[wavemode]

> there is no reason to support "sync I/O"

I disagree strongly.

From a performance perspective, asynchronous IO makes a lot of sense when you're dealing concurrently with a large number of tasks which each spend most of their time waiting for IO operations to complete. In this case, running those tasks in a single-threaded event loop is far more efficient than launching off thousands of individual threads.

However, if your application falls into literally any other category, then suddenly you are actually paying a performance penalty, since you need the overhead of running an event loop any time you just want to perform some IO.

Also, from a correctness perspective, non-concurrent code is simply a lot less complex and a lot harder to get wrong than concurrent code. So applications which don't need async also end up paying a maintainability, and in some cases memory safety / thread safety, penalty as well.

[simonask]

The beautiful thing about the “async” abstraction is that it doesn’t actually tie you to an event loop at all. Nothing about it implies that somebody is calling `epoll_wait` or similar anywhere in the stack.

It’s just a compiler feature that turns functions into state machines. It’s totally valid to have an async runtime that moves a task to a thread and blocks whenever it does I/O.

I do agree that async without memory safety and thread safety is a nightmare (just like all state machines are under those circumstances). Thankfully, we have languages now that all but completely solve those issues.

[wavemode]

You surely must be referring to Rust, the only multithreaded language with async-await in which data races aren't possible.

Rust is lovely and all, but is a bad example for the performance side of the argument, since in practice libraries usually have to decide on an async runtime, so in practice library users have to launch that runtime (usually Tokio) to execute the library's Futures.

[simonask]

Sure, but that’s a library limitation (no widespread common runtime interface that libraries such as Tokio implement), not a fundamental limitation of async.

Thread safety is also a lot easier to achieve in languages like C#, and then of course you have single-threaded environments like JS and Python.