Zero-copy in Go: sendfile, splice, and the cost of io.Copy

https://segflow.github.io/post/zero-copy-sendfile-splice/

by mrngm • 17 days ago

75 22 17 days ago

segflow.github.io

sanxiyn3• 17 days ago

A good reminder. It is surprising first time you encounter it.

Same for Rust. As https://doc.rust-lang.org/stable/std/io/fn.copy.html says, std::io::copy can use copy_file_range(2), sendfile(2), or splice(2).

mike_hock• 17 days ago

Zero-Copy in Go: Why magic is an antipattern, and: performance is observable behavior.

sanxiyn3• 17 days ago

What would you prefer?

I do think it is criminal this is not documented (https://pkg.go.dev/io#Copy), but I think io.Copy is fine as an API.

arccy• 17 days ago

it is documented by saying it calls ReadFrom or WriteTo

throwrioawfo• 17 days ago

Ugh, AI slop writing.

drivebyhooting3• 17 days ago

Definitely written by codex.

badc0ffee• 16 days ago

The lack of apostrophe in "dont" makes me think at least part of it was written manually.

It's also a totally readable article, really.

joaohaas• 17 days ago

Interesting premise for a post, but I had to stop midway due to the AI slop writing adding meaningless information.

inigyou• 17 days ago

This is almost like the expression problem. Copy is a new operation, and you introduced a new type, thus creating a new grid cell nobody from either side could have reasonably known about - except for the fact Copy is in the standard library so you could have known about it but not done anything.

drivebyhooting3• 17 days ago

How is the byte counting reader supposed to work in user space without putting the buffer in user space? The article claims there is a way but I want to see what is meant by counting bytes in that case.

flakes4• 17 days ago

sendfile(2) and io.ReaderFrom both return the number of bytes transmitted. The issue is that users are unaware of (or forget about) the optional interface upgrades and fail to define all the methods required for interface upgrades on their wrapper structs. You can definitely make a counting reader with a minimal performance loss, but the proper solution is less obvious than it ideally should be.

drivebyhooting3• 17 days ago

Isn’t this a symptom of structural/duck typing where interfaces are not declared? In Java for all its faults this wouldn’t happen because you’d be forced to implement all the interfaces.

sly010• 17 days ago

Beware, there are versions of go where sendfile is broken and only sends the first 4k of a file on macos.

majewsky• 16 days ago

If this is a thing you need to beware of, you should more importantly beware of using a version of Go that's been outdated for over 1.5 years and EOL for nearly a year.

Context: https://github.com/golang/go/issues/70000 fixed in 1.23.3 (released 2024-11-06), EOL for 1.23.x was 2025-08-06

Zero-copy in Go: sendfile, splice, and the cost of io.Copy

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