performance eventlooputilization utilization1 utilization2

• utilization1 {Object} The result of a previous call to eventLoopUtilization().
• utilization2 {Object} The result of a previous call to eventLoopUtilization() prior to utilization1.
• Returns {Object}
  • idle {number}
  • active {number}
  • utilization {number}

The eventLoopUtilization() method returns an object that contains the cumulative duration of time the event loop has been both idle and active as a high resolution milliseconds timer. The utilization value is the calculated Event Loop Utilization (ELU). If bootstrapping has not yet finished, the properties have the value of 0.

Both utilization1 and utilization2 are optional parameters.

If utilization1 is passed, then the delta between the current call’s active and idle times, as well as the corresponding utilization value are calculated and returned (similar to [process.hrtime()][]).

If utilization1 and utilization2 are both passed, then the delta is calculated between the two arguments. This is a convenience option because, unlike [process.hrtime()][], calculating the ELU is more complex than a single subtraction.

ELU is similar to CPU utilization, except that it only measures event loop statistics and not CPU usage. It represents the percentage of time the event loop has spent outside the event loop’s event provider (e.g. epoll_wait). No other CPU idle time is taken into consideration. The following is an example of how a mostly idle process will have a high ELU.

'use strict';
const { eventLoopUtilization } = require('perf_hooks').performance;
const { spawnSync } = require('child_process');
setImmediate(() => {
  const elu = eventLoopUtilization();
  spawnSync('sleep', ['5']);
  console.log(eventLoopUtilization(elu).utilization);
});

Although the CPU is mostly idle while running this script, the value of utilization is 1. This is because the call to [child_process.spawnSync()][] blocks the event loop from proceeding.

Passing in a user-defined object instead of the result of a previous call to eventLoopUtilization() will lead to undefined behavior. The return values are not guaranteed to reflect any correct state of the event loop.