GTF - Generalized Trace Facility

Enhanced Definition

The Generalized Trace Facility (GTF) is a powerful z/OS component used to capture detailed, event -driven trace data from various parts of the system. It records specific system events, program execution flow, and I/O operations, providing a granular view into system activity for problem determination and performance analysis.

Key Characteristics

- System-Wide Scope: GTF can trace events across the entire z/OS system, including multiple address spaces, the kernel, and I/O operations.
- Event-Driven: Tracing is highly customizable, allowing users to specify which types of events (e.g., SVCs, EXCPs, I/O, dispatcher activity, program calls) to capture.
- Output Destination: Trace records are typically written to a pre-allocated GTF trace dataset (a sequential dataset) or, in modern z/OS, can be directed to a system logger log stream.
- Operator Controlled: GTF is started and stopped via z/OS operator commands (START GTF with parameters) or through JCL, providing dynamic control over tracing activities.
- Performance Impact: While invaluable, GTF can introduce significant overhead if tracing too many events or running for extended periods, impacting system performance.
- Raw Data: The output is in a raw, unformatted binary format that requires post-processing utilities for human readability.

Use Cases

- Problem Determination: Diagnosing complex system abends, deadlocks, loops, or unexpected program behavior by tracing the sequence of events leading to the issue.
- Performance Analysis: Identifying performance bottlenecks, resource contention, excessive I/O activity, or inefficient program execution paths within an application or the system.
- Application Debugging: Tracing the flow of control within COBOL, PL/I, or assembler programs, especially when they interact heavily with z/OS services (e.g., SVCs, I/O).
- Security Auditing: Monitoring sensitive system calls or program fetches to detect unauthorized access attempts or suspicious system activity.
- Vendor Support: Often requested by IBM or other software vendors to gather detailed diagnostic information when troubleshooting issues with their products.

Related Concepts

GTF is a fundamental diagnostic tool that complements other z/OS monitoring facilities. Unlike SMF (System Management Facilities), which collects summary data for accounting and long-term trends, GTF provides highly detailed, event -level data for short-term, specific problem diagnosis. It often traces SVCs (Supervisor Calls) and EXCPs (Execute Channel Programs), which are critical for understanding how applications interact with the operating system and perform I/O. The raw trace data collected by GTF is typically formatted and analyzed using IPCS (Interactive Problem Control System), which provides powerful commands to interpret the trace records.

Best Practices:

Targeted Tracing: Always specify the minimum set of events and address spaces required to diagnose the problem to minimize system overhead.
Limited Duration: Run GTF for the shortest possible time necessary to capture the relevant events, as prolonged tracing can severely degrade performance.
Adequate Dataset Allocation: Pre-allocate GTF trace datasets with sufficient space to prevent trace data loss due to dataset full conditions.
Monitor System Resources: Continuously monitor CPU utilization, I/O rates, and available storage while GTF is active to detect and mitigate performance impacts.
Post-Processing: Utilize utilities like TRACEFMT or IPCS to effectively format, filter, and analyze the raw GTF trace data, making it readable and actionable.
Security Considerations: Restrict the use of GTF to authorized personnel, as it can capture highly sensitive system and application data.