Real-Time Systems Scheduling

Real-Time Systems Scheduling

In this lesson, you will learn how to implement a basic cooperative scheduler designed for constrained hardware where predictable execution flow is more critical than raw throughput.

The Cooperative Approach

On resource-constrained hardware, preemptive multitasking (where the hardware interrupts tasks) can introduce non-deterministic jitter. A cooperative scheduler instead relies on tasks explicitly yielding control back to a central orchestrator. Because Rust’s Future trait represents a state machine that can pause and resume, it serves as the ideal primitive for this.

To build a basic scheduler, we define a task as a pinned Future and manage a queue of these tasks. When a task is polled, it either completes or returns Poll::Pending. If pending, it must ensure the executor is notified to poll it again when progress can be made.

Implementing the Scheduler

On constrained hardware, we avoid heap allocation for our task queue by using a fixed-size buffer. Here is a simplified version of a cooperative loop:

use core::task::{Context, Poll};
use core::pin::Pin;
use futures::Future;

struct Task {
    future: Pin<Box<dyn Future<Output = ()>>>,
}

struct Scheduler {
    tasks: [Option<Task>; 4], // Fixed-size for memory safety
}

impl Scheduler {
    fn run(&mut self) {
        loop {
            for task in self.tasks.iter_mut() {
                if let Some(t) = task {
                    // Use a no-op waker or simple trigger
                    let mut cx = Context::from_waker(todo!("waker"));
                    if t.future.as_mut().poll(&mut cx).is_ready() {
                        *task = None; // Task finished
                    }
                }
            }
        }
    }
}

Constraints and Predictability

In real-time contexts, your scheduler loop should have a strictly bounded worst-case execution time. By avoiding dynamic allocation and utilizing Pin to keep tasks in stable memory locations, we prevent heap fragmentation. Each iteration of the loop effectively acts as a heartbeat. If your logic needs to respond to external hardware events, those events should trigger the Waker, which signals the scheduler that a specific task is ready to make progress, keeping the execution flow deterministic and efficient.

Key Takeaway: Cooperative scheduling in Rust allows you to orchestrate multiple tasks using the Future state machine, providing deterministic execution cycles essential for constrained, real-time hardware.