Priority numbering in FreeRTOS - Step-by-Step Execution

Concept Flow - Priority numbering in FreeRTOS

Define task priorities

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Assign priority numbers

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Scheduler compares priorities

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Highest priority task runs

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Lower priority tasks wait

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If higher priority task blocks

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Next highest priority task runs

Tasks are given priority numbers; the scheduler runs the highest priority task ready to run, lower priority tasks wait unless higher priority tasks block.

Execution Sample

FreeRTOS

xTaskCreate(task1, "Task1", 1000, NULL, 3, NULL);
xTaskCreate(task2, "Task2", 1000, NULL, 1, NULL);
xTaskCreate(task3, "Task3", 1000, NULL, 5, NULL);

Creates three tasks with priorities 3, 1, and 5; the scheduler runs the highest priority task first.

Execution Table

Step	Task Created	Priority Assigned	Scheduler Decision	Running Task
1	Task1	3	Task1 is highest priority so far	Task1
2	Task2	1	Task1 (3) > Task2 (1), Task1 runs	Task1
3	Task3	5	Task3 (5) > Task1 (3), Task3 runs	Task3
4	Task3 blocks or delays	N/A	Next highest priority ready task is Task1 (3)	Task1
5	Task1 blocks or delays	N/A	Next highest priority ready task is Task2 (1)	Task2
6	All tasks blocked	N/A	Idle task runs	Idle Task

💡 Execution stops when all tasks are blocked or system is idle

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Task1 Priority	N/A	3	3	3	3	3	3
Task2 Priority	N/A	N/A	1	1	1	1	1
Task3 Priority	N/A	N/A	N/A	5	5	5	5
Running Task	Idle Task	Task1	Task1	Task3	Task1	Task2	Idle Task

Key Moments - 3 Insights

Why does Task3 run instead of Task1 after it is created?

Because Task3 has a higher priority number (5) than Task1 (3), the scheduler runs the highest priority ready task as shown in step 3 of the execution_table.

What happens if the highest priority task blocks or delays?

The scheduler runs the next highest priority task that is ready, as shown in steps 4 and 5 where Task3 or Task1 blocks and the scheduler switches to the next ready task.

Is a higher priority number better or worse in FreeRTOS?

A higher priority number means higher priority and the task will run before lower priority tasks, as demonstrated by Task3 with priority 5 running before Task1 with priority 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, which task runs?

A Task2

B Task1

C Task3

D Idle Task

💡 Hint

Check the 'Running Task' column at step 3 in the execution_table

At which step does the scheduler run Task2?

A Step 4

B Step 5

C Step 2

D Step 6

💡 Hint

Look at the 'Running Task' column in the execution_table for when Task2 runs

If Task3 had priority 2 instead of 5, which task would run at step 3?

A Task1

B Task3

C Task2

D Idle Task

💡 Hint

Compare priorities in variable_tracker and see which is highest at step 3

Concept Snapshot

FreeRTOS uses numeric priorities for tasks.
Higher number means higher priority.
Scheduler runs highest priority ready task.
If highest priority blocks, next highest runs.
Idle task runs if no tasks ready.

Full Transcript

In FreeRTOS, each task is assigned a numeric priority. The scheduler always runs the task with the highest priority number that is ready to run. When a new task is created with a higher priority than the currently running task, the scheduler switches to the new higher priority task immediately. If the highest priority task blocks or delays, the scheduler runs the next highest priority task that is ready. If no tasks are ready, the idle task runs. This priority numbering system ensures that important tasks get CPU time before less important ones.