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# Performance Timing API#

Stability: 1 - Experimental

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

    const { performance } = require('perf_hooks');
    performance.mark('A');
    doSomeLongRunningProcess(() => {
      performance.mark('B');
      performance.measure('A to B', 'A', 'B');
      const measure = performance.getEntriesByName('A to B')[0];
      console.log(measure.duration);
      // Prints the number of milliseconds between Mark 'A' and Mark 'B'
    });

## Class: Performance#

Added in: v8.5.0

The `Performance` provides access to performance metric data. A single instance of this class is provided via the `performance` property.

### performance.clearEntries(name)#

Added in: v9.5.0

Remove all performance entry objects with `entryType` equal to `name` from the Performance Timeline.

### performance.clearFunctions([name])#

Added in: v8.5.0

- `name` <string>

If `name` is not provided, removes all `PerformanceFunction` objects from the Performance Timeline. If `name` is provided, removes entries with `name`.

### performance.clearGC()#

Added in: v8.5.0

Remove all performance entry objects with `entryType` equal to `gc` from the Performance Timeline.

### 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.clearMeasures([name])#

Added in: v8.5.0

- `name` <string>

If `name` is not provided, removes all `PerformanceMeasure` objects from the Performance Timeline. If `name` is provided, removes only objects whose `performanceEntry.name` matches `name`.

### performance.getEntries()#

Added in: v8.5.0

- Returns: <Array>

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

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

Added in: v8.5.0

- `name` <string>
- `type` <string>
- Returns: <Array>

Returns a list of all `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)#

Added in: v8.5.0

- `type` <string>
- Returns: <Array>

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

### 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.maxEntries#

Added in: REPLACEME

Value: <number>

The maximum number of Performance Entry items that should be added to the Performance Timeline. This limit is not strictly enforced, but a process warning will be emitted if the number of entries in the timeline exceeds this limit.

Defaults to 150.

### performance.measure(name, startMark, endMark)#

Added in: v8.5.0

- `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#

Added in: v8.5.0

- <PerformanceNodeTiming>

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.

### performance.timeOrigin#

Added in: v8.5.0

- <number>

The `timeOrigin` specifies the high resolution millisecond timestamp from which all performance metric durations are measured.

### performance.timerify(fn)#

Added in: v8.5.0

- `fn` <Function>

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

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

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

    const wrapped = performance.timerify(someFunction);

    const obs = new PerformanceObserver((list) => {
      console.log(list.getEntries()[0].duration);
      obs.disconnect();
      performance.clearFunctions();
    });
    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.name#

Added in: v8.5.0

- <string>

The name of the performance entry.

### performanceEntry.startTime#

Added in: v8.5.0

- <number>

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

### performanceEntry.entryType#

Added in: v8.5.0

- <string>

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

### performanceEntry.kind#

Added in: v8.5.0

- <number>

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

- `perf_hooks.constants.NODE_PERFORMANCE_GC_MAJOR`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_MINOR`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_INCREMENTAL`
- `perf_hooks.constants.NODE_PERFORMANCE_GC_WEAKCB`

## Class: PerformanceNodeTiming extends PerformanceEntry#

Added in: v8.5.0

Provides timing details for Node.js itself.

### performanceNodeTiming.bootstrapComplete#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which the Node.js process completed bootstrap.

### performanceNodeTiming.clusterSetupEnd#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which cluster processing ended.

### performanceNodeTiming.clusterSetupStart#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which cluster processing started.

### performanceNodeTiming.loopExit#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which the Node.js event loop exited.

### performanceNodeTiming.loopStart#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which the Node.js event loop started.

### performanceNodeTiming.moduleLoadEnd#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which main module load ended.

### performanceNodeTiming.moduleLoadStart#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which main module load started.

### performanceNodeTiming.nodeStart#

Added in: v8.5.0

- <number>

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

### performanceNodeTiming.preloadModuleLoadEnd#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which preload module load ended.

### performanceNodeTiming.preloadModuleLoadStart#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which preload module load started.

### performanceNodeTiming.thirdPartyMainEnd#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which third_party_main processing ended.

### performanceNodeTiming.thirdPartyMainStart#

Added in: v8.5.0

- <number>

The high resolution millisecond timestamp at which third_party_main processing started.

### performanceNodeTiming.v8Start#

Added in: v8.5.0

- <number>

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

## Class: PerformanceObserver(callback)#

Added in: v8.5.0

- `callback` <Function> A `PerformanceObserverCallback` callback function.

`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.

### Callback: PerformanceObserverCallback(list, observer)#

Added in: v8.5.0

- `list` <PerformanceObserverEntryList>
- `observer` <PerformanceObserver>

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

### Class: PerformanceObserverEntryList#

Added in: v8.5.0

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

#### performanceObserverEntryList.getEntries()#

Added in: v8.5.0

- Returns: <Array>

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: <Array>

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: <Array>

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

### 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` <Array> 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. Defaults to `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}`);

## Examples#

### Measuring the duration of async operations#

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

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

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

    const obs = new PerformanceObserver((list, observer) => {
      console.log(list.getEntries()[0]);
      performance.clearMarks();
      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:

    '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();
      // Free memory
      performance.clearFunctions();
    });
    obs.observe({ entryTypes: ['function'], buffered: true });

    require('some-module');