Zero-copy in Go: sendfile, splice, and the cost of io.Copy
https://segflow.github.io/post/zero-copy-sendfile-splice/
mrngm · 3 days ago
7 comments
https://segflow.github.io/post/zero-copy-sendfile-splice/
mrngm · 3 days ago
7 comments
sanxiyn · 3 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 · 3 days ago
Zero-Copy in Go: Why magic is an antipattern, and: performance is observable behavior.
sanxiyn · 3 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 · 3 days ago
it is documented by saying it calls ReadFrom or WriteTo
sanxiyn · 2 days ago
Sure, but it could be more explicit. See Rust for how it should be done.
throwrioawfo · 3 days ago
Ugh, AI slop writing.
drivebyhooting · 3 days ago
Definitely written by codex.
badc0ffee · 2 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 · 3 days ago
Interesting premise for a post, but I had to stop midway due to the AI slop writing adding meaningless information.
inigyou · 3 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.
drivebyhooting · 3 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.
flakes · 3 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.
drivebyhooting · 3 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.
flakes · 3 days ago
> Isn’t this a symptom of structural/duck typing where interfaces are not declared
Yes, essentially duck typing. See https://github.com/golang/go/blob/65504872cbca64d77f45828409...
The logic uses a type assertion to safely verify if the value backing the provided io.Reader interface also implements the io.ReaderFrom interface. If it matches, then it will use the more efficient implementation
if rf, ok := dst.(ReaderFrom); ok {
return rf.ReadFrom(src)
}
I don't think I would go that far. In Java, many libraries make heavy use of the `instanceof` keyword, which is more or less the same as Go type assertions.
pjmlp · 3 days ago
Yes, but contrary to Go, if you change an interface it will be a compiler error if additional methods are missing, unless they have default implementations.
In Java type assertions are mostly used when writing code pre-generics style, like downcasting from a common subclasse into the actual implementation, not to see if an interface is supported, as it is a given from the type system.
ericpauley · 2 days ago
It’s pretty common to type check interface implementations in Go using package-level casts.
pjmlp · 2 days ago
I know, it is yet another language hack, just like the iota/const dance, instead of proper language constructs.
flakes · 2 days ago
How is a package level interface check a “language hack”? They are straight forward and provide a user the same compile time guarantees as the Java implements keyword.
pjmlp · 2 days ago
Because they are a workaround for what should be a language feature in first place, just like on ML languages to type check structural types.
flakes · 2 days ago
What exactly isn’t a language feature? Or do you have issues with semantics?
var _ Foo = (*Bar)(nil)
I do agree it’s not as clean looking as the Java implements keyword, but it’s already a fairly terse pattern and IMO the inconvenience does not justify introducing new language syntax.
sly010 · 3 days ago
Beware, there are versions of go where sendfile is broken and only sends the first 4k of a file on macos.
majewsky · 2 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