Use `thread-priority` crate to set audio thread priority

[NeuralModder]

The thread-priority crate can be used to tell the OS' thread scheduler how to treat a thread. Adjusting the priority may or may not alleviate some of the crackling issues we've been having reported to us in Veloren.

I believe the relevant thread is the one spawned in crates/kira/src/backend/cpal/desktop/stream_manager.rs, so in this case we would replace the std::thread::spawn call with a thread_priority::spawn one, which takes an extra thread_priority::ThreadPriority argument.

As for what the ThreadPriority value should be, it could just be a higher than normal CrossPlatform value, or some kind of Deadline one.

See this commit for a simple example of how each value can be used in Veloren.

[tesselode]

Have you tried implementing this for Veloren to see if it actually yields the desired performance improvements? I'd be very surprised if using a higher priority on the thread you mentioned reduced audio crackling, because that's not the thread that does the audio processing. That thread is just for restarting the cpal stream if there's an error. The audio thread is created by cpal, and Kira has no control over it. I'd be very surprised if cpal wasn't already using the most appropriate thread priority for the audio thread.

[NeuralModder]

🤦 Thought that was the thread where the audio was processed for some reason... My bad 🫠

Cpal does set thread priorities, using a different crate... but it's gated behind a feature on cpal, realtime, which we don't have enabled. We can enable it ourselves, but do you think kira should enable it so that consumers don't have to? Or maybe make it a default-enabled feature in kira?

[tesselode]

Possibly! I'm curious why cpal doesn't always enable it. Can you try enabling cpal/realtime in Veloren to see if it improves the crackling?

[Imberflur]

This seems to have some explanation for why it is not enabled by default RustAudio/cpal#1203

[NeuralModder]

Possibly! I'm curious why cpal doesn't always enable it. Can you try enabling cpal/realtime in Veloren to see if it improves the crackling?

If only I knew of specific people who experience crackling that I could get to test :/

This seems to have some explanation for why it is not enabled by default RustAudio/cpal#1203

I mean aside from just "don't enable features by default", it seems there was only a specific issue with some alsa pcms, which they've needed to try and circumvent anyway otherwise it'd be a borked feature.

Reasonable though, they're a pretty low-level lib so it's good to be conservative. But for kira which is tailored for games it can be different.

[NeuralModder]

Nevermind I get stuttering around areas with lots of sound effects too. Enabling the realtime-dbus feature doesn't eliminate the stuttering but does seem to help a bit I think. You'll need to merge #159 before we/you can enable it though.