Being possible with minimal effort doesn't really preclude it from it not being the default. The issue I have is huge portions of Tokio's (and other async libs) API have a Send + Sync constraint that destroy the benefit of LocalSet / spawn_local. You can't build and application with the specialized thread-per core or single-threaded runtime thing if you wanted to because of pervasive incidental complexity.

I don't care that they have a good work-stealing event loop, I care that it's the default and their APIs all expect the work-stealing implementation and unnecessarily constrain cases where you don't use that implementation. It's frustrating and I go out of my way to avoid Tokio because of it.

Edit: the issues are in Axum, not the core Tokio API. Other libs have this problem too due to aforementioned defaults.

>You can't build and application with the specialized thread-per core or single-threaded runtime thing if you wanted to because of pervasive incidental complexity. [...] It's frustrating and I go out of my way to avoid Tokio because of it.

At $dayjob we have built a large codebase (high-throughput message broker) using the thread-per-core model with tokio (ie one worker thread per CPU, pinned to that CPU, driving a single-threaded tokio Runtime) and have not had any problems. Much of our async code is !Send or !Sync (Rc, RefCell, etc) precisely because we want it to benefit from not needing to run under the default tokio multi-threaded runtime.

We don't use many external libs for async though, which is what seems to be the source of your problems. Mostly just tokio and futures-* crates.

Single-threaded runtime doesn't require Send+Sync for spawned futures. AFAIK Tokio doesn't have a thread-per-core backend and as a sibling intimated you could build it yourself (or use something more suited for thread-per-core like Monoio or Glommio).