[nullwasamistake]

Threads are bad for high concurrency. Specifically when you need to call out to another service that has some latency.

Say you have 1000 threads. To handle a request each one needs to make 50ms of external or DB calls. In one second, each thread can handle 20 calls. So you can handle 20k requests/second with 1000 threads. But Rust is so fast it can serve 500k requests a second. So with regular threads, you need ~25,000 threads. The OS isn't going to like that.

With async you can run a single thread per core, with no concurrency limits. So you get your 500k requests without overhead. With fibers you just run 20k fibers which is a little bit of overhead but easy to do.

This is the core reason everyone is pushing async and fibers in fast languages. When you can push a ton of requests/second but each one has latency you can't control, regular threads will kneecap performance.

In "slow" languages like Python, Ruby, etc, async/fibers don't really matter because you can't handle enough requests to saturate a huge thread pool anyways.

[staticassertion]

Java services have managed for a long time to do just fine. Usually you just have dedicated threadpools for those db/ whatever calls.

But yes, eventually, for very heavy cases (more than what I would call "high") you will want async/await.

[pjmlp]

Which can still be done via java.util.concurrent (Callable, Futures, Promises, Flow) until Project Loom arrives.

[pcwalton]

25,000 threads are perfectly fine on Linux.

[je42]

it is the memory associated with a posix thread that becomes the limit.

[noncoml]

Why only 1000 threads? Why not 10k or 100k?

With 8k stack for each, you can easy have 10k-100k threads in a low-end system

[MrBuddyCasino]

Let's be real here, its not just the memory requirements, because context switching and the associated nuking of cpu caches are not free. You can go very far with it nowadays, but you can go much farther with async code, if you really need to.

[j88439h84]

In Python I use async instead of threads for reasons unrelated to performance. https://glyph.twistedmatrix.com/2014/02/unyielding.html