Multiple tasks running concurrently in FreeRTOS - Time & Space Complexity

Time Complexity: Multiple tasks running concurrently

O(n)

Understanding Time Complexity

When multiple tasks run at the same time in FreeRTOS, it's important to understand how the total work grows as we add more tasks.

We want to know how the time to manage all tasks changes when the number of tasks increases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void vTask1(void *pvParameters) {
    while(1) {
        // Task 1 work
        vTaskDelay(100 / portTICK_PERIOD_MS);
    }
}

void vTask2(void *pvParameters) {
    while(1) {
        // Task 2 work
        vTaskDelay(100 / portTICK_PERIOD_MS);
    }
}

int main() {
    xTaskCreate(vTask1, "Task1", 1000, NULL, 1, NULL);
    xTaskCreate(vTask2, "Task2", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    return 0;
}
    

This code creates two tasks that run repeatedly and share CPU time managed by FreeRTOS.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Each task runs an infinite loop doing work and then delaying.
• How many times: Each task repeats its loop forever, switching happens many times.

How Execution Grows With Input

As we add more tasks, the scheduler must switch between them more often, increasing the total management work.

Number of Tasks (n)	Approx. Scheduler Operations
2	Low switching overhead
10	More frequent context switches
100	Much more switching and management

Pattern observation: The scheduler's work grows roughly in proportion to the number of tasks.

Final Time Complexity

Time Complexity: O(n)

This means the time to manage all tasks grows linearly as we add more tasks.

Common Mistake

[X] Wrong: "Adding more tasks won't affect performance because tasks run independently."

[OK] Correct: The scheduler must switch between all tasks, so more tasks mean more switching and overhead.

Interview Connect

Understanding how task management scales helps you design efficient multitasking systems and shows you grasp real-time system behavior.

Self-Check

"What if tasks had different priorities? How would that affect the time complexity of scheduling?"

Practice

1. What is the main purpose of running multiple tasks concurrently in FreeRTOS?

easy

A. To allow the program to perform several operations at the same time

B. To make the program run slower

C. To use more memory without benefit

D. To avoid using any delays in the code

Solution

1. Step 1: Understand task concurrency

   Multiple tasks let the program handle different jobs at once, improving efficiency.

2. Step 2: Identify the purpose in FreeRTOS

   FreeRTOS switches between tasks automatically, enabling multitasking.

3. Final Answer:

   To allow the program to perform several operations at the same time -> Option A

4. Quick Check:

   Multiple tasks = multitasking = To allow the program to perform several operations at the same time [OK]

Hint: Multiple tasks mean doing many things at once [OK]

Common Mistakes:

• Thinking tasks slow down the program
• Believing tasks waste memory
• Ignoring the role of delays

2. Which of the following is the correct way to create a task in FreeRTOS?

easy

A. TaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

B. createTask(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

C. xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

D. xCreateTask(TaskFunction, "TaskName", stackSize, NULL, priority, NULL);

Solution

1. Step 1: Recall FreeRTOS task creation function

   The correct function to create a task is xTaskCreate.

2. Step 2: Match the exact syntax

   xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); matches the correct function name and parameters.

3. Final Answer:

   xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); -> Option C

4. Quick Check:

   Task creation function = xTaskCreate = xTaskCreate(TaskFunction, "TaskName", stackSize, NULL, priority, NULL); [OK]

Hint: Remember the function starts with 'xTaskCreate' [OK]

Common Mistakes:

• Using incorrect function names like createTask
• Mixing up parameter order
• Missing the 'x' prefix in function name

3. Consider this FreeRTOS code snippet:

void Task1(void *pvParameters) {
  for (;;) {
    printf("Task1 running\n");
    vTaskDelay(pdMS_TO_TICKS(1000));
  }
}

void Task2(void *pvParameters) {
  for (;;) {
    printf("Task2 running\n");
    vTaskDelay(pdMS_TO_TICKS(500));
  }
}

What will be the output pattern when both tasks run concurrently?

medium

A. Task1 prints every 1 second; Task2 prints every 0.5 seconds

B. Task1 and Task2 print at the same time every 1 second

C. Task1 prints every 0.5 seconds; Task2 prints every 1 second

D. Only Task1 prints because Task2 is blocked

Solution

1. Step 1: Analyze vTaskDelay timing for Task1

   Task1 delays 1000 ms (1 second) between prints.

2. Step 2: Analyze vTaskDelay timing for Task2

   Task2 delays 500 ms (0.5 seconds) between prints.

3. Step 3: Understand concurrent execution

   Both tasks run independently, so Task2 prints twice as often as Task1.

4. Final Answer:

   Task1 prints every 1 second; Task2 prints every 0.5 seconds -> Option A

5. Quick Check:

   Delays control print rate = Task1 prints every 1 second; Task2 prints every 0.5 seconds [OK]

Hint: Check each task's delay time to predict print frequency [OK]

Common Mistakes:

• Assuming tasks print simultaneously
• Mixing up delay times
• Thinking one task blocks the other

4. Given this code snippet, what is the error?

void TaskA(void *pvParameters) {
  while(1) {
    // Do something
    vTaskDelay(1000);
  }
}

xTaskCreate(TaskA, "TaskA", 1000, NULL, 1, NULL);

medium

A. Task function must return int, not void

B. vTaskDelay requires delay in ticks, not milliseconds

C. Stack size 1000 is too small

D. Task name must be longer than 5 characters

Solution

1. Step 1: Check vTaskDelay parameter

   vTaskDelay expects delay in ticks, not milliseconds.

2. Step 2: Identify correct delay usage

   Use pdMS_TO_TICKS(1000) to convert 1000 ms to ticks.

3. Final Answer:

   vTaskDelay requires delay in ticks, not milliseconds -> Option B

4. Quick Check:

   Delay unit must be ticks = vTaskDelay requires delay in ticks, not milliseconds [OK]

Hint: Use pdMS_TO_TICKS() to convert ms to ticks for vTaskDelay [OK]

Common Mistakes:

• Passing milliseconds directly to vTaskDelay
• Wrong return type for task function
• Incorrect stack size assumptions

5. You want two tasks to print messages alternately every 500 ms without overlapping prints. Which approach ensures this behavior in FreeRTOS?

hard

A. Use global variables without any synchronization

B. Use vTaskDelay with different delay times for each task

C. Run both tasks at the same priority without synchronization

D. Use a binary semaphore to synchronize tasks before printing

Solution

1. Step 1: Understand the need for synchronization

   To avoid overlapping prints, tasks must coordinate access to the printing resource.

2. Step 2: Identify synchronization method

   A binary semaphore allows one task to print while the other waits, alternating access.

3. Step 3: Evaluate other options

   Different delays or same priority without sync can cause overlapping prints; global variables alone don't prevent conflicts.

4. Final Answer:

   Use a binary semaphore to synchronize tasks before printing -> Option D

5. Quick Check:

   Synchronization via semaphore = Use a binary semaphore to synchronize tasks before printing [OK]

Hint: Use semaphores to control shared resource access [OK]

Common Mistakes:

• Relying only on delays for synchronization
• Ignoring task priority effects
• Not using any synchronization mechanism