# Scheduler and Task Management

> **The criterion.** The scheduler must guarantee execution timelines: preemptive fixed-priority scheduling
> (the industry standard), plus round-robin or time-triggered options, with deterministic, predictable
> context-switch and interrupt latency.

## How Revera answers it

Revera schedules **preemptive fixed-priority** by default, with **round-robin** among equal priorities and an
optional **EDF** (earliest-deadline-first) policy. Priority is an explicit number in `[TK_PRIO_MIN,
TK_PRIO_MAX]` (0..255), not a vague level — higher preempts lower, equal round-robins. On the MCU ports a
timer interrupt can switch at any instant; on the desktop the same code runs cooperatively. Every port
provides its own O(1)-style ready selection; the exact context-switch cycle count is a per-target property
(measure it — Revera does not publish a number it cannot back on your silicon).

## Feature · how to turn it on · how to use it · where it works

| Feature | Turn on with | Use these APIs | Where |
|---|---|---|---|
| Preemptive fixed-priority | default | `tk_thread_create` / `tk_thread_spawn` (priority arg), `TK_PRIO_MIN` / `TK_PRIO_MAX` | all |
| Round-robin (equal priority) | default | `tk_thread_set_quantum` (time slice) | all |
| Cooperative mode | `tk_kernel_set_preempt_mode(TK_COOPERATIVE)` | `tk_thread_yield` | all |
| EDF (deadline-driven) | `TK_CONFIG_ENABLE_EDF` + `tk_kernel_set_sched_policy(TK_SCHED_EDF)` | `tk_thread_set_deadline` | all |
| Sleep / yield (blocking, not busy-wait) | — | `tk_sleep_ms`, `tk_thread_yield` | all |
| Thread lifecycle | — | `tk_thread_create` / `spawn` / `exit` / `join` / `detach` / `self` | all |
| Per-thread config | setup | `tk_thread_set_quantum` / `set_privileged` / `set_affinity` / `pin` | all |

*Full signatures and contracts: [API reference — Threads / Kernel lifecycle](../reference/api.md).*

## Limits & roadmap

- **Context-switch / interrupt-latency cycle counts are not published** — they are arch- and board-specific;
  measure on your target. A tickless / time-triggered mode is not in 0.1.0.
- EDF is optional and off by default (fixed-priority is the safe, deterministic default).

## Learn more

- **How it works:** [tick policy](../architecture/tick-policy.md) · [timekeeping](../architecture/timekeeping.md).
- **Run it:** `examples/threads` (priority + round-robin) · `examples/threads_smp`.
- **Full API:** [API reference](../reference/api.md).