Bun Benchmarking

Benchmarking

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 can be found in the /bench directory of the Bun repo.

Measuring time

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

1. The Web-standard 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 tools

When writing your own benchmarks, it's important to choose the right tool.

• For microbenchmarks, a great general-purpose tool is 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:
  • bombardier
  • oha
  • http_load_test
• For benchmarking scripts or CLI commands, we recommend hyperfine.

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:

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

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 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. 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)

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

Once imported, you should see something like this:

The web debugger also offers the timeline feature which allows you to track and examine the memory usage of the running debug session.

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.

MIMALLOC_SHOW_STATS=1 bun script.js

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.

bun --cpu-prof script.js

This generates 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:

bun --cpu-prof-md script.js

Both --cpu-prof and --cpu-prof-md can be used together to generate both formats at once:

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

You can also trigger profiling via the BUN_OPTIONS environment variable:

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

Options

bun --cpu-prof --cpu-prof-name my-profile.cpuprofile script.js
bun --cpu-prof --cpu-prof-dir ./profiles script.js

Heap profiling

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

bun --heap-prof script.js

This generates a V8 .heapsnapshot file that can be loaded in Chrome DevTools (Memory tab → Load).

Markdown output

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

bun --heap-prof-md script.js

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

Options

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