Time-slicing for equal priority tasks in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create two tasks with equal priority in FreeRTOS.

FreeRTOS

xTaskCreate(task1, "Task1", 1000, NULL, [1], NULL);

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A0

B1

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using different priority numbers for tasks that should share CPU time equally.

Confusing stack size with priority.

Priority 1 is assigned to the task, which is a common base priority for equal priority tasks.

2fill in blank

medium

Complete the code to start the FreeRTOS scheduler after creating tasks.

FreeRTOS

vTaskStart[1]();

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AStartScheduler

BScheduler

CStart

DRunScheduler

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names like vTaskStart or vTaskRun.

Forgetting to start the scheduler after creating tasks.

The function vTaskStartScheduler() starts the FreeRTOS scheduler to begin task switching.

3fill in blank

hard

Fix the error in the task function to allow time-slicing by yielding the processor.

FreeRTOS

void task1(void *pvParameters) {
    for(;;) {
        // Task code here
        [1]();
    }
}

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AvTaskDelay

BvTaskSuspend

CvTaskDelete

DtaskYIELD

Attempts:

3 left

💡 Hint

Common Mistakes

Using vTaskDelay without a delay time, which causes errors.

Using vTaskSuspend or vTaskDelete which stop the task instead of yielding.

Calling taskYIELD() inside a task allows the scheduler to switch to another task of equal priority, enabling time-slicing.

4fill in blank

hard

Fill both blanks to create two tasks with equal priority and start the scheduler.

FreeRTOS

xTaskCreate(task1, "Task1", 1000, NULL, [1], NULL);
xTaskCreate(task2, "Task2", 1000, NULL, [2], NULL);
vTaskStartScheduler();

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A2

B1

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning different priorities to the tasks, which disables time-slicing.

Using zero priority which may cause unexpected behavior.

Both tasks must have the same priority number (1) to enable time-slicing between them.

5fill in blank

hard

Fill all three blanks to create a task that yields, create two equal priority tasks, and start the scheduler.

FreeRTOS

void task1(void *pvParameters) {
    for(;;) {
        // Task code
        [1]();
    }
}
xTaskCreate(task1, "Task1", 1000, NULL, [2], NULL);
xTaskCreate(task2, "Task2", 1000, NULL, [3], NULL);
vTaskStartScheduler();

Drag options to blanks, or click blank then click option'

AvTaskDelay

BtaskYIELD

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using vTaskDelay without parameters inside the task.

Assigning different priorities to tasks.

Forgetting to start the scheduler.

The task calls taskYIELD() to allow time-slicing. Both tasks have priority 1 to share CPU time equally. The scheduler is started to begin multitasking.

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