Time-slicing for equal priority tasks in FreeRTOS

Introduction

Time-slicing lets tasks with the same priority share the CPU fairly by taking turns. This keeps your program responsive and balanced.

When you have multiple tasks that do different jobs but have the same importance.

When you want to make sure no single task hogs the CPU and others get a chance to run.

When tasks need to run smoothly without waiting too long for CPU time.

When you want to keep your system responsive even with many tasks running.

Syntax

FreeRTOS

vTaskStartScheduler();
// FreeRTOS automatically time-slices tasks of equal priority
// No special code needed to enable time-slicing

FreeRTOS automatically switches between tasks of the same priority every tick.

You just create tasks with the same priority; the scheduler handles the rest.

Examples

Two tasks with priority 2 will share CPU time equally by time-slicing.

FreeRTOS

xTaskCreate(Task1, "Task 1", 1000, NULL, 2, NULL);
xTaskCreate(Task2, "Task 2", 1000, NULL, 2, NULL);
vTaskStartScheduler();

Tasks A and B share CPU time by time-slicing because they have the same priority 1. Task C (priority 3) has higher priority and preempts A and B when ready.

FreeRTOS

xTaskCreate(TaskA, "Task A", 1000, NULL, 1, NULL);
xTaskCreate(TaskB, "Task B", 1000, NULL, 1, NULL);
xTaskCreate(TaskC, "Task C", 1000, NULL, 3, NULL);
vTaskStartScheduler();

Sample Program

This program creates two tasks with the same priority. FreeRTOS switches between them regularly, so both print their messages repeatedly.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void Task1(void *pvParameters) {
    for (;;) {
        printf("Task 1 running\n");
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void Task2(void *pvParameters) {
    for (;;) {
        printf("Task 2 running\n");
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

int main(void) {
    xTaskCreate(Task1, "Task 1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task 2", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    for(;;); // Should never reach here
    return 0;
}

Important Notes

Time-slicing depends on the tick interrupt frequency; faster ticks mean more frequent switches.

If tasks block or delay themselves, time-slicing pauses until they are ready again.

Tasks with higher priority run before time-slicing happens among equal priority tasks.

Summary

Time-slicing lets tasks of the same priority share CPU time fairly.

FreeRTOS handles time-slicing automatically; just assign equal priorities.

This keeps your system responsive and balanced when running multiple tasks.

Practice

1. What does time-slicing mean in FreeRTOS when tasks have equal priority?

easy

A. Higher priority tasks are paused to let lower priority tasks run.

B. The CPU time is shared fairly among tasks with the same priority.

C. Tasks with equal priority run one after another without switching.

D. Only the first created task runs until it finishes.

Solution

1. Step 1: Understand time-slicing concept

   Time-slicing means the CPU divides its time between tasks that have the same priority.

2. Step 2: Apply to FreeRTOS behavior

   FreeRTOS automatically switches between equal priority tasks to share CPU time fairly.

3. Final Answer:

   The CPU time is shared fairly among tasks with the same priority. -> Option B

4. Quick Check:

   Time-slicing = fair CPU sharing [OK]

Hint: Equal priority means tasks share CPU time fairly [OK]

Common Mistakes:

• Thinking only one task runs until it finishes
• Believing tasks run sequentially without switching
• Assuming lower priority tasks pause higher priority ones

2. Which of the following is the correct way to set two tasks to have equal priority in FreeRTOS?

easy

A. xTaskCreate(task1, "Task1", 1000, NULL, 2, NULL); xTaskCreate(task2, "Task2", 1000, NULL, 2, NULL);

B. xTaskCreate(task1, "Task1", 1000, NULL, 1, NULL); xTaskCreate(task2, "Task2", 1000, NULL, 3, NULL);

C. xTaskCreate(task1, "Task1", 1000, NULL, 0, NULL); xTaskCreate(task2, "Task2", 1000, NULL, 1, NULL);

D. xTaskCreate(task1, "Task1", 1000, NULL, 5, NULL); xTaskCreate(task2, "Task2", 1000, NULL, 4, NULL);

Solution

1. Step 1: Check task priority parameters

   In FreeRTOS, the fifth parameter in xTaskCreate is the task priority.

2. Step 2: Identify equal priorities

   Both tasks have priority 2 in xTaskCreate(task1, "Task1", 1000, NULL, 2, NULL); xTaskCreate(task2, "Task2", 1000, NULL, 2, NULL);, making them equal priority tasks.

3. Final Answer:

   Both tasks have priority 2, so they have equal priority. -> Option A

4. Quick Check:

   Same priority number = equal priority [OK]

Hint: Same priority number means equal priority [OK]

Common Mistakes:

• Assigning different priority numbers to tasks
• Confusing task stack size with priority
• Using priority 0 and 1 as equal

3. Given two tasks with equal priority in FreeRTOS, what is the expected behavior when both are ready to run?

medium

A. Both tasks run simultaneously on the same CPU core.

B. One task runs to completion before the other starts.

C. The second task never runs because the first task blocks it.

D. The scheduler switches between tasks periodically to share CPU time.

Solution

1. Step 1: Understand FreeRTOS scheduling for equal priority tasks

   FreeRTOS uses time-slicing to switch between tasks of equal priority.

2. Step 2: Predict scheduler behavior

   The scheduler switches tasks periodically, giving each task CPU time in turn.

3. Final Answer:

   The scheduler switches between tasks periodically to share CPU time. -> Option D

4. Quick Check:

   Equal priority tasks share CPU via switching [OK]

Hint: Equal priority tasks get CPU time by switching [OK]

Common Mistakes:

• Thinking tasks run one after another without switching
• Assuming tasks run simultaneously on one core
• Believing one task blocks the other forever

4. You have two tasks with equal priority, but one task never seems to run. What is the most likely cause?

medium

A. The task that never runs has a higher priority number.

B. FreeRTOS does not support time-slicing for equal priority tasks.

C. The task that never runs is blocked or waiting indefinitely.

D. The scheduler is disabled, so no task switching occurs.

Solution

1. Step 1: Check task states in FreeRTOS

   Even with equal priority, a task that is blocked or waiting will not run.

2. Step 2: Identify why a task never runs

   If a task never runs, it is likely blocked or waiting indefinitely, not ready to run.

3. Final Answer:

   The task that never runs is blocked or waiting indefinitely. -> Option C

4. Quick Check:

   Blocked task = no CPU time [OK]

Hint: Blocked tasks don't get CPU time even if priority is equal [OK]

Common Mistakes:

• Assuming higher priority task blocks lower priority always
• Believing FreeRTOS lacks time-slicing
• Ignoring task states like blocked or suspended

5. You have three tasks with equal priority in FreeRTOS. Task A runs for 10ms, Task B for 20ms, and Task C for 30ms before yielding. How does time-slicing affect their CPU time distribution?

hard

A. Each task gets equal CPU time slices regardless of run duration before yielding.

B. Tasks that run longer before yielding get more CPU time overall.

C. Task C will starve Tasks A and B because it runs longest.

D. FreeRTOS runs tasks in creation order ignoring time-slicing.

Solution

1. Step 1: Understand time-slicing with equal priority tasks

   FreeRTOS scheduler switches tasks periodically to share CPU time fairly among equal priority tasks.

2. Step 2: Analyze effect of different run durations before yielding

   Even if tasks run different times before yielding, the scheduler enforces time slices to balance CPU time.

3. Final Answer:

   Each task gets equal CPU time slices regardless of run duration before yielding. -> Option A

4. Quick Check:

   Time-slicing ensures fair CPU time despite run length [OK]

Hint: Time-slicing balances CPU time regardless of task run length [OK]

Common Mistakes:

• Assuming longer running tasks get more CPU time
• Thinking tasks can starve others with longer runs
• Believing task creation order controls CPU time