How to get the pipe performance of $ foo | bar with Bun.spawn?

[adworacz]

Hello all - I'm not sure if I've discovered a bug/memory leak, or if things are working as intended and I'm just using something wrong.

I'm currently writing scripts that pipe some ffmpeg + ffplay commands together, and to my surprise, I started getting OOM errors and process kills due to resource exhaustion. Looking a little further, it appears to be related to my use of Bun.spawn.

Example that OOMs + shows ~3% CPU usage in Bun alone (presumably due to memory copies?)

const command_args = ['ffmpeg' , '-i', 'foo', ...]

const ffmpeg = Bun.spawn(command_args)
const {exited} = Bun.spawn(['ffplay', '-'], {stdin: ffmpeg.stdout})
await exited

Example that doesn't OOM + shows 0% CPU usage in Bun

await $`ffmpeg -i foo ... | ffplay -`

I'd honestly expect these two to be equivalent under the hood, but that doesn't appear to be the case. Is there a way to optimize the former to behave like the latter?

[amyssnippet]

This is happening because Method A (Bun.spawn) routes the data through the JavaScript runtime, whereas Method B ($) uses a direct OS-level pipe.

The Technical Difference

• Bun.spawn Piping: When you pass stdin: ffmpeg.stdout, Bun currently treats ffmpeg.stdout as a JavaScript ReadableStream. This means every chunk of video data is allocated in JS memory, passed through the event loop, and then written to ffplay. For high-throughput binary data (like ffmpeg), the Garbage Collector cannot keep up with the allocation rate, leading to the OOM and CPU spike.
• $ Shell: This sets up the pipe at the file descriptor level (Kernel Space) before the processes start. The data flows directly from process to process without ever touching the JavaScript thread.

The Fix

You can keep the native performance of $ while still using dynamic arguments (variables) by passing arrays directly into the template literal. Bun handles the escaping automatically:

const input = "foo.mp4";
const args = ["-i", input, ...otherFlags]; // Your dynamic args

// This uses native pipes (0% JS CPU overhead)
// Bun automatically escapes the array elements
await $`ffmpeg ${args} - | ffplay -`;

[adworacz]

Okay, this seems to be a limitation of the existing API, since the answer is effectively “use a different API”.

Thanks for the details though, I didn’t know that Bun would auto expand the parameter array like that. It’s enough to get me off the ground.