Node.js v17.0.0-rc.3 documentation

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.

'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[, options]])#

• 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.

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

• 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.

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[, options])#

• 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.

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();

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()#

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

Class: PerformanceEntry#

performanceEntry.detail#

• <any>

Additional detail specific to the entryType.

performanceEntry.duration#

• <number>

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

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.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

performanceEntry.name#

• <string>

The name of the performance entry.

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.startTime#

• <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.

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.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.

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.

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.

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>
  • 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 PerformanceEntrys 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:

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

const obs = new PerformanceObserver((list, observer) => {
  // Called three times synchronously. `list` contains one item.
});
obs.observe({ type: 'mark' });

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.

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
   *   }
   * ]
   */
  observer.disconnect();
});
obs.observe({ type: 'mark' });

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

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.

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

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

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.

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
   *   }
   * ]
   */
  observer.disconnect();
});
obs.observe({ type: 'mark' });

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

perf_hooks.createHistogram([options])#

• options <Object>
  • min <number> | <bigint> The minimum recordable value. Must be an integer value greater than 0. Default: 1.
  • max <number> | <bigint> The maximum recordable value. Must be an integer value greater than min. 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])#

• 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.

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#

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 in the range (0, 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.

Class: IntervalHistogram extends Histogram#

A Histogram that is periodically updated on a given interval.

histogram.disable()#

• Returns: <boolean>

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

histogram.enable()#

• 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#

histogram.record(val)#

• val <number> | <bigint> The amount to record in the histogram.

histogram.recordDelta()#

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).

'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');