Time-slicing for equal priority tasks in FreeRTOS - Step-by-Step Execution

Concept Flow - Time-slicing for equal priority tasks

Start Scheduler

↓

Ready Tasks with Equal Priority

↓

Select Task 1

→Run Task 1 for Time Slice

↓

Time Slice Ends

↓

Select Task 2

→Run Task 2 for Time Slice

↓

Time Slice Ends

... (repeat for all equal priority tasks)↓

Cycle repeats until tasks block or finish

↩... (repeat for all equal priority tasks)

The scheduler cycles through tasks of equal priority, giving each a fixed time slice to run before switching to the next, repeating this round-robin until tasks block or complete.

Execution Sample

FreeRTOS

void Task1(void *pvParameters) {
  for(;;) {
    // Task1 work
    vTaskDelay(10);
  }
}

void Task2(void *pvParameters) {
  for(;;) {
    // Task2 work
    vTaskDelay(10);
  }
}

Two tasks with equal priority run in turn, each getting a time slice before the scheduler switches to the other.

Execution Table

Step	Current Task	Time Slice Remaining	Action	Next Task
1	Task1	TickCountToTimeSlice	Task1 runs	Task1
2	Task1	0	Time slice ends, switch	Task2
3	Task2	TickCountToTimeSlice	Task2 runs	Task2
4	Task2	0	Time slice ends, switch	Task1
5	Task1	TickCountToTimeSlice	Task1 runs	Task1
6	Task1	0	Time slice ends, switch	Task2
7	Task2	TickCountToTimeSlice	Task2 runs	Task2
8	Task2	0	Time slice ends, switch	Task1
9	Task1	TickCountToTimeSlice	Task1 runs	Task1
10	Task1	0	Time slice ends, switch	Task2
11	-	-	Tasks block or finish, scheduler idles or switches	-

💡 Tasks block or finish, so scheduler stops time-slicing and switches context accordingly.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
Current Task	Task1	Task2	Task1	Task2	Task1	-
Time Slice Remaining	TickCountToTimeSlice	0	TickCountToTimeSlice	0	TickCountToTimeSlice	-

Key Moments - 2 Insights

Why does the scheduler switch tasks even if the current task is not blocked?

Because tasks have equal priority, the scheduler uses time-slicing to give each task a fixed time slice, then switches to the next task as shown in steps 2, 4, 6, etc.

What happens if a task blocks before its time slice ends?

The scheduler immediately switches to the next ready task, skipping the remaining time slice, which is why the exit note mentions tasks blocking or finishing.

Visual Quiz - 3 Questions

Look at the execution table, what is the current task at step 4?

A Task1

B Task2

C Idle Task

D Scheduler

💡 Hint

Check the 'Current Task' column at step 4 in the execution_table.

At which step does the time slice for Task1 end for the first time?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look for when 'Time Slice Remaining' becomes 0 for Task1 in the execution_table.

If Task1 blocks early, how would the execution table change?

A Scheduler would stop all tasks.

B Scheduler would continue running Task1 until time slice ends.

C Scheduler would immediately switch to Task2, skipping Task1's remaining time slice.

D Scheduler would run Task1 and Task2 simultaneously.

💡 Hint

Refer to the key moment about task blocking and the exit note in the execution_table.

Concept Snapshot

Time-slicing lets FreeRTOS share CPU time fairly among equal priority tasks.
Each task runs for a fixed time slice.
When time slice ends, scheduler switches to next equal priority task.
If a task blocks, scheduler switches immediately.
This creates a round-robin execution pattern.
Helps prevent one task from starving others.

Full Transcript

In FreeRTOS, when multiple tasks have the same priority, the scheduler uses time-slicing to share CPU time fairly. It runs each task for a fixed time slice, then switches to the next task of equal priority. This cycle repeats, creating a round-robin pattern. If a task blocks or finishes early, the scheduler immediately switches to the next ready task, skipping the remaining time slice. This ensures all equal priority tasks get a chance to run without one blocking the others. The execution table shows how the scheduler switches tasks after each time slice ends, alternating between Task1 and Task2. Variables track which task is running and how much time slice remains. This method keeps the system responsive and fair.

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