Schedulers in RTOS

Nice, let’s go deep into Schedulers in RTOS. Think of scheduler as the traffic police of the CPU – it decides which task runs and when.

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What is a Scheduler?

• In RTOS, multiple tasks exist, but only one runs on CPU at a time.
• The scheduler decides which task gets CPU based on priority, state, and algorithm.
• FreeRTOS uses a priority-based preemptive scheduler (but can be configured).

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Types of Schedulers in RTOS

1. Cooperative Scheduling

• A running task must yield (give up CPU) voluntarily.
• No preemption – task runs until it finishes or calls taskYIELD().
• Simple but risky: if a task never yields, others starve.

Example (FreeRTOS with configUSE_PREEMPTION=0):

for(;;)
{
    // Do work
    taskYIELD(); // must call this to let others run
}

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2. Preemptive Scheduling

• RTOS forces context switch if a higher-priority task becomes ready.
• Ensures real-time responsiveness.
• Default in FreeRTOS (configUSE_PREEMPTION=1).

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3. Round Robin Scheduling (within same priority)

• If multiple tasks share same priority, FreeRTOS gives CPU in time slices.
• Controlled by configUSE_TIME_SLICING.

Example:

• Task A and Task B both priority 2.
• They take turns in running when time slice expires.

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4. Priority-Based Scheduling (Main type in FreeRTOS)

• Always runs the highest priority ready task.
• If multiple with same priority → round robin.

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5. Rate Monotonic Scheduling (RMS)

• A fixed-priority algorithm:

  • Shorter period task → higher priority.
• Common in hard real-time systems.
• FreeRTOS doesn’t implement RMS directly, but you can assign priorities that way.

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6. Earliest Deadline First (EDF)

• Dynamic priority: task with the earliest deadline runs first.
• Not built into FreeRTOS, but possible with custom scheduler.
• Used in hard real-time apps like medical devices, automotive.

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Summary Table

Scheduler Type	Preemption	Who decides CPU use?	Use Case
Cooperative	No	Task itself	Simpler, non-critical apps
Preemptive (FreeRTOS)	Yes	Scheduler (priority)	Most RTOS apps
Round Robin	Yes (same priority)	Scheduler (time slice)	Fairness among same-level tasks
Rate Monotonic (RMS)	Yes	Fixed priority (period-based)	Hard real-time
Earliest Deadline First (EDF)	Yes	Dynamic deadline-based	Hard real-time with varying deadlines

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In FreeRTOS interviews, they usually expect you to know:

• Preemptive vs Cooperative
• Round robin for same priority tasks
• How time slicing works

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