nodejs/doc/api/perf_hooks.md

Performance Timing API

Stability: 1 - Experimental

The Performance Timing API provides an implementation of the W3C Performance Timeline specification. The purpose of the API is to support collection of high resolution performance metrics. This is the same Performance API as implemented in modern Web browsers.

const { PerformanceObserver, performance } = require('perf_hooks');

const obs = new PerformanceObserver((items) => {
  console.log(items.getEntries()[0].duration);
  performance.clearMarks();
});
obs.observe({ entryTypes: ['measure'] });

performance.mark('A');
doSomeLongRunningProcess(() => {
  performance.mark('B');
  performance.measure('A to B', 'A', 'B');
});

Class: Performance

performance.clearMarks([name])

• name {string}

If name is not provided, removes all PerformanceMark objects from the Performance Timeline. If name is provided, removes only the named mark.

performance.mark([name])

• name {string}

Creates a new PerformanceMark entry in the Performance Timeline. A PerformanceMark is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'mark', and whose performanceEntry.duration is always 0. Performance marks are used to mark specific significant moments in the Performance Timeline.

performance.measure(name, startMark, endMark)

• name {string}
• startMark {string}
• endMark {string}

Creates a new PerformanceMeasure entry in the Performance Timeline. A PerformanceMeasure is a subclass of PerformanceEntry whose performanceEntry.entryType is always 'measure', and whose performanceEntry.duration measures the number of milliseconds elapsed since startMark and endMark.

The startMark argument may identify any existing PerformanceMark in the Performance Timeline, or may identify any of the timestamp properties provided by the PerformanceNodeTiming class. If the named startMark does not exist, then startMark is set to timeOrigin by default.

The endMark argument must identify any existing PerformanceMark in the Performance Timeline or any of the timestamp properties provided by the PerformanceNodeTiming class. If the named endMark does not exist, an error will be thrown.

performance.nodeTiming

• {PerformanceNodeTiming}

An instance of the PerformanceNodeTiming class that provides performance metrics for specific Node.js operational milestones.

performance.now()

• Returns: {number}

Returns the current high resolution millisecond timestamp, where 0 represents the start of the current node process.

performance.timeOrigin

• {number}

The timeOrigin specifies the high resolution millisecond timestamp at which the current node process began, measured in Unix time.

performance.timerify(fn)

• fn {Function}

Wraps a function within a new function that measures the running time of the wrapped function. A PerformanceObserver must be subscribed to the 'function' event type in order for the timing details to be accessed.

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

function someFunction() {
  console.log('hello world');
}

const wrapped = performance.timerify(someFunction);

const obs = new PerformanceObserver((list) => {
  console.log(list.getEntries()[0].duration);
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

// A performance timeline entry will be created
wrapped();

Class: PerformanceEntry

performanceEntry.duration

• {number}

The total number of milliseconds elapsed for this entry. This value will not be meaningful for all Performance Entry types.

performanceEntry.name

• {string}

The name of the performance entry.

performanceEntry.startTime

• {number}

The high resolution millisecond timestamp marking the starting time of the Performance Entry.

performanceEntry.entryType

• {string}

The type of the performance entry. Currently it may be one of: 'node', 'mark', 'measure', 'gc', 'function', or 'http2'.

performanceEntry.kind

• {number}

When performanceEntry.entryType is equal to 'gc', the performance.kind property identifies the type of garbage collection operation that occurred. The value may be one of:

• perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR
• perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR
• perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL
• perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB

Class: PerformanceNodeTiming extends PerformanceEntry

Provides timing details for Node.js itself.

performanceNodeTiming.bootstrapComplete

• {number}

The high resolution millisecond timestamp at which the Node.js process completed bootstrapping. If bootstrapping has not yet finished, the property has the value of -1.

performanceNodeTiming.loopExit

• {number}

The high resolution millisecond timestamp at which the Node.js event loop exited. If the event loop has not yet exited, the property has the value of -1. It can only have a value of not -1 in a handler of the 'exit' event.

performanceNodeTiming.loopStart

• {number}

The high resolution millisecond timestamp at which the Node.js event loop started. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of -1.

performanceNodeTiming.nodeStart

• {number}

The high resolution millisecond timestamp at which the Node.js process was initialized.

performanceNodeTiming.v8Start

• {number}

The high resolution millisecond timestamp at which the V8 platform was initialized.

Class: PerformanceObserver

new PerformanceObserver(callback)

• callback {Function}
  • list {PerformanceObserverEntryList}
  • observer {PerformanceObserver}

PerformanceObserver objects provide notifications when new PerformanceEntry instances have been added to the Performance Timeline.

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark'], buffered: true });

performance.mark('test');

Because PerformanceObserver instances introduce their own additional performance overhead, instances should not be left subscribed to notifications indefinitely. Users should disconnect observers as soon as they are no longer needed.

The callback is invoked when a PerformanceObserver is notified about new PerformanceEntry instances. The callback receives a PerformanceObserverEntryList instance and a reference to the PerformanceObserver.

performanceObserver.disconnect()

Disconnects the PerformanceObserver instance from all notifications.

performanceObserver.observe(options)

• options {Object}
  • entryTypes {string[]} An array of strings identifying the types of PerformanceEntry instances the observer is interested in. If not provided an error will be thrown.
  • buffered {boolean} If true, the notification callback will be called using setImmediate() and multiple PerformanceEntry instance notifications will be buffered internally. If false, notifications will be immediate and synchronous. Default: false.

Subscribes the PerformanceObserver instance to notifications of new PerformanceEntry instances identified by options.entryTypes.

When options.buffered is false, the callback will be invoked once for every PerformanceEntry instance:

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // Called three times synchronously. list contains one item
});
obs.observe({ entryTypes: ['mark'] });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const obs = new PerformanceObserver((list, observer) => {
  // called once. list contains three items
});
obs.observe({ entryTypes: ['mark'], buffered: true });

for (let n = 0; n < 3; n++)
  performance.mark(`test${n}`);

Class: PerformanceObserverEntryList

The PerformanceObserverEntryList class is used to provide access to the PerformanceEntry instances passed to a PerformanceObserver.

performanceObserverEntryList.getEntries()

• Returns: {PerformanceEntry[]}

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime.

performanceObserverEntryList.getEntriesByName(name[, type])

• name {string}
• type {string}
• Returns: {PerformanceEntry[]}

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.name is equal to name, and optionally, whose performanceEntry.entryType is equal to type.

performanceObserverEntryList.getEntriesByType(type)

• type {string}
• Returns: {PerformanceEntry[]}

Returns a list of PerformanceEntry objects in chronological order with respect to performanceEntry.startTime whose performanceEntry.entryType is equal to type.

perf_hooks.monitorEventLoopDelay([options])

• options {Object}
  • resolution {number} The sampling rate in milliseconds. Must be greater than zero. Default: 10.
• Returns: {Histogram}

Creates a Histogram object that samples and reports the event loop delay over time.

Using a timer to detect approximate event loop delay works because the execution of timers is tied specifically to the lifecycle of the libuv event loop. That is, a delay in the loop will cause a delay in the execution of the timer, and those delays are specifically what this API is intended to detect.

const { monitorEventLoopDelay } = require('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));

Class: Histogram

Tracks the event loop delay at a given sampling rate.

histogram.disable()

• Returns: {boolean}

Disables the event loop delay sample timer. Returns true if the timer was stopped, false if it was already stopped.

histogram.enable()

• Returns: {boolean}

Enables the event loop delay sample timer. Returns true if the timer was started, false if it was already started.

histogram.exceeds

• {number}

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

histogram.max

• {number}

The maximum recorded event loop delay.

histogram.mean

• {number}

The mean of the recorded event loop delays.

histogram.min

• {number}

The minimum recorded event loop delay.

histogram.percentile(percentile)

• percentile {number} A percentile value between 1 and 100.
• Returns: {number}

Returns the value at the given percentile.

histogram.percentiles

• {Map}

Returns a Map object detailing the accumulated percentile distribution.

histogram.reset()

Resets the collected histogram data.

histogram.stddev

• {number}

The standard deviation of the recorded event loop delays.

Examples

Measuring the duration of async operations

The following example uses the Async Hooks and Performance APIs to measure the actual duration of a Timeout operation (including the amount of time it to execute the callback).

'use strict';
const async_hooks = require('async_hooks');
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');

const set = new Set();
const hook = async_hooks.createHook({
  init(id, type) {
    if (type === 'Timeout') {
      performance.mark(`Timeout-${id}-Init`);
      set.add(id);
    }
  },
  destroy(id) {
    if (set.has(id)) {
      set.delete(id);
      performance.mark(`Timeout-${id}-Destroy`);
      performance.measure(`Timeout-${id}`,
                          `Timeout-${id}-Init`,
                          `Timeout-${id}-Destroy`);
    }
  }
});
hook.enable();

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries()[0]);
  performance.clearMarks();
  observer.disconnect();
});
obs.observe({ entryTypes: ['measure'], buffered: true });

setTimeout(() => {}, 1000);

Measuring how long it takes to load dependencies

The following example measures the duration of require() operations to load dependencies:

'use strict';
const {
  performance,
  PerformanceObserver
} = require('perf_hooks');
const mod = require('module');

// Monkey patch the require function
mod.Module.prototype.require =
  performance.timerify(mod.Module.prototype.require);
require = performance.timerify(require);

// Activate the observer
const obs = new PerformanceObserver((list) => {
  const entries = list.getEntries();
  entries.forEach((entry) => {
    console.log(`require('${entry[0]}')`, entry.duration);
  });
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');