The equeue library

A "Swiss Army knife" for scheduling on embedded systems, the equeue library is a simple but powerful libarary for scheduling events on composable event queues.

#include "equeue.h"
#include <stdio.h>

int main() {
    // creates a queue with space for 32 basic events
    equeue_t queue;
    equeue_create(&queue, 32*EQUEUE_EVENT_SIZE);

    // events can be simple callbacks
    equeue_call(&queue, print, "called immediately");
    equeue_call_in(&queue, 2000, print, "called in 2 seconds");
    equeue_call_every(&queue, 1000, print, "called every 1 seconds");

    // events are executed in equeue_dispatch
    equeue_dispatch(&queue, 3000);

    print("called after 3 seconds");

    equeue_destroy(&queue);
}

The equeue library can be used as a normal event loop, or it can be backgrounded on a single hardware timer or even another event loop. It is both thread and irq safe, and provides functions for easily composing multiple queues.

The equeue library can act as a drop-in scheduler, provide synchronization between multiple threads, or just act as a mechanism for moving events out of interrupt contexts.

Documentation

The in-depth documentation on specific functions can be found in equeue.h.

The core of the equeue library is the equeue_t type which represents a single event queue, and the equeue_dispatch function which runs the equeue, providing the context for executing events.

On top of this, equeue_call, equeue_call_in, and equeue_call_every provide easy methods for posting events to execute in the context of the equeue_dispatch function.

#include "equeue.h"
#include "game.h"

equeue_t queue;
struct game game;

// button_isr may be in interrupt context
void button_isr(void) {
    equeue_call(&queue, game_button_update, &game);
}

// a simple user-interface framework
int main() {
    equeue_create(&queue, 4096);
    game_create(&game);

    // call game_screen_udpate at 60 Hz
    equeue_call_every(&queue, 1000/60, game_screen_update, &game);

    // dispatch forever
    equeue_dispatch(&queue, -1);
}

In addition to simple callbacks, an event can be manually allocated with equeue_alloc and posted with equeue_post to allow passing an arbitrary amount of context to the execution of the event. This memory is allocated out of the equeue's buffer, and dynamic memory can be completely avoided.

The equeue allocator is designed to minimize jitter in interrupt contexts as well as avoid memory fragmentation on small devices. The allocator achieves both constant-runtime and zero-fragmentation for fixed-size events, however grows linearly as the quantity of differently-sized allocations increases.

#include "equeue.h"

equeue_t queue;

// arbitrary data can be moved to a different context
int enet_consume(void *buffer, int size) {
    if (size > 512) {
        size = 512;
    }

    void *data = equeue_alloc(&queue, 512);
    memcpy(data, buffer, size);
    equeue_post(&queue, handle_data_elsewhere, data);

    return size;
}

Additionally, in-flight events can be cancelled with equeue_cancel. Events are given unique ids on post, allowing safe cancellation of expired events.

#include "equeue.h"

equeue_t queue;
int sonar_value;
int sonar_timeout_id;

void sonar_isr(int value) {
    equeue_cancel(&queue, sonar_timeout_id);
    sonar_value = value;
}

void sonar_timeout(void *) {
    sonar_value = -1;
}

void sonar_read(void) {
    sonar_timeout_id = equeue_call_in(&queue, 300, sonar_timeout, 0);
    sonar_start();
}

From an architectural standpoint, event queues easily align with module boundaries, where internal state can be implicitly synchronized through event dispatch.

On platforms where multiple threads are unavailable, multiple modules can use independent event queues and still be composed through the equeue_chain function.

#include "equeue.h"

// run a simultaneous localization and mapping loop in one queue
struct slam {
    equeue_t queue;
};

void slam_create(struct slam *s, equeue_t *target) {
    equeue_create(&s->queue, 4096);
    equeue_chain(&s->queue, target);
    equeue_call_every(&s->queue, 100, slam_filter);
}

// run a sonar with it's own queue
struct sonar {
    equeue_t equeue;
    struct slam *slam;
};

void sonar_create(struct sonar *s, equeue_t *target) {
    equeue_create(&s->queue, 64);
    equeue_chain(&s->queue, target);
    equeue_call_in(&s->queue, 5, sonar_update, s);
}

// all of the above queues can be combined into a single thread of execution
int main() {
    equeue_t queue;
    equeue_create(&queue, 1024);

    struct sonar s1, s2, s3;
    sonar_create(&s1, &queue);
    sonar_create(&s2, &queue);
    sonar_create(&s3, &queue);

    struct slam slam;
    slam_create(&slam, &queue);

    // dispatches events from all of the modules
    equeue_dispatch(&queue, -1);
}

Design

See DESIGN.md for more information on the underlying design of the event queue and the tradeoffs related to trying to provide a simple and robust scheduler.

Platform

The equeue library has a minimal porting layer that is flexible depending on the requirements of the underlying platform. Platform specific declarations and more information can be found in equeue_platform.h.

Tests

The equeue library uses a set of local tests based on the posix implementation.

Runtime tests are located in tests.c:

make test

Profiling tests based on rdtsc are located in prof.c:

make prof

To make profiling results more tangible, the profiler also supports percentage comparison with previous runs:

make prof | tee results.txt
cat results.txt | make prof