function

perf_hooks.monitorEventLoopDelay

options?: EventLoopMonitorOptions

This property is an extension by Node.js. It is not available in Web browsers.

Creates a histogram object that samples and reports the event loop delay over time. The delays will be reported in nanoseconds.

By default, the histogram is updated by a timer using the configured resolution. When samplePerIteration is true, samples are taken once per event loop iteration using uv_prepare_t and uv_check_t hooks. In that mode, the histogram does not keep the loop alive or force additional iterations when the application is idle. The two sampling modes produce significantly different results and should not be compared directly.

import { monitorEventLoopDelay } from 'node:perf_hooks';
const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();
// Do something.
h.disable();
console.log(h.min);
console.log(h.max);
console.log(h.mean);
console.log(h.stddev);
console.log(h.percentiles);
console.log(h.percentile(50));
console.log(h.percentile(99));

Referenced types

interface EventLoopMonitorOptions

  • resolution?: number

    The sampling rate in milliseconds for interval-based sampling. Must be greater than zero. This option is ignored when samplePerIteration is true. Default: 10.

  • samplePerIteration?: boolean

    When true, samples are taken once per event loop iteration. Default: false.

interface ELDHistogram

A Histogram that records event loop delay, returned by perf_hooks.monitorEventLoopDelay().

  • readonly count: number

    The number of samples recorded by the histogram.

  • readonly countBigInt: bigint

    The number of samples recorded by the histogram. v17.4.0, v16.14.0

  • readonly exceeds: number

    The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

  • readonly exceedsBigInt: bigint

    The number of times the event loop delay exceeded the maximum 1 hour event loop delay threshold.

  • readonly max: number

    The maximum recorded event loop delay.

  • readonly maxBigInt: number

    The maximum recorded event loop delay. v17.4.0, v16.14.0

  • readonly mean: number

    The mean of the recorded event loop delays.

  • readonly min: number

    The minimum recorded event loop delay.

  • readonly minBigInt: bigint

    The minimum recorded event loop delay. v17.4.0, v16.14.0

  • readonly percentiles: Map<number, number>

    Returns a Map object detailing the accumulated percentile distribution.

  • readonly percentilesBigInt: Map<bigint, bigint>

    Returns a Map object detailing the accumulated percentile distribution.

  • readonly stddev: number

    The standard deviation of the recorded event loop delays.

  • Disables event loop delay sampling when the histogram is disposed.

    const { monitorEventLoopDelay } = require('node:perf_hooks');
{
  using hist = monitorEventLoopDelay({ resolution: 20 });
  hist.enable();
  // The histogram will be disabled when the block is exited.
}
  • disable(): boolean;

    Disables event loop delay sampling. Returns true if sampling was stopped, false if it was already stopped.

  • enable(): boolean;

    Enables event loop delay sampling. Returns true if sampling was started, false if it was already started.

  • percentile: number
    ): number;

    Returns the value at the given percentile.

    @param percentile

    A percentile value in the range (0, 100].

  • percentile: number
    ): bigint;

    Returns the value at the given percentile.

    @param percentile

    A percentile value in the range (0, 100].

  • reset(): void;

    Resets the collected histogram data.