Elapsed Time

Enhanced Definition

Elapsed time, in the context of IBM z/OS, refers to the total "wall-clock" duration from the initiation to the completion of a job, task, transaction, or process. It encompasses all time spent, including CPU execution, I/O operations, waiting for resources, and queuing for dispatch. In z/OS, **elapsed time** refers to the total wall-clock time that passes from the initiation to the completion of a job, task, program, or transaction. It encompasses all processing time, I/O wait time, CPU wait time, and any other delays encountered during its execution, representing the real-world duration observed by an external clock.

Key Characteristics

- Wall-Clock Measurement: It measures the actual time that passes, as observed by a clock, from start to finish, irrespective of whether the process was actively using the CPU.
- Includes Wait Times: Unlike CPU time, elapsed time includes periods where the job or task is waiting for I/O completion, system resources (e.g., enqueues, locks), or CPU dispatch.
- Performance Indicator: It is a critical metric for evaluating the overall performance and responsiveness of batch jobs, online transactions (e.g., CICS, IMS), and system utilities.
- Affected by Workload: System-wide workload, resource contention, and I/O subsystem performance directly impact the elapsed time of individual processes.
- Service Level Agreement (SLA) Metric: Often a primary metric used in Service Level Agreements (SLAs) to define acceptable completion times for critical workloads.

Use Cases

- Batch Job Monitoring: Tracking the elapsed time of daily, weekly, or monthly batch jobs to ensure they complete within their scheduled windows and meet business requirements.
- Online Transaction Response Time: Measuring the elapsed time for CICS or IMS transactions from user input to screen refresh, directly impacting user experience.
- System Throughput Analysis: Analyzing the elapsed times of various workloads to understand overall system capacity and identify bottlenecks that hinder parallel processing.
- Problem Diagnosis: Investigating unexpected increases in elapsed time for specific jobs or transactions to pinpoint performance issues like I/O contention, CPU saturation, or inefficient code.
- Resource Management: Using elapsed time data to justify resource upgrades or reallocate existing resources more effectively via Workload Manager (WLM) policies.

Related Concepts

Elapsed time is a broader measure than CPU time, as CPU time is only the portion of elapsed time during which the CPU was actively executing instructions for the process. For online systems, elapsed time is often synonymous with response time. It is a key metric that the Workload Manager (WLM) uses to manage system resources, prioritizing workloads to meet defined service goals, which are frequently expressed in terms of elapsed time or response time. Efficient JCL and optimized COBOL code can significantly reduce both CPU time and I/O waits, thereby improving elapsed time.

Best Practices:

Establish Baselines: Regularly monitor and establish baseline elapsed times for critical batch jobs and online transactions to easily identify deviations.
Set Realistic Goals: Define achievable elapsed time goals in SLAs and WLM policies, considering the workload's importance and available system resources.
Utilize Monitoring Tools: Employ performance monitoring tools like RMF, SMF, OMEGAMON, or CICS PA to collect, analyze, and report on elapsed time metrics.
Investigate Anomalies: Promptly investigate any significant increases in elapsed time, using system logs and performance data to diagnose root causes such as enqueue contention, I/O bottlenecks, or CPU overutilization.
Optimize Workloads: Continuously optimize JCL, COBOL programs, DB2 queries, and IMS database access to reduce resource consumption and minimize wait times, thereby improving elapsed time.