How to detect and measure event loop blocking in node.js?

Code

this code will measure the time in nanoseconds it took for the event loop to trigger. it measures the time between the current process and the next tick.

var time = process.hrtime();
process.nextTick(function() {
   var diff = process.hrtime(time);


   console.log('benchmark took %d nanoseconds', diff[0] * 1e9 + diff[1]);
   // benchmark took 1000000527 nanoseconds
});

EDIT: added explanation,

process.hrtime([time])

Returns the current high-resolution real time in a [seconds, nanoseconds] tuple Array. time is an optional parameter that must be the result of a previous process.hrtime() call (and therefore, a real time in a [seconds, nanoseconds] tuple Array containing a previous time) to diff with the current time. These times are relative to an arbitrary time in the past, and not related to the time of day and therefore not subject to clock drift. The primary use is for measuring performance between intervals.

process.nextTick(callback[, arg][, ...])

Once the current event loop turn runs to completion, call the callback function.

This is not a simple alias to setTimeout(fn, 0), it's much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.

Check out this plugin https://github.com/tj/node-blocked I'm using it now and it seems to do what you want.

let blocked = require("blocked");

blocked(ms => {
  console.log("EVENT LOOP Blocked", ms);
});

Will print out how long in ms the event loop is blocked for

"Is there a better way to get this information?" I don't have a better way to test the eventloop than checking the time delay of SetImmediate, but you can get better precision using node's high resolution timer instead of Date.now()

var interval = 500;
var interval = setInterval(function() {
    var last = process.hrtime();          // replace Date.now()        
    setImmediate(function() {
        var delta = process.hrtime(last); // with process.hrtime()
        if (delta > blockDelta) {
            report("node.eventloop_blocked", delta);
        }
    });
}, interval);

NOTE: delta will be a tuple Array [seconds, nanoseconds].

For more details on process.hrtime(): https://nodejs.org/api/all.html#all_process_hrtime

"The primary use is for measuring performance between intervals."