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The cpuload package provides a way to estimate the cpuload. It gives

an estimated percentage load for the last 100 milliseconds, 1 second

and 10 seconds.

The package allows the CPU load to be estimated. The measurement code

must first be calibrated to the target it is running on. Once this

has been performed the measurement process can be started. This is a

continuous process, so always providing the most up to data

measurements. The process can be stopped at any time if required. Once

the process is active, the results can be retrieved.

Note that if the target/processor performs any power saving actions,

such as reducing the clock speed, or halting until the next interrupt

etc, these will interfere with the CPU load measurement. Under these

conditions the measurement results are undefined. The synthetic target

is one such system. See the implementation details at the foot of this

page for further information.  

SMP systems are not supported, only uniprocessor system.

 The API for load measuring functions can be

found in the file cyg/cpuload/cpuload.h.

This function is used to calibrate the cpu load measurement code. It

makes a measurement to determine the CPU properties while idle.

void cyg_cpuload_calibrate(cyg_uint32  *calibration);

The function returns the calibration value at the location pointed to

by calibration.

This function is quite unusual. For it to work correctly a few

conditions must be met. The function makes use of the two highest

thread priorities. No other threads must be using these priorities

while the function is being used. The kernel scheduler must be started

and not disabled. The function takes 100ms to complete during which

time no other threads will be run.

This function starts the CPU load measurments.

void cyg_cpuload_create(cyg_cpuload_t *cpuload,

                        cyg_uint32 calibrate,

                        cyg_handle_t *handle);

The measurement process is started and a handle to it is returned in

*handle. This handle is used to access the

results and the stop the measurement process. 

This function stops the measurement process.

void cyg_cpuload_delete(cyg_handle_t handle);

handle should be the value returned by the create function.

This function returns the latest measurements.

void cyg_cpuload_get(cyg_handle_t handle,

                 cyg_uint32 *average_point1s,

                 cyg_uint32 *average_1s,

                 cyg_uint32 *average_10s);

handle should be the value returned by the

create function. The load measurements for the last 100ms, 1s and 10s

are returned in

*average_point1s,*average_1s

and *average_10s respectively.

This section gives a few details of how the measurements are

made. This should help to understand what the results mean.

When there are no other threads runnable, eCos will execute the idle

thread. This thread is always runnable and uses the lowest thread

priority. The idle thread does little. It is an endless loop which

increments the variable, idle_thread_loops and

executes the macro HAL_IDLE_THREAD_ACTION. The cpu

load measurement code makes use of the variable. It periodically

examines the value of the variable and sees how much it has

changed. The idler the system, the more it will have incremented. From

this it is simple to determine the load of the system.

The function cyg_cpuload_calibrate executes the

idle thread for 100ms to determine how much

idle_thread_loops is incremented on a system idle

for 100ms. cyg_cpuload_create starts an alarm which

every 100ms calls an alarm function. This function looks at the

difference in idle_thread_loops since the last

invocation of the alarm function and so calculated how idle or busy

the system has been. The structure cyg_cpuload is

updated during the alarm functions with the new results.  The 100ms

result is simply the result from the last measurement period. A simple

filter is used to average the load over a period of time, namely 1s

and 10s. Due to rounding errors, the 1s and 10s value will probably

never reach 100% on a fully loaded system, but 99% is often seen.

As stated above, clever power management code will interfere with

these measurements. The basic assumption is that the idle thread will

be executed un-hindered and under the same conditions as when the

calibration function was executed. If the CPU clock rate is reduced,

the idle thread counter will be incremented less and so the CPU load

measurements will give values too high. If the CPU is halted entirely,

100% cpu load will be measured.