> Context switching is virtually free, comparable to a function call.
If you’re counting that low, then you need to count carefully.
A coroutine switch, however well implemented, inevitably breaks the branch predictor’s idea of your return stack, but the effect of mispredicted returns will be smeared over the target coroutine’s execution rather than concentrated at the point of the switch. (Similar issues exist with e.g. measuring the effect of blowing the cache on a CPU migration.) I’m actually not sure if Zig’s async design even uses hardware call/return pairs when a (monomorphized-as-)async function calls another one, or if every return just gets translated to an indirect jump. (This option affords what I think is a cleaner design for coroutines with compact frames, but it is much less friendly to the CPU.)
So a foolproof benchmark would require one to compare the total execution time of a (compute-bound) program that constantly switches between (say) two tasks to that of an equivalent program that not only does not switch but (given what little I know about Zig’s “colorless” async) does not run under an async executor(?) at all. Those tasks would also need to yield on a non-trivial call stack each time. Seems quite tricky all in all.
Stackful coroutines make sense when you have the RAM for it.
I've been using Zig for embedded (ARM Cortex-M4, 256KB RAM) mainly for memory safety with C interop. The explicitness around calling conventions catches ABI mismatches at compile-time instead of runtime crashes.
I actually prefer colored async (like Rust) over this approach. The "illusion of synchronous code" feels magical, but magic becomes a gotcha in larger codebases when you can't tell what's blocking and what isn't.
Isn't this a bad time to be embracing Zig? It's currently going through an intrusive upheaval of its I/O model. My impression is that it was going to take a few years for things to shake out. Is that wrong?
I am still mystified as to why callback-based async seems to have become the standard. What this and e.g. libtask[1] do seems so much cleaner to me.
The Rust folks adopted async with callbacks, and they were essentially starting from scratch so had no need to do it that way, and they are smarter than I (both individually and collectively) so I'm sure they have a reason; I just don't know what it is.
Mostly out of curiosity, a read on a TCP connection could easily block for a month - how does the I/O timeout interface look like ? e.g. if you want to send an application level heartbeat when a read has blocked for 30 seconds.
I really need to play with Zig. I got really into Rust a few months ago, and I was actually extremely impressed by Tokio, so if this library also gives me Go-style concurrency without having to rely on a garbage collector, then I am likely to enjoy it.
> In the previous C++ version, I used Qt, which might seem very strange for a server software, but I wanted a nice way of doing asynchronous I/O and Qt allowed me to do that. It was callback-based, but Qt has a lot of support for making callbacks usable. In the newer prototypes, I used Go, specifically for the ease of networking and concurrency. With Zig, I was stuck.
For me, I want these for Rust, especially what Zig has because I use KDE. I know about https://github.com/KDAB/cxx-qt and it is the only maintained effort for Rust that is left standing after all these years. But I don't want QML. I definitely don't want C++ or CMake. I just want Rust and Cargo.
Honestly, have been excited about Zig for quite a while, dabbled a bit a while back and was waiting for it getting closer to 1.0 to actually do a deep dive... but that moment doesn't seem to come.
I don't mind, it's up to the maintainers on how they want to proceed. However, I would greatly appreciate if Zig news was a bit clearer on what's happening, timelines etc.
I think it takes relatively little time to do so, but optics would be so much better.
perhaps it's a trivial observation that people tend to conflate the programming language in the strict sense (syntax, semantics, compiler implementation etc.) with its standard and/or community libraries and tooling.
of course these are very important, but perhaps i'm just a language nerd/pedant who gets confused when an article about a programming language tends to be about async i/o libraries.
How I turned Zig into my favorite language to write network programs in
(lalinsky.com)318 points by 0x1997 27 October 2025 | 137 comments
Comments
If you’re counting that low, then you need to count carefully.
A coroutine switch, however well implemented, inevitably breaks the branch predictor’s idea of your return stack, but the effect of mispredicted returns will be smeared over the target coroutine’s execution rather than concentrated at the point of the switch. (Similar issues exist with e.g. measuring the effect of blowing the cache on a CPU migration.) I’m actually not sure if Zig’s async design even uses hardware call/return pairs when a (monomorphized-as-)async function calls another one, or if every return just gets translated to an indirect jump. (This option affords what I think is a cleaner design for coroutines with compact frames, but it is much less friendly to the CPU.)
So a foolproof benchmark would require one to compare the total execution time of a (compute-bound) program that constantly switches between (say) two tasks to that of an equivalent program that not only does not switch but (given what little I know about Zig’s “colorless” async) does not run under an async executor(?) at all. Those tasks would also need to yield on a non-trivial call stack each time. Seems quite tricky all in all.
I've been using Zig for embedded (ARM Cortex-M4, 256KB RAM) mainly for memory safety with C interop. The explicitness around calling conventions catches ABI mismatches at compile-time instead of runtime crashes.
I actually prefer colored async (like Rust) over this approach. The "illusion of synchronous code" feels magical, but magic becomes a gotcha in larger codebases when you can't tell what's blocking and what isn't.
The Rust folks adopted async with callbacks, and they were essentially starting from scratch so had no need to do it that way, and they are smarter than I (both individually and collectively) so I'm sure they have a reason; I just don't know what it is.
1: https://swtch.com/libtask/
There are new Qt bindings for these. Go has https://github.com/mappu/miqt and Zig has https://github.com/rcalixte/libqt6zig. I wonder if the author knew about them. I don't know enough about either language to speak on the async parts.
For me, I want these for Rust, especially what Zig has because I use KDE. I know about https://github.com/KDAB/cxx-qt and it is the only maintained effort for Rust that is left standing after all these years. But I don't want QML. I definitely don't want C++ or CMake. I just want Rust and Cargo.
This looks interesting but I'm not familiar with NATS
I don't mind, it's up to the maintainers on how they want to proceed. However, I would greatly appreciate if Zig news was a bit clearer on what's happening, timelines etc.
I think it takes relatively little time to do so, but optics would be so much better.
of course these are very important, but perhaps i'm just a language nerd/pedant who gets confused when an article about a programming language tends to be about async i/o libraries.