node/doc/api/perf_hooks.md

# Performance measurement APIs

<!--introduced_in=v8.5.0-->

> Stability: 2 - Stable

<!-- source_link=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][]

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

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

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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])`

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* `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.clearMeasures([name])`

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* `name` {string}

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

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

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* `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.

```js
'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.getEntries()`

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* Returns: {PerformanceEntry\[]}

Returns a list of `PerformanceEntry` objects in chronological order with
respect to `performanceEntry.startTime`. If you are only interested in
performance entries of certain types or that have certain names, see
`performance.getEntriesByType()` and `performance.getEntriesByName()`.

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

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* `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`.

### `performance.getEntriesByType(type)`

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* `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`.

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

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    pr-url: https://github.com/nodejs/node/pull/37136
    description: Updated to conform to the User Timing Level 3 specification.
-->

* `name` {string}
* `options` {Object}
  * `detail` {any} Additional optional detail to include with the mark.
  * `startTime` {number} An optional timestamp to be used as the mark time.
    **Defaults**: `performance.now()`.

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.

The created `PerformanceMark` entry is put in the global Performance Timeline
and can be queried with `performance.getEntries`,
`performance.getEntriesByName`, and `performance.getEntriesByType`. When the
observation is performed, the entries should be cleared from the global
Performance Timeline manually with `performance.clearMarks`.

### `performance.measure(name[, startMarkOrOptions[, endMark]])`

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    pr-url: https://github.com/nodejs/node/pull/37136
    description: Updated to conform to the User Timing Level 3 specification.
  - version:
      - v13.13.0
      - v12.16.3
    pr-url: https://github.com/nodejs/node/pull/32651
    description: Make `startMark` and `endMark` parameters optional.
-->

* `name` {string}
* `startMarkOrOptions` {string|Object} Optional.
  * `detail` {any} Additional optional detail to include with the measure.
  * `duration` {number} Duration between start and end times.
  * `end` {number|string} Timestamp to be used as the end time, or a string
    identifying a previously recorded mark.
  * `start` {number|string} Timestamp to be used as the start time, or a string
    identifying a previously recorded mark.
* `endMark` {string} Optional. Must be omitted if `startMarkOrOptions` is an
  {Object}.

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, an error is thrown.

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.

The created `PerformanceMeasure` entry is put in the global Performance Timeline
and can be queried with `performance.getEntries`,
`performance.getEntriesByName`, and `performance.getEntriesByType`. When the
observation is performed, the entries should be cleared from the global
Performance Timeline manually with `performance.clearMeasures`.

### `performance.nodeTiming`

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* {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()`

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* Returns: {number}

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

### `performance.timeOrigin`

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* {number}

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

### `performance.timerify(fn[, options])`

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added: v8.5.0
changes:
  - version: v16.0.0
    pr-url: https://github.com/nodejs/node/pull/37475
    description: Added the histogram option.
  - version: v16.0.0
    pr-url: https://github.com/nodejs/node/pull/37136
    description: Re-implemented to use pure-JavaScript and the ability
                 to time async functions.
-->

* `fn` {Function}
* `options` {Object}
  * `histogram` {RecordableHistogram} A histogram object created using
    `perf_hooks.createHistogram()` that will record runtime durations in
    nanoseconds.

_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.

```js
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);

  performance.clearMarks();
  performance.clearMeasures();
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

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

If the wrapped function returns a promise, a finally handler will be attached
to the promise and the duration will be reported once the finally handler is
invoked.

### `performance.toJSON()`

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An object which is JSON representation of the `performance` object. It
is similar to [`window.performance.toJSON`][] in browsers.

## Class: `PerformanceEntry`

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### `performanceEntry.detail`

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* {any}

Additional detail specific to the `entryType`.

### `performanceEntry.duration`

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* {number}

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

### `performanceEntry.entryType`

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* {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`

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                 when entryType is 'gc'.
-->

* {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`

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* {string}

The name of the performance entry.

### `performanceEntry.kind`

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    description: Runtime deprecated. Now moved to the detail property
                 when entryType is 'gc'.
-->

* {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`

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* {number}

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

### Garbage Collection ('gc') Details

When `performanceEntry.type` is equal to `'gc'`, the `performanceEntry.detail`
property will be an {Object} with two properties:

* `kind` {number} 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`
* `flags` {number} 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`

### HTTP/2 ('http2') Details

When `performanceEntry.type` is equal to `'http2'`, the
`performanceEntry.detail` property will be an {Object} containing
additional performance information.

If `performanceEntry.name` is equal to `Http2Stream`, the `detail`
will contain the following properties:

* `bytesRead` {number} The number of `DATA` frame bytes received for this
  `Http2Stream`.
* `bytesWritten` {number} The number of `DATA` frame bytes sent for this
  `Http2Stream`.
* `id` {number} The identifier of the associated `Http2Stream`
* `timeToFirstByte` {number} The number of milliseconds elapsed between the
  `PerformanceEntry` `startTime` and the reception of the first `DATA` frame.
* `timeToFirstByteSent` {number} The number of milliseconds elapsed between
  the `PerformanceEntry` `startTime` and sending of the first `DATA` frame.
* `timeToFirstHeader` {number} The number of milliseconds elapsed between the
  `PerformanceEntry` `startTime` and the reception of the first header.

If `performanceEntry.name` is equal to `Http2Session`, the `detail` will
contain the following properties:

* `bytesRead` {number} The number of bytes received for this `Http2Session`.
* `bytesWritten` {number} The number of bytes sent for this `Http2Session`.
* `framesReceived` {number} The number of HTTP/2 frames received by the
  `Http2Session`.
* `framesSent` {number} The number of HTTP/2 frames sent by the `Http2Session`.
* `maxConcurrentStreams` {number} The maximum number of streams concurrently
  open during the lifetime of the `Http2Session`.
* `pingRTT` {number} The number of milliseconds elapsed since the transmission
  of a `PING` frame and the reception of its acknowledgment. Only present if
  a `PING` frame has been sent on the `Http2Session`.
* `streamAverageDuration` {number} The average duration (in milliseconds) for
  all `Http2Stream` instances.
* `streamCount` {number} The number of `Http2Stream` instances processed by
  the `Http2Session`.
* `type` {string} Either `'server'` or `'client'` to identify the type of
  `Http2Session`.

### Timerify ('function') Details

When `performanceEntry.type` is equal to `'function'`, the
`performanceEntry.detail` property will be an {Array} listing
the input arguments to the timed function.

### Net ('net') Details

When `performanceEntry.type` is equal to `'net'`, the
`performanceEntry.detail` property will be an {Object} containing
additional information.

If `performanceEntry.name` is equal to `connect`, the `detail`
will contain the following properties: `host`, `port`.

### DNS ('dns') Details

When `performanceEntry.type` is equal to `'dns'`, the
`performanceEntry.detail` property will be an {Object} containing
additional information.

If `performanceEntry.name` is equal to `lookup`, the `detail`
will contain the following properties: `hostname`, `family`, `hints`, `verbatim`.

If `performanceEntry.name` is equal to `lookupService`, the `detail` will
contain the following properties: `host`, `port`.

If `performanceEntry.name` is equal to `queryxxx` or `getHostByAddr`, the `detail` will
contain the following properties: `host`, `ttl`.

## Class: `PerformanceNodeTiming`

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* 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`

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* {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`

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* {number}

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

### `performanceNodeTiming.idleTime`

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  - v12.19.0
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* {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`

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* {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`

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* {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`

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* {number}

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

### `performanceNodeTiming.v8Start`

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* {number}

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

## Class: `perf_hooks.PerformanceObserver`

### `new PerformanceObserver(callback)`

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                 now throws `ERR_INVALID_ARG_TYPE` instead of
                 `ERR_INVALID_CALLBACK`.
-->

* `callback` {Function}
  * `list` {PerformanceObserverEntryList}
  * `observer` {PerformanceObserver}

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

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

const obs = new PerformanceObserver((list, observer) => {
  console.log(list.getEntries());

  performance.clearMarks();
  performance.clearMeasures();
  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()`

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Disconnects the `PerformanceObserver` instance from all notifications.

### `performanceObserver.observe(options)`

<!-- YAML
added: v8.5.0
changes:
  - version: v16.7.0
    pr-url: https://github.com/nodejs/node/pull/39297
    description: Updated to conform to Performance Timeline Level 2. The
                 buffered option has been added back.
  - version: v16.0.0
    pr-url: https://github.com/nodejs/node/pull/37136
    description: Updated to conform to User Timing Level 3. The
                 buffered option has been removed.
-->

* `options` {Object}
  * `type` {string} A single {PerformanceEntry} type. Must not be given
    if `entryTypes` is already specified.
  * `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 observer callback is called with a
    list global `PerformanceEntry` buffered entries. If false, only
    `PerformanceEntry`s created after the time point are sent to the
    observer callback. **Default:** `false`.

Subscribes the {PerformanceObserver} instance to notifications of new
{PerformanceEntry} instances identified either by `options.entryTypes`
or `options.type`:

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

const obs = new PerformanceObserver((list, observer) => {
  // Called once asynchronously. `list` contains three items.
});
obs.observe({ type: 'mark' });

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

## Class: `PerformanceObserverEntryList`

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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()`

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* Returns: {PerformanceEntry\[]}

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

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

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntries());
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 81.465639,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 81.860064,
   *     duration: 0
   *   }
   * ]
   */

  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ type: 'mark' });

performance.mark('test');
performance.mark('meow');
```

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

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* `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`.

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

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntriesByName('meow'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 98.545991,
   *     duration: 0
   *   }
   * ]
   */
  console.log(perfObserverList.getEntriesByName('nope')); // []

  console.log(perfObserverList.getEntriesByName('test', 'mark'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 63.518931,
   *     duration: 0
   *   }
   * ]
   */
  console.log(perfObserverList.getEntriesByName('test', 'measure')); // []

  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ entryTypes: ['mark', 'measure'] });

performance.mark('test');
performance.mark('meow');
```

### `performanceObserverEntryList.getEntriesByType(type)`

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* `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`.

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

const obs = new PerformanceObserver((perfObserverList, observer) => {
  console.log(perfObserverList.getEntriesByType('mark'));
  /**
   * [
   *   PerformanceEntry {
   *     name: 'test',
   *     entryType: 'mark',
   *     startTime: 55.897834,
   *     duration: 0
   *   },
   *   PerformanceEntry {
   *     name: 'meow',
   *     entryType: 'mark',
   *     startTime: 56.350146,
   *     duration: 0
   *   }
   * ]
   */
  performance.clearMarks();
  performance.clearMeasures();
  observer.disconnect();
});
obs.observe({ type: 'mark' });

performance.mark('test');
performance.mark('meow');
```

## `perf_hooks.createHistogram([options])`

<!-- YAML
added:
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* `options` {Object}
  * `lowest` {number|bigint} The lowest discernible value. Must be an integer
    value greater than 0. **Default:** `1`.
  * `highest` {number|bigint} The highest recordable value. Must be an integer
    value that is equal to or greater than two times `lowest`.
    **Default:** `Number.MAX_SAFE_INTEGER`.
  * `figures` {number} The number of accuracy digits. Must be a number between
    `1` and `5`. **Default:** `3`.
* Returns {RecordableHistogram}

Returns a {RecordableHistogram}.

## `perf_hooks.monitorEventLoopDelay([options])`

<!-- YAML
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* `options` {Object}
  * `resolution` {number} The sampling rate in milliseconds. Must be greater
    than zero. **Default:** `10`.
* Returns: {IntervalHistogram}

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

Creates an `IntervalHistogram` 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.

```js
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`

<!-- YAML
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### `histogram.count`

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* {number}

The number of samples recorded by the histogram.

### `histogram.countBigInt`

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* {bigint}

The number of samples recorded by the histogram.

### `histogram.exceeds`

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* {number}

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

### `histogram.exceedsBigInt`

<!-- YAML
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* {bigint}

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

### `histogram.max`

<!-- YAML
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-->

* {number}

The maximum recorded event loop delay.

### `histogram.maxBigInt`

<!-- YAML
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-->

* {bigint}

The maximum recorded event loop delay.

### `histogram.mean`

<!-- YAML
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-->

* {number}

The mean of the recorded event loop delays.

### `histogram.min`

<!-- YAML
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-->

* {number}

The minimum recorded event loop delay.

### `histogram.minBigInt`

<!-- YAML
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-->

* {bigint}

The minimum recorded event loop delay.

### `histogram.percentile(percentile)`

<!-- YAML
added: v11.10.0
-->

* `percentile` {number} A percentile value in the range (0, 100].
* Returns: {number}

Returns the value at the given percentile.

### `histogram.percentileBigInt(percentile)`

<!-- YAML
added:
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-->

* `percentile` {number} A percentile value in the range (0, 100].
* Returns: {bigint}

Returns the value at the given percentile.

### `histogram.percentiles`

<!-- YAML
added: v11.10.0
-->

* {Map}

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

### `histogram.percentilesBigInt`

<!-- YAML
added:
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  - v16.14.0
-->

* {Map}

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

### `histogram.reset()`

<!-- YAML
added: v11.10.0
-->

Resets the collected histogram data.

### `histogram.stddev`

<!-- YAML
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-->

* {number}

The standard deviation of the recorded event loop delays.

## Class: `IntervalHistogram extends Histogram`

A `Histogram` that is periodically updated on a given interval.

### `histogram.disable()`

<!-- YAML
added: v11.10.0
-->

* Returns: {boolean}

Disables the update interval timer. Returns `true` if the timer was
stopped, `false` if it was already stopped.

### `histogram.enable()`

<!-- YAML
added: v11.10.0
-->

* Returns: {boolean}

Enables the update interval timer. Returns `true` if the timer was
started, `false` if it was already started.

### Cloning an `IntervalHistogram`

{IntervalHistogram} instances can be cloned via {MessagePort}. On the receiving
end, the histogram is cloned as a plain {Histogram} object that does not
implement the `enable()` and `disable()` methods.

## Class: `RecordableHistogram extends Histogram`

<!-- YAML
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### `histogram.add(other)`

<!-- YAML
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* `other` {RecordableHistogram}

Adds the values from `other` to this histogram.

### `histogram.record(val)`

<!-- YAML
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* `val` {number|bigint} The amount to record in the histogram.

### `histogram.recordDelta()`

<!-- YAML
added:
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  - v14.18.0
-->

Calculates the amount of time (in nanoseconds) that has passed since the
previous call to `recordDelta()` and records that amount in the histogram.

## 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).

```js
'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();
  performance.clearMeasures();
  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:

<!-- eslint-disable no-global-assign -->

```js
'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);
  });
  performance.clearMarks();
  performance.clearMeasures();
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'], buffered: true });

require('some-module');
```

### Measuring how long one HTTP round-trip takes

The following example is used to trace the time spent by HTTP client
(`OutgoingMessage`) and HTTP request (`IncomingMessage`). For HTTP client,
it means the time interval between starting the request and receiving the
response, and for HTTP request, it means the time interval between receiving
the request and sending the response:

```js
'use strict';
const { PerformanceObserver } = require('perf_hooks');
const http = require('http');

const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});

obs.observe({ entryTypes: ['http'] });

const PORT = 8080;

http.createServer((req, res) => {
  res.end('ok');
}).listen(PORT, () => {
  http.get(`http://127.0.0.1:${PORT}`);
});
```

### Measuring how long the `net.connect` (only for TCP) takes when the connection is successful

```js
'use strict';
const { PerformanceObserver } = require('perf_hooks');
const net = require('net');
const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});
obs.observe({ entryTypes: ['net'] });
const PORT = 8080;
net.createServer((socket) => {
  socket.destroy();
}).listen(PORT, () => {
  net.connect(PORT);
});
```

### Measuring how long the DNS takes when the request is successful

```js
'use strict';
const { PerformanceObserver } = require('perf_hooks');
const dns = require('dns');
const obs = new PerformanceObserver((items) => {
  items.getEntries().forEach((item) => {
    console.log(item);
  });
});
obs.observe({ entryTypes: ['dns'] });
dns.lookup('localhost', () => {});
dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[High Resolution Time]: https://www.w3.org/TR/hr-time-2
[Performance Timeline]: https://w3c.github.io/performance-timeline/
[User Timing]: https://www.w3.org/TR/user-timing/
[Web Performance APIs]: https://w3c.github.io/perf-timing-primer/
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`process.hrtime()`]: process.md#processhrtimetime
[`timeOrigin`]: https://w3c.github.io/hr-time/#dom-performance-timeorigin
[`window.performance.toJSON`]: https://developer.mozilla.org/en-US/docs/Web/API/Performance/toJSON
[`window.performance`]: https://developer.mozilla.org/en-US/docs/Web/API/Window/performance