parfeval overhead

function [x, pasi] = hybrid_jacobi_gauss_seidel (A, b, err)
    p = gcp('nocreate');
    if isempty(p)
        num_workers = 0;
    else
        num_workers = p.NumWorkers;
    end

    n = length(A);

    bucket_size = floor(sqrt(n));
    which_bucket = zeros(1, n);
    ends_of_buckets = [];
    for i = 1:n
        if i == 1
            remaining = bucket_size;
            which_bucket(i) = 1;
        elseif remaining > 0
            which_bucket(i) = which_bucket(i-1);
        else
            remaining = bucket_size;
            which_bucket(i) = which_bucket(i-1) + 1;
            ends_of_buckets(end+1) = i-1;
        end
        remaining = remaining - 1;
    end
    ends_of_buckets(end+1) = n;

    x = zeros(n, 1);
    pasi = 0;

    while pasi == 0 || max(abs(x - x_last)) > err
        x_last = x;
        %the outer for is serial
        for current_bucket = 1:length(ends_of_buckets)
            buck_bound = 0;
            if current_bucket > 1
                buck_bound = ends_of_buckets(current_bucket-1);
            end

            %worker_length = UPPER BOUND for how many indexes need to be calculated by one worker
            worker_length = floor((ends_of_buckets(current_bucket) - buck_bound + num_workers - 1) / num_workers);

            future_results(1:num_workers) = parallel.FevalFuture;
            for i = 1:num_workers
                start_itv = buck_bound+1 + (i - 1) * worker_length;
                end_itv = min(buck_bound+1 + i * worker_length - 1, ends_of_buckets(current_bucket));
                future_results(i) = parfeval(p, @hybrid_parallel_function, 3, A, b, x, x_last, buck_bound, n, start_itv, end_itv);
            end

            for i = 1:num_workers
                [~, arr, start_itv, end_itv] = fetchNext(future_results(i));
                x(start_itv:end_itv) = arr;
            end
        end

        pasi = pasi + 1;
    end
end

function [x_par, start_itv, end_itv] = hybrid_parallel_function (A, b, x, x_last, buck_bound, n, start_itv, end_itv)
    x_par = zeros(end_itv - start_itv + 1, 1);
    for i = start_itv:end_itv
        x_par(i-start_itv+1) = b(i);
        x_par(i-start_itv+1) = x_par(i-start_itv+1) - A(i, 1:buck_bound) * x(1:buck_bound);
        x_par(i-start_itv+1) = x_par(i-start_itv+1) - A(i, buck_bound+1:i-1) * x_last(buck_bound+1:i-1);
        x_par(i-start_itv+1) = x_par(i-start_itv+1) - A(i, i+1:n) * x_last(i+1:n);
        x_par(i-start_itv+1) = x_par(i-start_itv+1) / A(i, i);
    end
end