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✏️ Editing: perf_hooks.md

# Performance measurement APIs

> Stability: 2 - Stable

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({ 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`

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])`

* `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]])`

* `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.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} 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`

* {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()`

* 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}

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

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

### `performance.eventLoopUtilization([util1][,util2])`

* `util1` {Object} The result of a previous call to `eventLoopUtilization()`
* `util2` {Object} The result of a previous call to `eventLoopUtilization()`
    prior to `util1`
* Returns {Object}
  * `idle` {number}
  * `active` {number}
  * `utilization` {number}

`util1` and `util2` are optional parameters.

If `util1` is passed then the delta between the current call's `active` and
`idle` times are calculated and returned (similar to [`process.hrtime()`][]).
Likewise the adjusted `utilization` value is calculated.

If `util1` and `util2` are both passed then the calculation adjustments are
done between the two arguments. This is a convenience option because unlike
[`process.hrtime()`][] additional work is done to calculate the ELU.

ELU is similar to CPU utilization except that it is calculated using high
precision wall-clock time. 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.

## 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. 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.kind`

* {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.flags

* {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`

## Class: `PerformanceNodeTiming 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`

* {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`

* {number}

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

### `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.

### `performanceNodeTiming.idleTime`

* {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.

## Class: `perf_hooks.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.

```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()`

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:

```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`

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()`

* 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}

_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.

```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`

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()`

* 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 took
to execute the callback).

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

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:

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

MR SKB