NodeJS Buffer Size Logic: why 8KB as a default and not just the data size?

If you read a file smaller than 8KB in size NodeJS allocates 8 KB for it.

This isn’t intuitive or expected, we are getting "more" bytes than we put. But it isn’t a bug either at least in the sense that it is documented.

Minimal NodeJS Example

const { Buffer } = require("node:buffer");

// Create a buffer smaller than `Buffer.poolSize`.
const nodeBuffer = Buffer.from(new Uint8Array([0, 1, 2, 3, 4]));

// size is 8KB, not 5 bytes.
console.log(nodeBuffer.buffer);

Question

• Why would 8KB buffer size be preferred over what we expect to have (the size of the data in the file.) ?

Files over 8KB would have an allocated space that matches the number of bytes in the file.

Other Details

I’m aware that we can use the offsets for reading:

// 1 KB
const buffer = fs.readFileSync(fileName, buffer.byteOffset, buffer.byteLength)

but the question isn’t this.

>Solution :

The clue is in the name of the property you linked, poolSize.

Elsewhere on the same page:

The Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe(), Buffer.from(array), and Buffer.concat() only when size is less than Buffer.poolSize >>> 1 (floor of Buffer.poolSize divided by two).

In other words, it’s for performance reasons, since allocating and deallocating memory can be slow; instead, there’s a pool of memory the module dips into for suitably small allocations. If you want a smaller buffer, it’d probably still be less efficient to allocate it from out of the pool than to have a buffer that’s a bit bigger than what you need.