perf_hooks: add binary export/import to Histogram#65434

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perf_hooks: add binary export/import to Histogram #65434

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perf_hooks: add CBOR export/import for histogram exchange
jasnell Aug 20, 2026
b784466
doc: move histogram.burnRate to correct location in doc
jasnell Aug 20, 2026
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130 changes: 99 additions & 31 deletions doc/api/perf_hooks.md
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
[`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
(exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
(exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down
29 changes: 29 additions & 0 deletions lib/internal/histogram.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
validateObject,
} = require('internal/validators');

const {
isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
return this[kHandle]?.exceedsBigInt();
}

/**
* Serializes the histogram to a CBOR-encoded Uint8Array suitable for
* transmission or storage. The data can be reconstructed into a new
* histogram using `importHistogram()`.
* @returns {Uint8Array}
*/
export() {
if (!isHistogram(this))
throw new ERR_INVALID_THIS('Histogram');
return this[kHandle]?.export();
}

/**
* Returns the exponentially weighted moving average of recorded values.
* Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
* Reconstructs a histogram from a CBOR-encoded Uint8Array previously
* produced by `histogram.export()`.
* @param {Uint8Array} data
* @returns {RecordableHistogram}
*/
function importHistogram(data) {
if (!isUint8Array(data))
throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
Histogram,
RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
kHandle,
kSkipThrow,
createHistogram,
importHistogram,
};
2 changes: 2 additions & 0 deletions lib/perf_hooks.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
createHistogram,
importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
eventLoopUtilization,
timerify,
createHistogram,
importHistogram,
performance,
};

Expand Down
Loading
Loading
, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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perf_hooks: add binary export/import to Histogram#65434

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perf_hooks: add binary export/import to Histogram
#65434
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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

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Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
perf_hooks: add binary export/import to Histogram#65434

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perf_hooks: add binary export/import to Histogram
#65434
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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

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Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
perf_hooks: add binary export/import to Histogram#65434

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perf_hooks: add binary export/import to Histogram
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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
perf_hooks: add binary export/import to Histogram#65434

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 Closed

jasnell
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perf_hooks: add binary export/import to Histogram
#65434
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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

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Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

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Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
Loading
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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perf_hooks: add binary export/import to Histogram#65434

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#65434
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1290158

perf_hooks: add CBOR export/import for histogram exchange


jasnell Aug 20, 2026
b784466

doc: move histogram.burnRate to correct location in doc


jasnell Aug 20, 2026

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130 changes: 99 additions & 31 deletions
doc/api/perf_hooks.md

View file


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Original file line numberDiff line numberDiff line change
Expand Up@@ -1639,6 +1639,34 @@ added:

Returns a {RecordableHistogram}.

## `perf_hooks.importHistogram(data)`

<!-- YAML
added: REPLACEME
-->

* `data` {Uint8Array} A CBOR-encoded histogram previously produced by
 [`histogram.export()`][].
* Returns: {RecordableHistogram}

Reconstructs a histogram from a CBOR-encoded `Uint8Array`. The returned
histogram is a full {RecordableHistogram} with all bucket data, configuration,
and EWMA state restored. New values can be recorded into it.

```js
const { createHistogram, importHistogram } = require('node:perf_hooks');

const h = createHistogram();
for (let i = 1; i <= 1000; i++) h.record(i);

// Serialize and reconstruct
const data = h.export();
const h2 = importHistogram(data);

console.log(h2.count); // 1000
console.log(h2.percentile(99)); // Same as h.percentile(99)
```

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

<!-- YAML
Expand DownExpand Up@@ -1854,6 +1882,37 @@ invoked.
added: v11.10.0
-->

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.count`

<!-- YAML
Expand DownExpand Up@@ -1971,6 +2030,43 @@ added:
The number of times the event loop delay exceeded the maximum 1 hour event
loop delay threshold.

### `histogram.export()`

<!-- YAML
added: REPLACEME
-->

* Returns: {Uint8Array}

Serializes the histogram to a [CBOR][]-encoded (RFC 8949) `Uint8Array`
suitable for transmission or persistent storage. The encoding uses a
delta-encoded sparse representation of the bucket counts, so the output size
scales with the number of distinct recorded values rather than the total
bucket count.

The output includes all histogram configuration, bucket data, and EWMA
state (when enabled). It can be reconstructed into a new histogram using
[`perf_hooks.importHistogram()`][].

The CBOR payload is a map with integer keys:

| Key | Type | Field |
| --- | ------- | --------------------------------------------- |
| 0 | uint | Format version (currently 1) |
| 1 | uint | Lowest discernible value |
| 2 | uint | Highest trackable value |
| 3 | uint | Significant figures |
| 4 | uint | Total count |
| 5 | uint | Min value |
| 6 | uint | Max value |
| 7 | uint | Normalizing index offset |
| 8 | float64 | Conversion ratio |
| 9 | uint | Counts array length |
| 10 | array | Delta-encoded sparse counts `[delta, c, ...]` |
| 11 | map | EWMA state (omitted when disabled) |

Any standard CBOR decoder can parse the output.

### `histogram.ewmaMean`

<!-- YAML
Expand DownExpand Up@@ -2008,37 +2104,6 @@ The EWMA-smoothed probability of a recorded value exceeding the configured
and `threshold` options. Returns `0` when not enabled or no values have been
recorded.

### `histogram.burnRate(sloTarget)`

<!-- YAML
added: REPLACEME
-->

* `sloTarget` {number} The SLO target as a fraction between 0 and 1
 (exclusive). For example, `0.999` for a 99.9% SLO.
* Returns: {number}

Returns the SLO burn rate: `ewmaErrorRate / (1 - sloTarget)`. A burn rate
of 1 means the error budget will be exactly exhausted over the SLO window.
A burn rate greater than 1 means it is being consumed faster than allowed.
Requires the histogram to have been created with both `halfLife` and
`threshold` options.

```js
const { createHistogram } = require('node:perf_hooks');

// Track latency with a 200ms SLO threshold, half-life of 100 samples
const h = createHistogram({ halfLife: 100, threshold: 200_000_000 });

// ... record latency values ...

// Check burn rate against a 99.9% SLO
const rate = h.burnRate(0.999);
if (rate > 1) {
 console.log(`SLO burn rate: ${rate.toFixed(2)}x — error budget depleting`);
}
```

### `histogram.ksTest(other)`

<!-- YAML
Expand DownExpand Up@@ -2846,6 +2911,7 @@ dns.promises.resolve('localhost');
```

[Async Hooks]: async_hooks.md
[CBOR]: https://www.rfc-editor.org/rfc/rfc8949
[Cliff's delta]: https://en.wikipedia.org/wiki/Effect_size#Cliff's_delta
[Cohen's d]: https://en.wikipedia.org/wiki/Effect_size#Cohen's_d
[Fetch Response Body Info]: https://fetch.spec.whatwg.org/#response-body-info
Expand All@@ -2860,7 +2926,9 @@ dns.promises.resolve('localhost');
[Worker threads]: worker_threads.md#worker-threads
[`'exit'`]: process.md#event-exit
[`child_process.spawnSync()`]: child_process.md#child_processspawnsynccommand-args-options
[`histogram.export()`]: #histogramexport
[`perf_hooks.eventLoopUtilization()`]: #perf_hookseventlooputilizationutilization1-utilization2
[`perf_hooks.importHistogram()`]: #perf_hooksimporthistogramdata
[`perf_hooks.monitorEventLoopDelay()`]: #perf_hooksmonitoreventloopdelayoptions
[`perf_hooks.timerify()`]: #perf_hookstimerifyfn-options
[`process.hrtime()`]: process.md#processhrtimetime
Expand Down

29 changes: 29 additions & 0 deletions
lib/internal/histogram.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -38,6 +38,10 @@ const {
 validateObject,
} = require('internal/validators');

const {
 isUint8Array,
} = require('internal/util/types');

const kDestroy = Symbol('kDestroy');
const kHandle = Symbol('kHandle');
const kRecordable = Symbol('kRecordable');
Expand DownExpand Up@@ -213,6 +217,18 @@ class Histogram {
 return this[kHandle]?.exceedsBigInt();
 }

 /**
 * Serializes the histogram to a CBOR-encoded Uint8Array suitable for
 * transmission or storage. The data can be reconstructed into a new
 * histogram using `importHistogram()`.
 * @returns {Uint8Array}
 */
 export() {
 if (!isHistogram(this))
 throw new ERR_INVALID_THIS('Histogram');
 return this[kHandle]?.export();
 }

 /**
 * Returns the exponentially weighted moving average of recorded values.
 * Only active when the histogram was created with a `halfLife` option.
Expand DownExpand Up@@ -756,6 +772,18 @@ function createHistogram(options = kEmptyObject) {
 new _Histogram(lowest, highest, figures, halfLife, threshold));
}

/**
 * Reconstructs a histogram from a CBOR-encoded Uint8Array previously
 * produced by `histogram.export()`.
 * @param {Uint8Array} data
 * @returns {RecordableHistogram}
 */
function importHistogram(data) {
 if (!isUint8Array(data))
 throw new ERR_INVALID_ARG_TYPE('data', 'Uint8Array', data);
 return createRecordableHistogram(_Histogram.import(data));
}

module.exports = {
 Histogram,
 RecordableHistogram,
Expand All@@ -766,4 +794,5 @@ module.exports = {
 kHandle,
 kSkipThrow,
 createHistogram,
 importHistogram,
};

2 changes: 2 additions & 0 deletions
lib/perf_hooks.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -25,6 +25,7 @@ const {

const {
 createHistogram,
 importHistogram,
} = require('internal/histogram');

const monitorEventLoopDelay = require('internal/perf/event_loop_delay');
Expand All@@ -43,6 +44,7 @@ module.exports = {
 eventLoopUtilization,
 timerify,
 createHistogram,
 importHistogram,
 performance,
};

Expand Down
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