Respecting the scheduler in Erlang NIFs

int enif_consume_timeslice(ErlNifEnv* env, int percent) {
    Process *proc;
    Sint reds;

    execution_state(env, &proc, NULL);

    ASSERT(is_proc_bound(env) && percent >= 1 && percent <= 100);
    if (percent < 1) percent = 1;
    else if (percent > 100) percent = 100;

    reds = ((CONTEXT_REDS+99) / 100) * percent;
    ASSERT(reds > 0 && reds <= CONTEXT_REDS);
    BUMP_REDS(proc, reds);
    return ERTS_BIF_REDS_LEFT(proc) == 0;
}

// The first thing you'll want to do is create a struct to maintain any
// internal state you want to pass between calls to your NIF.
// In this case, I need to keep track of my matrix, the input strings,
// and which x, y position I had iterated up to.
struct LevenshteinState {
    // The matrix being used to calculate the distance
    unsigned int *matrix;

    // The input strings + their sizes
    unsigned char *s1;
    unsigned s1len;
    unsigned char *s2;
    unsigned s2len;

    // The index of the last processed row of the matrix,
    // so that the next iteration can pick up where we left off
    unsigned int lastX;
    unsigned int lastY;
};

// Now, let's rewrite our entry point so that all it does is read in the
// command arguments, validate them and then yield a call to our internal NIF
// that will do all the actual work.
static ERL_NIF_TERM erl_levenshtein(ErlNifEnv* env, int argc, const
                                    ERL_NIF_TERM argv[]) {
    // Not pictured: verifying argc, the type of the arguments,
    // and casting the binaries to ErlNifBinary structs.

    // Retrieve the state resource descriptor from our priv data,
    // and allocate a new structure
    // PrivData here is a custom struct, initialized during our module load
    // callback. See the code on github for the full implementation with
    // regards to this.
    struct PrivData *priv_data = enif_priv_data(env);
    struct LevenshteinState* state = enif_alloc_resource(
        priv_data->levenshtein_state_resource,
        sizeof(struct LevenshteinState)
    );

    //// Initialize the calculation state
    // Allocate our matrix
    size_t matrix_size = (
        sizeof(unsigned int) * (binary1.size + 1) * (binary2.size + 1)
    );
    state->matrix = malloc(matrix_size);
    // Copy the binary term info
    state->s1 = binary1.data;
    state->s1len = binary1.size;
    state->s2 = binary2.data;
    state->s2len = binary2.size;
    // Set our initial X and Y values
    state->lastX = 1;
    state->lastY = 1;

    // In the full version, here is where I also initialize the first row and
    // column of the matrix, in order to simplify the code in the helper NIF.

    // Now that the setup is complete, we can call erl_schedule_nif to
    // tell the beam our continuation function.
    // First, we need to wrap our data resource so that it can be passed
    // through the BEAM. enif_make_resource takes our state pointer and returns
    // an ERL_NIF_TERM.
    ERL_NIF_TERM state_term = enif_make_resource(env, state);
    // The NIF name here does not seem to be used for determining what code to
    // call, and is likely only used when debugging what code is running.
    return enif_schedule_nif(
        env,
        "levenshtein_yielding", // NIF to call
        0, // Flags
        erl_levenshtein_yielding,
        1,
        args
    );

static ERL_NIF_TERM erl_levenshtein_yielding(ErlNifEnv* env, int argc,
                                             const ERL_NIF_TERM argv[]) {
    // Not pictured: argc check

    // Extract the state term. In the same way we wrapped it before, we need
    // to now unwrap the resource we used to pass our struct through.
    struct PrivData *priv_data = enif_priv_data(env);
    struct LevenshteinState* state;
    if (!enif_get_resource(env, argv[0],
                           priv_data->levenshtein_state_resource,
                           ((void*) (&state)))) {
        return mk_error(env, "bad_internal_state");
    }

    // Start processing wherever the previous slice left off
    const unsigned int xsize = state->s1len + 1;
    unsigned int x = state->lastX;
    unsigned int y = state->lastY;

    // Specs for tracking function runtime
    struct timespec start_time;
    struct timespec current_time;

    // Grab the function start time
    clock_gettime(CLOCK_MONOTONIC, &start_time);

    // Create a tracker for the number of loop iterations we've done.
    // This operation count will act as a punctuator for us to check
    // whether it's time for us to yield again.
    unsigned long operations = 0;

    // This is a bit slimy, but is the simplest way to preload
    // the x and y loop vars for the first inner loop iteration
    goto loop_inner;

    // Loop over the matrix
    for (x = state->lastX; x <= state->s2len; x++) {
        for (y = 1; y <= state->s1len; y++) {
loop_inner:
            // Ordinary Levenshtein implementation
            MATRIX_ELEMENT(state->matrix, xsize, x, y) = MIN3(
                MATRIX_ELEMENT(state->matrix, xsize, x-1, y) + 1,
                MATRIX_ELEMENT(state->matrix, xsize, x, y-1) + 1,
                MATRIX_ELEMENT(state->matrix, xsize, x-1, y-1) +
                    (state->s1[y-1] == state->s2[x-1] ? 0 : 1)
            );

            // For each cell, increment the op count until we hit our
            // check threshold.
            if (unlikely(operations++ > OPERATIONS_BETWEN_TIMECHEKS)) {
                // When we do, get the current time
                clock_gettime(CLOCK_MONOTONIC, &current_time);

                // Figure out how many nanoseconds have passed since we started
                // calculating
                unsigned long nanoseconds_diff = (
                    (current_time.tv_nsec - start_time.tv_nsec) +
                    (current_time.tv_sec - start_time.tv_sec) * 1000000000
                );

                // Convert that to a percentage of a timeslice, assuming that
                // a time slice is 1 millisecond.
                int slice_percent = (nanoseconds_diff * 100) / TIMESLICE_NANOSECONDS;

                // enif_consume_timeslice requires a percentage in the range
                // 1 <= timeslice <= 100
                if (slice_percent < 1) {
                    slice_percent = 1;
                } else if (slice_percent > 100) {
                    slice_percent = 100;
                }

                // Consume that amount of a timeslice.
                // If the result is 1, then we have consumed the entire slice and
                // should now yield.
                if (enif_consume_timeslice(env, slice_percent)) {
                    // Break out of both loops
                    goto loop_exit;
                }

                // If we're not done, shift the times over and keep looping
                start_time.tv_sec = current_time.tv_sec;
                start_time.tv_nsec = current_time.tv_nsec;
                operations = 0;
            }
        }
    }

loop_exit:
    // If we exited the loop via jump, we must have run out of time
    // in this slice. Update our state and yield the next cycle.
    if (likely(x <= state->s2len || y <= state->s1len)) {
        // Update the state with the next row value to process
        state->lastX = x;
        state->lastY = y;

        // Yield another call to ourselves.
        // We can re-use our argv, since we're reusing the same state struct.
        return enif_schedule_nif(
            env,
            "levenshtein_yielding", // NIF to call
            0, // Flags
            erl_levenshtein_yielding,
            1,
            argv
        );
    }

    // If we are done, grab the result
    unsigned int result = MATRIX_ELEMENT(
        state->matrix, xsize, state->s2len, state->s1len);

    // We've finished, so it's time to free the work state
    // state.
    free(state->matrix);
    enif_release_resource(state);

    // Return the calculated value
    return enif_make_int(env, result);
}

realtime_test() ->
    % Allocate two large binaries
    A = << <<0>> || _ <- lists:seq(1, 10000) >>,
    B = << <<1>> || _ <- lists:seq(1, 10000) >>,

    % Create a printer process that tries to print regularly
    _Printer = spawn_link(fun() -> realtime_printer(os:system_time()) end),

    % Create enough adversarial worker processes to saturate all cores
    _Workers = [
        spawn_link(fun() -> realtime_worker(A, B) end)
        || _ <- lists:seq(1, erlang:system_info(logical_processors_available))
    ],
    ok.

% Spins forever, running our NIF on the input strings
realtime_worker(A, B) ->
    levenshtein:levenshtein(A, B),
    realtime_worker(A, B).

% Attempt to run exactly every second, and print how much we were off by.
realtime_printer(LastRan) ->
    timer:sleep(1000),
    Delta = os:system_time() - LastRan,
    DeltaMs = Delta / 1000000,
    Jitter = 1000000000 - Delta,
    JitterMs = Jitter / 1000000,
    io:format("Time since last schedule: ~p ms, Jitter: ~p ms~n", [
        DeltaMs, abs(JitterMs)
    ]),
    realtime_printer(os:system_time()).