lag-meter.js

const history = Array(1500);
let history_total = 0;
const MDELAY_MS = 1;
let counter = 0;

function measure() {
    setTimeout((start) => {
        const diff = process.hrtime(start);
        const lag = Math.max(0, diff[0] * 1e3 + diff[1] / 1e6 - MDELAY_MS);
        const index = counter++ % history.length;
        history[index] = lag;
        history_total += lag;
    }, MDELAY_MS, process.hrtime());
}

let timeout = (function test() {
    setTimeout(() => {
        measure();
        timeout = test();
    }, 150 + Math.random() * 100);
})();

module.exports = function lagstats() {
    const sorted = history.filter(v => typeof v != 'undefined').sort();
    const N = sorted.length;
    const sum = sorted.reduce((s, v) => s + v, 0);
    const mean = sum / N;
    const sd = Math.sqrt(sorted.reduce((s, v) => s + Math.pow(v - mean, 2), 0) / N);
    const perc = {
        50: sorted[Math.floor(N * .50) - 1],
        90: sorted[Math.floor(N * .90) - 1],
        95: sorted[Math.floor(N * .95) - 1],
        98: sorted[Math.floor(N * .98) - 1],
        99: sorted[Math.floor(N * .99) - 1],
        100: sorted[N - 1],
    };

    return {mean, sd, perc, total: history_total};
};