Why is the xTimerPeriodInTicks affecting the code's runtime? - c++

Question

So for a report I'm trying to time the number of cycles it takes to execute a couple seperate functions in a program running on an ESP32, using freeRTOS. The code looks something like this with init_input_generator() being the first function called, here i'm timing onesecwindow.fifo_iir_filter_datapoint():
float geninputarray[SAMPLING_RATE]; //stores the values of an artificial signal we can use to test the algorithm
fifo_buffer onesecwindow = fifo_buffer(SAMPLING_RATE);
esp_err_t init_input_generator() {
dsps_tone_gen_f32(geninputarray, SAMPLING_RATE, 4, 0.04, 0); //10Hz
TimerHandle_t input_signal_timer =
xTimerCreate("input_timer", pdMS_TO_TICKS(10), pdTRUE, (void *)0, input_generator_callback);
xTimerStart(input_signal_timer, 0);
return ESP_OK;
}
void input_generator_callback(TimerHandle_t xTimer)
{
xTaskCreatePinnedToCore(
input_counter,
"input_generator",
4096,
NULL,
tskIDLE_PRIORITY + 11,
NULL,
PRO_CPU_NUM);
}
void input_counter(TimerHandle_t xTimer)
{
stepcounter = (stepcounter + 1) % SAMPLING_RATE;
insert_new_data(geninputarray[stepcounter]);
vTaskDelete(nullptr); // to gracefully end the task as returning is not allowed
}
esp_err_t insert_new_data(float datapoint)
{
onesecwindow.fifo_write(datapoint); // writes the new datapoint into the onesec window
unsigned int start_time = dsp_get_cpu_cycle_count();
onesecwindow.fifo_iir_filter_datapoint();
unsigned int end_time = dsp_get_cpu_cycle_count();
printf("%i\n", (end_time-start_time));
}
The thing that i'm noticing is that whenever I change the xTimerPeriodInTicks to a larger number, I get significantly longer time results, which just really confuses me and get's in the way of proper timing. Although the function doesn't scale with the SAMPLING_RATE and thus should give quite consistent results for each loop, output in cycles with a 10ms timer period looks something like this (every 5 or 6 "loops" for some reason it's longer):
1567, 630, 607, 624, 591, 619, 649, 1606, 607
with a 40ms timer period I get this as a typical output:
1904, 600, 1894, 616, 1928, 1928, 607, 1897, 628
As such I'm confused by the output, as they use the same freeRTOS taskcreate to run with the same priority on the same core, I don't see why there would be any difference. Perhaps I'm misunderstanding something basic/fundamental here so any help would be greatly appreciated.
Update
So based on the comments by #Tarmo i have restructured to approach to having a recurring task, unfortunately the output still seems to suffer from the same problem. The code now looks like this:
#include <esp_dsp.h> //Official ESP-DSP library
float geninputarray[SAMPLING_RATE]; //stores the values of an artificial signal we can use to test the algorithm
fifo_buffer onesecwindow = fifo_buffer(SAMPLING_RATE);
esp_err_t init_input_generator() {
dsps_tone_gen_f32(geninputarray, SAMPLING_RATE, 4, 0.04, 0); //10Hz
xTaskCreatePinnedToCore(
input_counter, // the actual function to be called
"input_generator",
4096,
NULL,
tskIDLE_PRIORITY + 5,
NULL,
APP_CPU_NUM);
return ESP_OK;
}
void input_counter(TimerHandle_t xTimer)
{
while (true)
{
stepcounter = (stepcounter + 1) % SAMPLING_RATE;
insert_new_data(geninputarray[stepcounter]);
vTaskDelay(4);
}
}
esp_err_t insert_new_data(float datapoint)
{
onesecwindow.fifo_write(datapoint); // writes the new datapoint into the onesec window
unsigned int start_time = dsp_get_cpu_cycle_count();
onesecwindow.fifo_iir_filter_datapoint();
unsigned int end_time = dsp_get_cpu_cycle_count();
printf("%i\n", (end_time-start_time));
}