# Performance Timing API

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> Stability: 2 - Stable

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.

```js
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`
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### `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.mark([name])`
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* `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)`
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* `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`
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* {PerformanceNodeTiming}

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)`
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* `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.

```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);
  obs.disconnect();
});
obs.observe({ entryTypes: ['function'] });

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

## Class: `PerformanceEntry`
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### `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.name`
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* {string}

The name of the performance entry.

### `performanceEntry.startTime`
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* {number}

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

### `performanceEntry.entryType`
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* {string}

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

### `performanceEntry.kind`
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* {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`

### performanceEntry.flags
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* {number}

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`

## Class: `PerformanceNodeTiming extends PerformanceEntry`
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Provides timing details for Node.js itself.

### `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.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: `PerformanceObserver`

### `new PerformanceObserver(callback)`
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* `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());
  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)`
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* `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:

```js
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}`);
```

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

### `performanceObserverEntryList.getEntries()`
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* Returns: {PerformanceEntry[]}

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

### `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`.

### `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`.

## `perf_hooks.monitorEventLoopDelay([options])`
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* `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. 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`
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Tracks the event loop delay at a given sampling rate.

#### `histogram.disable()`
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* Returns: {boolean}

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

#### `histogram.enable()`
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* Returns: {boolean}

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

#### `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.max`
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* {number}

The maximum recorded event loop delay.

#### `histogram.mean`
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* {number}

The mean of the recorded event loop delays.

#### `histogram.min`
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* {number}

The minimum recorded event loop delay.

#### `histogram.percentile(percentile)`
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* `percentile` {number} A percentile value between 1 and 100.
* Returns: {number}

Returns the value at the given percentile.

#### `histogram.percentiles`
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* {Map}

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

#### `histogram.reset()`
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Resets the collected histogram data.

#### `histogram.stddev`
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* {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).

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

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

[`'exit'`]: process.html#process_event_exit
[`timeOrigin`]: https://w3c.github.io/hr-time/#dom-performance-timeorigin
[Async Hooks]: async_hooks.html
[W3C Performance Timeline]: https://w3c.github.io/performance-timeline/