Benchmarking

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 directory of the Bun repo.

Measuring time

To measure time precisely, Bun offers two runtime APIs:

The Web-standard performance.now() function
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.
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 are not fast enough. We recommend one of the following:
For benchmarking scripts or CLI commands, we recommend 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:

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

View example statistics

{
  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
  }
}

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:

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:

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):

Open the Developer Tools
Click “Timeline”
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
Click “Import” and select your heap snapshot JSON

Once imported, you should see something like this:

The web debugger 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().

Bun.unsafe.mimallocDump();

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.

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:

terminal

bun --cpu-prof-md script.js

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

terminal

bun --cpu-prof --cpu-prof-md script.js

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

terminal

BUN_OPTIONS="--cpu-prof-md" bun script.js

Options

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

Generate heap snapshots on exit to analyze memory usage and find memory leaks.

terminal

bun --heap-prof script.js

--heap-prof writes a V8 .heapsnapshot file you can load in Chrome DevTools (Memory tab → Load).

Markdown output

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

terminal

bun --heap-prof-md script.js

If both --heap-prof and --heap-prof-md are specified, the markdown format is used.

Options

terminal

bun --heap-prof --heap-prof-name my-snapshot.heapsnapshot script.js
bun --heap-prof --heap-prof-dir ./profiles script.js

Flag	Description
`--heap-prof`	Generate a V8 `.heapsnapshot` 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

​Measuring time

​Benchmarking tools

​Measuring memory usage

​JavaScript heap stats

​Native heap stats

​CPU profiling

​Markdown output

​Options

​Heap profiling

​Markdown output

​Options

Measuring time

Benchmarking tools

Measuring memory usage

JavaScript heap stats

Native heap stats

CPU profiling

Markdown output

Options

Heap profiling

Markdown output

Options