Contention

Enhanced Definition

Contention in the mainframe context refers to the competition among multiple tasks, jobs, or users for access to a shared system resource. It occurs when two or more entities simultaneously attempt to acquire or modify the same resource, leading to delays, performance degradation, and potential bottlenecks. Contention in the mainframe context refers to the competition among multiple tasks, programs, or address spaces for exclusive or shared access to a limited system resource. It occurs when two or more entities attempt to acquire the same resource simultaneously, leading to one or more entities being delayed while waiting for the resource to become available. This competition can significantly impact system performance and transaction response times.

Key Characteristics

- Resource Types: Can involve various mainframe resources such as CPU, I/O devices (DASD, tape), memory, datasets (VSAM, sequential), control blocks, CICS resources (files, programs, transactions), and DB2/IMS database locks.
- Serialization Mechanisms: z/OS and its subsystems (CICS, DB2, IMS) employ various serialization techniques like enqueues (ENQ), latches, and locking protocols to manage concurrent access and prevent data corruption.
- Performance Impact: High contention can significantly increase transaction response times, batch job elapsed times, CPU utilization, and I/O wait times, reducing overall system throughput.
- Detection: Identified through performance monitoring tools such as RMF (Resource Measurement Facility), SMF (System Management Facilities), OMEGAMON, and specific CICS/DB2/IMS monitors.
- Resolution: Often requires application redesign, system parameter tuning, workload balancing, or optimizing resource access patterns.

Use Cases

- Dataset Contention: Multiple batch jobs or CICS regions attempting to update the same VSAM KSDS or sequential dataset concurrently, requiring exclusive access.
- DB2 Lock Contention: Two or more DB2 transactions trying to update the same row, page, or table, resulting in one transaction waiting for the other to release its lock.
- CICS Resource Contention: Multiple CICS transactions competing for a limited number of CICS tasks, program copies, file buffers, or transient data queues.
- Global Resource Serialization (GRS) Contention: When jobs across different z/OS systems in a sysplex compete for an ENQ on a shared resource (e.g., a critical dataset or control block).
- CPU Contention: Occurs when the number of ready-to-run tasks exceeds the available CPU capacity, leading to tasks waiting in the dispatch queue.

Related Concepts

Contention is intrinsically linked to serialization and locking mechanisms (e.g., ENQ/DEQ, latches, DB2 locks) which are designed to manage and mitigate it. It directly impacts system throughput and response time, making performance monitoring tools like RMF and SMF crucial for its detection and analysis. Severe contention can lead to deadlocks, a specific scenario where two or more tasks are perpetually blocked, each waiting for a resource held by another.

Best Practices:

Design for Concurrency: Develop COBOL or PL/I applications to minimize the duration of resource holding and access shared resources efficiently.
Optimize Resource Access: Reduce the scope and duration of locks; for example, in DB2, commit frequently to release locks, or use appropriate isolation levels.
Monitor and Analyze: Regularly use tools like RMF, SMF, and OMEGAMON to identify contention hotspots, analyze wait states, and pinpoint the specific resources involved.
Tune Subsystems: Configure CICS, DB2, IMS, and other subsystems to optimize buffer pools, thread limits, and task dispatching to reduce internal contention.
Effective GRS Management: Properly define ENQ scopes and resource names when sharing resources across a sysplex to prevent unnecessary contention or deadlocks.