std::process::Command::output is unacceptably slow

[safinaskar]

std::process::Command::output is unacceptably slow. Consider this code:

https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=333b57442699553257f0273bb1bba3ac

When trying this code, make sure you have enabled release mode. On my local machine I see this output:

1565 ms
100000000
103 ms
100000000

So std::process::Command::output is 15x slower than std::io::BufReader::read_to_end(). My rustc version is rustc 1.69.0-nightly (75a0be98f 2023-02-05). My OS is Debian Stretch x86_64

[safinaskar]

@rustbot label +A-process +I-slow

[Noratrieb]

On windows they are both equally fast, so it seems to be a unix-specific thing. Also note that these two programs are not equivalent, as output() also reads stderr, while you don't (this probably introduces some overhead). Still seems worth at least investigating.

[KittyBorgX]

What's the magnitude of slowness here? and @Nilstrieb what's the time taken on windows?
Here's it on my computer, built with release mode

614 ms
100000000
36 ms
100000000

My computer : Apple M1 Mac Mini, 16gb RAM and 8 CPU Cores

[safinaskar]

@tmiasko . I just compared old rustc (rustc 1.69.0-nightly (e972bc808 2023-01-29)) and new one, with this fix (rustc 1.69.0-nightly (c5c7d2b37 2023-02-24)). I see that the bug is actually fixed, thanks a lot! Moreover, both benchmarks became faster