Priority-based scheduling in FreeRTOS - Step-by-Step Execution

Concept Flow - Priority-based scheduling

Ready Tasks Queue

↓

Check Highest Priority Task

↓

Run Highest Priority Task

↓

Task Completes or Blocks

↓

Update Ready Queue

↩Back to Check Highest Priority Task

The scheduler always picks the ready task with the highest priority to run next. When a task finishes or blocks, the scheduler updates the queue and picks the next highest priority task.

Execution Sample

FreeRTOS

void vTask1(void *pvParameters) {
  for(;;) {
    // Task 1 code
    vTaskDelay(100 / portTICK_PERIOD_MS);
  }
}

void vTask2(void *pvParameters) {
  for(;;) {
    // Task 2 code
    vTaskDelay(50 / portTICK_PERIOD_MS);
  }
}

Two tasks run with different priorities; the scheduler runs the higher priority task first whenever it is ready.

Execution Table

Step	Ready Tasks	Highest Priority Task	Action	Output/State
1	Task1(Priority 2), Task2(Priority 1)	Task1	Run Task1	Task1 runs
2	Task1 (delayed), Task2 (ready)	Task2	Run Task2	Task2 runs
3	Task1 (ready), Task2 (delayed)	Task1	Run Task1	Task1 runs
4	Task1 (delayed), Task2 (ready)	Task2	Run Task2	Task2 runs
5	Task1 (ready), Task2 (delayed)	Task1	Run Task1	Task1 runs
6	Task1 (delayed), Task2 (ready)	Task2	Run Task2	Task2 runs
7	Task1 (ready), Task2 (delayed)	Task1	Run Task1	Task1 runs
8	Task1 (delayed), Task2 (ready)	Task2	Run Task2	Task2 runs
9	Task1 (ready), Task2 (delayed)	Task1	Run Task1	Task1 runs
10	Task1 (delayed), Task2 (ready)	Task2	Run Task2	Task2 runs
11	No ready tasks	None	Idle	CPU idle, waiting for tasks
Exit	No ready tasks	None	Stop	No tasks ready to run

💡 No tasks are ready; scheduler idles or waits for interrupts.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	Final
Task1 State	Ready	Running	Delayed	Ready	Delayed	Ready	Delayed	Ready	Delayed	Ready	Delayed	Idle
Task2 State	Ready	Ready	Running	Delayed	Ready	Delayed	Ready	Delayed	Ready	Delayed	Ready	Idle

Key Moments - 3 Insights

Why does Task2 run immediately after Task1 delays, even though Task1 has higher priority?

Because Task1 is delayed (blocked), it is not ready to run. The scheduler picks the highest priority task that is ready, which is Task2 at that moment (see execution_table rows 2 and 4).

What happens if both tasks are ready at the same time?

The scheduler always runs the task with the higher priority first. Here, Task1 has priority 2 and Task2 has priority 1, so Task1 runs first whenever both are ready (see execution_table rows 1, 3, 5).

Why does the CPU become idle at the end?

When no tasks are ready to run (all delayed or blocked), the scheduler idles the CPU waiting for an event or interrupt (see execution_table row 11).

Visual Quiz - 3 Questions

Look at the execution table, which task runs at Step 3?

A Task2

B Task1

C No task runs

D Both tasks run simultaneously

💡 Hint

Check the 'Highest Priority Task' column at Step 3 in the execution_table.

At which step does the CPU become idle according to the execution table?

A Step 11

B Step 9

C Step 10

D Step 1

💡 Hint

Look for the row where 'Highest Priority Task' is 'None' and action is 'Idle'.

If Task1 priority was lower than Task2, which task would run first when both are ready?

A They run alternately

B Task1

C Task2

D CPU idles

💡 Hint

Priority-based scheduling always runs the highest priority ready task first (see key_moments about priority).

Concept Snapshot

Priority-based scheduling in FreeRTOS:
- Scheduler picks the ready task with highest priority.
- If higher priority task is blocked, lower priority runs.
- Tasks run until they block, delay, or yield.
- CPU idles if no tasks are ready.
- Priorities are numeric; higher number means higher priority.

Full Transcript

Priority-based scheduling in FreeRTOS means the scheduler always runs the ready task with the highest priority number. If that task blocks or delays, the scheduler picks the next highest priority ready task. Tasks run in a loop, and when no tasks are ready, the CPU idles. This example shows two tasks with different priorities running alternately as they delay. The execution table traces which task runs at each step, and the variable tracker shows task states changing from ready to running to delayed. Key moments clarify why lower priority tasks run only when higher priority ones are blocked. The visual quiz tests understanding of task selection and CPU idling based on priority and readiness.