simultaneously, often sharing system resources, to maximize throughput and resource utilization on the powerful z/Architecture.

Key Characteristics

- Shared Resources: Concurrent tasks frequently access and potentially update shared resources such as CPU, memory, I/O devices, datasets (e.g., VSAM, sequential files), and database objects (DB2 tables, IMS segments).
- Serialization: Mechanisms like locks, latches, and ENQ/DEQ services are essential to prevent data corruption and ensure data integrity when multiple concurrent tasks attempt to modify the same shared resource.
- Multitasking and Multiprogramming: Concurrency is a direct result of z/OS's core capabilities to manage multiple programs in memory and dispatch them across available processors, giving the illusion of simultaneous execution.
- Workload Management (WLM): z/OS WLM plays a critical role in managing concurrent workloads by dynamically adjusting dispatching priorities and resource allocation to ensure that critical tasks meet their performance goals.
- Application Design: Applications, especially those in COBOL or PL/I for online transaction processing, must be designed to be reentrant or serially reusable to support multiple concurrent users executing the same program code.
- Transaction Processing Focus: High-volume transaction managers like CICS and IMS Transaction Manager are specifically engineered to handle thousands of concurrent transactions efficiently.

Use Cases

- Online Transaction Processing (OLTP): Thousands of CICS or IMS transactions running simultaneously, each processing a user request, accessing shared databases, and updating records.
- Batch Job Execution: Multiple independent batch jobs running concurrently, often accessing different datasets, but sometimes sharing read-only data or even updating different parts of the same database.
- Database Access: Numerous applications, utilities, and TSO users concurrently querying and updating shared DB2 or IMS databases.
- Multi-user TSO Sessions: Many users logged into TSO, each running their own commands, editing files, or executing programs concurrently without interfering with others.
- Middleware Services: IBM MQSeries queue managers processing messages from multiple sending applications and delivering them to multiple receiving applications concurrently.

Related Concepts

Concurrency is intrinsically linked to serialization mechanisms (e.g., ENQ/DEQ services, DB2 locks, IMS locks) which are vital for maintaining data integrity and preventing race conditions when shared resources are accessed. It is a fundamental outcome of z/OS's multitasking and multiprogramming capabilities, allowing multiple programs to reside in memory and share CPU time. The Workload Manager (WLM) is crucial for managing concurrent workloads by dynamically adjusting dispatching priorities and resource allocation. Furthermore, reentrancy and reusability are key application design principles that enable programs (especially in CICS/IMS) to be executed concurrently by multiple users.

Best Practices:

Minimize Lock Contention: Design applications and databases to reduce the duration and scope of locks. Use row-level locking in DB2 where appropriate, and optimize transaction commit frequency to release locks quickly.
Utilize z/OS Serialization Services: Employ ENQ/DEQ for system-wide resource serialization and consider application-level locks or semaphores for finer-grained control within a program.
Design for Reentrancy: Write COBOL, PL/I, or C programs to be reentrant, particularly for CICS or IMS transactions, to allow multiple users to execute the same program copy concurrently without data conflicts.
Optimize I/O Operations: Efficient I/O design, including proper buffer pool sizing (e.g., VSAM LSR/GSR), helps reduce I/O wait times, allowing the CPU to dispatch other concurrent tasks.
Monitor and Tune: Regularly use performance monitoring tools (e.g., RMF, SMF, CICS/DB2 monitors) to identify bottlenecks, lock contention, and resource contention arising from high concurrency, and tune system parameters or application logic accordingly.