33. CPU Usage Statistics

33.1. Introduction

The CPU usage statistics manager is an RTEMS support component that provides a convenient way to manipulate the CPU usage information associated with each task. The routines provided by the CPU usage statistics manager are:

33.2. Background

When analyzing and debugging real-time applications, it is important to be able to know how much CPU time each task in the system consumes. This support component provides a mechanism to easily obtain this information with little burden placed on the target.

The raw data is gathered as part of performing a context switch. RTEMS keeps track of how many clock ticks have occurred which the task being switched out has been executing. If the task has been running less than 1 clock tick, then for the purposes of the statistics, it is assumed to have executed 1 clock tick. This results in some inaccuracy but the alternative is for the task to have appeared to execute 0 clock ticks.

RTEMS versions newer than the 4.7 release series, support the ability to obtain timestamps with nanosecond granularity if the BSP provides support. It is a desirable enhancement to change the way the usage data is gathered to take advantage of this recently added capability. Please consider sponsoring the core RTEMS development team to add this capability.

33.3. Operations

33.3.1. Report CPU Usage Statistics

The application may dynamically report the CPU usage for every task in the system by calling the rtems_cpu_usage_report routine. This routine prints a table with the following information per task:

  • task id

  • task name

  • number of clock ticks executed

  • percentage of time consumed by this task

The following is an example of the report generated:

+------------------------------------------------------------------------------+
|CPU USAGE BY THREAD                                                           |
+-----------+----------------------------------------+-------------------------+
|ID         | NAME                                   | SECONDS       | PERCENT |
+-----------+----------------------------------------+---------------+---------+
|0x04010001 | IDLE                                   |             0 |   0.000 |
+-----------+----------------------------------------+---------------+---------+
|0x08010002 | TA1                                    |          1203 |   0.748 |
+-----------+----------------------------------------+---------------+---------+
|0x08010003 | TA2                                    |           203 |   0.126 |
+-----------+----------------------------------------+---------------+---------+
|0x08010004 | TA3                                    |           202 |   0.126 |
+-----------+----------------------------------------+---------------+---------+
|TICKS SINCE LAST SYSTEM RESET:                                           1600 |
|TOTAL UNITS:                                                             1608 |
+------------------------------------------------------------------------------+

Notice that the TOTAL UNITS is greater than the ticks per reset. This is an artifact of the way in which RTEMS keeps track of CPU usage. When a task is context switched into the CPU, the number of clock ticks it has executed is incremented. While the task is executing, this number is incremented on each clock tick. Otherwise, if a task begins and completes execution between successive clock ticks, there would be no way to tell that it executed at all.

Another thing to keep in mind when looking at idle time, is that many systems - especially during debug - have a task providing some type of debug interface. It is usually fine to think of the total idle time as being the sum of the IDLE task and a debug task that will not be included in a production build of an application.

33.3.2. Reset CPU Usage Statistics

Invoking the rtems_cpu_usage_reset routine resets the CPU usage statistics for all tasks in the system.

33.4. Directives

This section details the CPU usage statistics manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

33.4.1. cpu_usage_report - Report CPU Usage Statistics

CALLING SEQUENCE:
void rtems_cpu_usage_report( void );
STATUS CODES:

NONE

DESCRIPTION:

This routine prints out a table detailing the CPU usage statistics for all tasks in the system.

NOTES:

The table is printed using the printk routine.

33.4.2. cpu_usage_reset - Reset CPU Usage Statistics

CALLING SEQUENCE:
void rtems_cpu_usage_reset( void );
STATUS CODES:

NONE

DESCRIPTION:

This routine re-initializes the CPU usage statistics for all tasks in the system to their initial state. The initial state is that a task has not executed and thus has consumed no CPU time.

NOTES:

NONE