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# Performance measurement APIs#

Stability: 2 - Stable

Source Code: lib/perf_hooks.js

This module provides an implementation of a subset of the W3C Web Performance APIs as well as additional APIs for Node.js-specific performance measurements.

Node.js supports the following Web Performance APIs:

- High Resolution Time
- Performance Timeline
- User Timing

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

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

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

      performance.mark('B');
      performance.measure('A to B', 'A', 'B');
    });

## `perf_hooks.performance`#

Added in: v8.5.0

An object that can be used to collect performance metrics from the current Node.js instance. It is similar to `window.performance` in browsers.

### `performance.clearMarks([name])`#

Added in: v8.5.0

- `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.eventLoopUtilization([utilization1[, utilization2]])`#

Added in: v14.10.0, v12.19.0

- `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 on the main thread the properties have the value of `0`. The ELU is immediately available on Worker threads since bootstrap happens within the event loop.

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.

### `performance.mark([name])`#

Added in: v8.5.0

- `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]])`#

History

Version | Changes
--- | ---
v13.13.0, v12.16.3 | Make `startMark` and `endMark` parameters optional.
v8.5.0 | Added in: v8.5.0

- `name` <string>
- `startMark` <string> Optional.
- `endMark` <string> Optional.

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 optional `endMark` argument must identify any existing `PerformanceMark` in the Performance Timeline or any of the timestamp properties provided by the `PerformanceNodeTiming` class. `endMark` will be `performance.now()` if no parameter is passed, otherwise if the named `endMark` does not exist, an error will be thrown.

### `performance.nodeTiming`#

Added in: v8.5.0

- <PerformanceNodeTiming>

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

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

### `performance.now()`#

Added in: v8.5.0

- Returns: <number>

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

### `performance.timeOrigin`#

Added in: v8.5.0

- <number>

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

### `performance.timerify(fn)`#

Added in: v8.5.0

- `fn` <Function>

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

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`#

Added in: v8.5.0

### `performanceEntry.duration`#

Added in: v8.5.0

- <number>

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

### `performanceEntry.entryType`#

Added in: v8.5.0

- <string>

The type of the performance entry. It may be one of:

- `'node'` (Node.js only)
- `'mark'` (available on the Web)
- `'measure'` (available on the Web)
- `'gc'` (Node.js only)
- `'function'` (Node.js only)
- `'http2'` (Node.js only)
- `'http'` (Node.js only)

### performanceEntry.flags#

Added in: v13.9.0, v12.17.0

- <number>

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

When `performanceEntry.entryType` is equal to `'gc'`, the `performance.flags` property contains additional information about garbage collection operation. The value may be one of:

- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_NO`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_CONSTRUCT_RETAINED`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_FORCED`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SYNCHRONOUS_PHANTOM_PROCESSING`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_AVAILABLE_GARBAGE`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_ALL_EXTERNAL_MEMORY`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_FLAGS_SCHEDULE_IDLE`

### `performanceEntry.name`#

Added in: v8.5.0

- <string>

The name of the performance entry.

### `performanceEntry.kind`#

Added in: v8.5.0

- <number>

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

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`

### `performanceEntry.startTime`#

Added in: v8.5.0

- <number>

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

## Class: `PerformanceNodeTiming`#

Added in: v8.5.0

- Extends: <PerformanceEntry>

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

Provides timing details for Node.js itself. The constructor of this class is not exposed to users.

### `performanceNodeTiming.bootstrapComplete`#

Added in: v8.5.0

- <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.environment`#

Added in: v8.5.0

- <number>

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

### `performanceNodeTiming.idleTime`#

Added in: v14.10.0, v12.19.0

- <number>

The high resolution millisecond timestamp of the amount of time the event loop has been idle within the event loop's event provider (e.g. `epoll_wait`). This does not take CPU usage into consideration. If the event loop has not yet started (e.g., in the first tick of the main script), the property has the value of 0.

### `performanceNodeTiming.loopExit`#

Added in: v8.5.0

- <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`#

Added in: v8.5.0

- <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`#

Added in: v8.5.0

- <number>

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

### `performanceNodeTiming.v8Start`#

Added in: v8.5.0

- <number>

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

## Class: `perf_hooks.PerformanceObserver`#

### `new PerformanceObserver(callback)`#

Added in: v8.5.0

- `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()`#

Added in: v8.5.0

Disconnects the `PerformanceObserver` instance from all notifications.

### `performanceObserver.observe(options)`#

Added in: v8.5.0

- `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`#

Added in: v8.5.0

The `PerformanceObserverEntryList` class is used to provide access to the `PerformanceEntry` instances passed to a `PerformanceObserver`. The constructor of this class is not exposed to users.

### `performanceObserverEntryList.getEntries()`#

Added in: v8.5.0

- Returns: <PerformanceEntry[]>

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

### `performanceObserverEntryList.getEntriesByName(name[, type])`#

Added in: v8.5.0

- `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)`#

Added in: v8.5.0

- `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])`#

Added in: v11.10.0

- `options` <Object>
  - `resolution` <number> The sampling rate in milliseconds. Must be greater than zero. Default: `10`.
- Returns: <Histogram>

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.

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`#

Added in: v11.10.0

Tracks the event loop delay at a given sampling rate. The constructor of this class not exposed to users.

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

#### `histogram.disable()`#

Added in: v11.10.0

- Returns: <boolean>

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

#### `histogram.enable()`#

Added in: v11.10.0

- Returns: <boolean>

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

#### `histogram.exceeds`#

Added in: v11.10.0

- <number>

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

#### `histogram.max`#

Added in: v11.10.0

- <number>

The maximum recorded event loop delay.

#### `histogram.mean`#

Added in: v11.10.0

- <number>

The mean of the recorded event loop delays.

#### `histogram.min`#

Added in: v11.10.0

- <number>

The minimum recorded event loop delay.

#### `histogram.percentile(percentile)`#

Added in: v11.10.0

- `percentile` <number> A percentile value between 1 and 100.
- Returns: <number>

Returns the value at the given percentile.

#### `histogram.percentiles`#

Added in: v11.10.0

- <Map>

Returns a `Map` object detailing the accumulated percentile distribution.

#### `histogram.reset()`#

Added in: v11.10.0

Resets the collected histogram data.

#### `histogram.stddev`#

Added in: v11.10.0

- <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 took 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');