bun.sh/docs/project/profiling.md

To precisely measure time, Bun offers two runtime APIs functions:

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

## Benchmarking `Bun.serve`

You will need an HTTP client which is at least as fast as `Bun.serve()`.

That means popular Node.js-based benchmarking tools like **autocannon is not fast enough**.

Recommended HTTP clients:

- [`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)

## Measuring memory usage

Bun has two heaps. One heap is for the JavaScript runtime and the other heap is 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());

// will show something like this:
{
  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,
    JSProxy: 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.

You can force garbage collection to run manually by calling:

```js
const synchronously = true;
Bun.gc(synchronously);
```

### JavaScript heap snapshot

Heap snapshots let you inspect what objects are not being freed. You can use the `bun:jsc` module to take a heap snapshot and then view it with Safari or WebKit GTK developer tools.

{% image  alt="image" src="https://user-images.githubusercontent.com/709451/204429337-b0d8935f-3509-4071-b991-217794d1fb27.png" /%}

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

![image](https://user-images.githubusercontent.com/709451/204428943-ba999e8f-8984-4f23-97cb-b4e3e280363e.png)

### Native heap stats

Bun uses mimalloc for the other heap. To report a summary of non-JavaScript memory usage, set the `MIMALLOC_SHOW_STATS=1` environment variable. and stats will print on exit.

```js
MIMALLOC_SHOW_STATS=1 bun script.js

# will show something like this:
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
```