---
title: Benchmarking
description: How to benchmark Bun
---

Bun is designed for speed. Hot paths are extensively profiled and benchmarked. The source code for all of Bun's public benchmarks is in the [`/bench`](https://github.com/oven-sh/bun/tree/main/bench) directory of the Bun repo.

## Measuring time

To measure time precisely, Bun offers two runtime APIs:

1. The Web-standard [`performance.now()`](https://developer.mozilla.org/en-US/docs/Web/API/Performance/now) function
2. `Bun.nanoseconds()`, which is like `performance.now()` except it returns the time since the application started in nanoseconds. Use `performance.timeOrigin` to convert this to a Unix timestamp.

## Benchmarking tools

- For microbenchmarks, we recommend [`mitata`](https://github.com/evanwashere/mitata).
- For load testing, you _must use_ an HTTP benchmarking tool that is at least as fast as `Bun.serve()`, or your results will be skewed. Some popular Node.js-based benchmarking tools like [`autocannon`](https://github.com/mcollina/autocannon) are not fast enough. We recommend one of the following:
  - [`bombardier`](https://github.com/codesenberg/bombardier)
  - [`oha`](https://github.com/hatoo/oha)
  - [`http_load_test`](https://github.com/uNetworking/uSockets/blob/master/examples/http_load_test.c)
- For benchmarking scripts or CLI commands, we recommend [`hyperfine`](https://github.com/sharkdp/hyperfine).

## Measuring memory usage

Bun has two heaps: one for the JavaScript runtime, and one for everything else.

### JavaScript heap stats

The `bun:jsc` module exposes a few functions for measuring memory usage:

```ts
import { heapStats } from "bun:jsc";
console.log(heapStats());
```

<Accordion title="View example statistics">

```ts expandable icon="/icons/typescript.svg"
{
  heapSize: 1657575,
  heapCapacity: 2872775,
  extraMemorySize: 598199,
  objectCount: 13790,
  protectedObjectCount: 62,
  globalObjectCount: 1,
  protectedGlobalObjectCount: 1,
  // A count of every object type in the heap
  objectTypeCounts: {
    CallbackObject: 25,
    FunctionExecutable: 2078,
    AsyncGeneratorFunction: 2,
    'RegExp String Iterator': 1,
    FunctionCodeBlock: 188,
    ModuleProgramExecutable: 13,
    String: 1,
    UnlinkedModuleProgramCodeBlock: 13,
    JSON: 1,
    AsyncGenerator: 1,
    Symbol: 1,
    GetterSetter: 68,
    ImportMeta: 10,
    DOMAttributeGetterSetter: 1,
    UnlinkedFunctionCodeBlock: 174,
    RegExp: 52,
    ModuleLoader: 1,
    Intl: 1,
    WeakMap: 4,
    Generator: 2,
    PropertyTable: 95,
    'Array Iterator': 1,
    JSLexicalEnvironment: 75,
    UnlinkedFunctionExecutable: 2067,
    WeakSet: 1,
    console: 1,
    Map: 23,
    SparseArrayValueMap: 14,
    StructureChain: 19,
    Set: 18,
    'String Iterator': 1,
    FunctionRareData: 3,
    JSGlobalLexicalEnvironment: 1,
    Object: 481,
    BigInt: 2,
    StructureRareData: 55,
    Array: 179,
    AbortController: 2,
    ModuleNamespaceObject: 11,
    ShadowRealm: 1,
    'Immutable Butterfly': 103,
    Primordials: 1,
    'Set Iterator': 1,
    JSGlobalProxy: 1,
    AsyncFromSyncIterator: 1,
    ModuleRecord: 13,
    FinalizationRegistry: 1,
    AsyncIterator: 1,
    InternalPromise: 22,
    Iterator: 1,
    CustomGetterSetter: 65,
    Promise: 19,
    WeakRef: 1,
    InternalPromisePrototype: 1,
    Function: 2381,
    AsyncFunction: 2,
    GlobalObject: 1,
    ArrayBuffer: 2,
    Boolean: 1,
    Math: 1,
    CallbackConstructor: 1,
    Error: 2,
    JSModuleEnvironment: 13,
    WebAssembly: 1,
    HashMapBucket: 300,
    Callee: 3,
    symbol: 37,
    string: 2484,
    Performance: 1,
    ModuleProgramCodeBlock: 12,
    JSSourceCode: 13,
    JSPropertyNameEnumerator: 3,
    NativeExecutable: 290,
    Number: 1,
    Structure: 1550,
    SymbolTable: 108,
    GeneratorFunction: 2,
    'Map Iterator': 1
  },
  protectedObjectTypeCounts: {
    CallbackConstructor: 1,
    BigInt: 1,
    RegExp: 2,
    GlobalObject: 1,
    UnlinkedModuleProgramCodeBlock: 13,
    HashMapBucket: 2,
    Structure: 41,
    JSPropertyNameEnumerator: 1
  }
}
```

</Accordion>

JavaScript is a garbage-collected language, not reference counted. It's normal and correct for objects to not be freed immediately in all cases, though it's not normal for objects to never be freed.

To force garbage collection to run manually:

```ts
Bun.gc(true); // synchronous
Bun.gc(false); // asynchronous
```

Heap snapshots show which objects are not being freed. Use `Bun.generateHeapSnapshot()` to take a heap snapshot, then view it with Safari or WebKit GTK developer tools. To generate a heap snapshot:

```ts
import { generateHeapSnapshot } from "bun";

const snapshot = generateHeapSnapshot();
await Bun.write("heap.json", JSON.stringify(snapshot, null, 2));
```

To view the snapshot, open the `heap.json` file in Safari's Developer Tools (or WebKit GTK):

1. Open the Developer Tools
2. Click "Timeline"
3. Click "JavaScript Allocations" in the menu on the left. It might not be visible until you click the pencil icon to show all the timelines
4. Click "Import" and select your heap snapshot JSON

<Frame>
  <img
    src="https://user-images.githubusercontent.com/709451/204428943-ba999e8f-8984-4f23-97cb-b4e3e280363e.png"
    alt="Importing a heap snapshot"
  />
</Frame>

Once imported, you should see something like this:

<Frame>
  <img
    alt="Viewing heap snapshot in Safari"
    src="https://user-images.githubusercontent.com/709451/204429337-b0d8935f-3509-4071-b991-217794d1fb27.png"
    caption="Viewing heap snapshot in Safari Dev Tools"
  />
</Frame>

> The [web debugger](/runtime/debugger#inspect) timeline also tracks the memory usage of the running debug session.

### Native heap stats

Bun uses mimalloc for the other heap. To print a summary of non-JavaScript memory usage, call `Bun.unsafe.mimallocDump()`.

```ts
Bun.unsafe.mimallocDump();
```

```txt
heap stats:    peak      total      freed    current       unit      count
  reserved:   64.0 MiB   64.0 MiB      0       64.0 MiB                        not all freed!
 committed:   64.0 MiB   64.0 MiB      0       64.0 MiB                        not all freed!
     reset:      0          0          0          0                            ok
   touched:  128.5 KiB  128.5 KiB    5.4 MiB   -5.3 MiB                        ok
  segments:      1          1          0          1                            not all freed!
-abandoned:      0          0          0          0                            ok
   -cached:      0          0          0          0                            ok
     pages:      0          0         53        -53                            ok
-abandoned:      0          0          0          0                            ok
 -extended:      0
 -noretire:      0
     mmaps:      0
   commits:      0
   threads:      0          0          0          0                            ok
  searches:     0.0 avg
numa nodes:       1
   elapsed:       0.068 s
   process: user: 0.061 s, system: 0.014 s, faults: 0, rss: 57.4 MiB, commit: 64.0 MiB
```

## CPU profiling

Profile JavaScript execution to identify performance bottlenecks with the `--cpu-prof` flag.

```sh terminal icon="terminal"
bun --cpu-prof script.js
```

`--cpu-prof` writes a `.cpuprofile` file you can open in Chrome DevTools (Performance tab → Load profile) or VS Code's CPU profiler.

### Markdown output

Use `--cpu-prof-md` to generate a markdown CPU profile, which is grep-friendly and designed for LLM analysis:

```sh terminal icon="terminal"
bun --cpu-prof-md script.js
```

Combine `--cpu-prof` and `--cpu-prof-md` to generate both formats at once:

```sh terminal icon="terminal"
bun --cpu-prof --cpu-prof-md script.js
```

You can also pass the flag through the `BUN_OPTIONS` environment variable:

```sh terminal icon="terminal"
BUN_OPTIONS="--cpu-prof-md" bun script.js
```

### Options

```sh terminal icon="terminal"
bun --cpu-prof --cpu-prof-name my-profile.cpuprofile script.js
bun --cpu-prof --cpu-prof-dir ./profiles script.js
```

| Flag                         | Description                                                 |
| ---------------------------- | ----------------------------------------------------------- |
| `--cpu-prof`                 | Generate a `.cpuprofile` JSON file (Chrome DevTools format) |
| `--cpu-prof-md`              | Generate a markdown CPU profile (grep/LLM-friendly)         |
| `--cpu-prof-name <filename>` | Set output filename                                         |
| `--cpu-prof-dir <dir>`       | Set output directory                                        |

## Heap profiling

Write a heap profile on exit to analyze memory usage and find memory leaks.

```sh terminal icon="terminal"
bun --heap-prof script.js
```

`--heap-prof` writes a full V8-format heap snapshot on exit, using Node.js's
diagnostic filename format
(`Heap.<yyyymmdd>.<hhmmss>.<pid>.<tid>.<seq>.heapprofile`). The content is the
same as `v8.writeHeapSnapshot()` / `Bun.generateHeapSnapshot("v8")`: load it in
Chrome DevTools via Memory tab → Load (pick "All Files" or rename to
`.heapsnapshot` — the extension follows Node's `--heap-prof` filename contract,
which the harness and tooling key on).

### Markdown output

Use `--heap-prof-md` to generate a markdown heap profile for CLI analysis:

```sh terminal icon="terminal"
bun --heap-prof-md script.js
```

<Note>If both `--heap-prof` and `--heap-prof-md` are specified, the markdown format is used.</Note>

### Options

```sh terminal icon="terminal"
bun --heap-prof --heap-prof-name my-profile.heapprofile script.js
bun --heap-prof --heap-prof-dir ./profiles script.js
```

| Flag                           | Description                                                                                                                                |
| ------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------ |
| `--heap-prof`                  | Write a `.heapprofile` file on exit                                                                                                        |
| `--heap-prof-md`               | Generate a markdown heap profile on exit                                                                                                   |
| `--heap-prof-name <filename>`  | Set output filename                                                                                                                        |
| `--heap-prof-dir <dir>`        | Set output directory                                                                                                                       |
| `--heap-prof-interval <bytes>` | Accepted for Node.js compatibility (the snapshot is taken once at exit; JavaScriptCore has no allocation sampling to apply an interval to) |
