Chapter 6: Analyze the System Program Counter

This chapter provides instructions for performing the following tasks using CA Performance Agent System Program Counter (PC) Analysis:

Collecting system-wide PC data using the ADVISE COLLECT SYSTEM_PC command
Reporting on the collected data using the ADVISE COLLECT REPORT SYSTEM_PC command
Using System PC data to investigate how system CPU time is spent
Using System PC data to isolate the cause of High Interrupt Stack activity

System-wide PC (program counter) statistics can be used as a detailed CPU measurement tool. Whenever a performance investigation indicates that CPU usage is high or that the CPU may be a bottleneck, system-wide PC statistics can pinpoint exactly where (PC) and how [IPL (interrupt priority level) or access mode] the CPU is being used.

System-wide PC statistics can be used to answer the following questions:

Why is kernel mode time so high?
- Is the time attributable to any particular process?
- If so, is the time being spent calling system services or is a lot of code executed in kernel mode?
- What amount of kernel time is accounted for in driver code or FDT routines? (FDT routines perform the device-dependent processing of an I/O request.)
- What amount of CPU time is spent in each driver and in RMS routines?
A real-time system is unable to meet its processing requirements but CPU utilization is low. The following questions may come to mind:
- Are real-time interrupts being blocked by code executing at elevated IPL?
- Are time-critical parts of user-mode code being stretched by elevated IPL code executing in response to device interrupts, fork processing, I/O post-processing, or AST delivery?

If you encounter any of the following symptoms, you may want to use the PC Analysis facility to further investigate CPU usage:

Lost CPU time indicated by a value greater than 10% for the Pg+Swp Wait field
High Interrupt Stack activity indicated by a value of the Inter Stack field in the tabular report greater than 20%
Excessive CPU usage indicated by a value of less than 15% Total Idle Time on the tabular report

The following sections tell you how to collect and report system-wide PC data, and provide examples of evaluating System PC reports to investigate the symptoms. For detailed descriptions of the PC Analysis commands and reports, see Description of the Program Counter Analysis Report in this chapter.

This section contains the following topics:

Use the System PC Analysis Facility

Description of the Program Counter Analysis Report