Question about per-host runtime determinism

[mlowicki]

Hi there,

I’ve read a bit of the code to understand how things work under the hood and have question about determinism. Every host uses its own tokio’s single-threaded executor and Sim simply iterates over hosts in a deterministic order and ticks every host. Is determinism of per-host runtime guaranteed somehow? In other words how it’s achieved that host has some tasks to run and execution of those will be deterministic?

[mcches]

Hi, thanks for taking a look at turmoil! Within each host we use a paused runtime that we step forward deterministically with each step using the LocalSet. Our use of the current thread, LocalSet, emulated sockets, and this timing mechanism make all of this work.

Here is where all the magic happens: https://github.com/tokio-rs/turmoil/blob/main/src/rt.rs#L126-L148

That said, its up to the software (just a future) that runs on each host to do the rest. Things like iterating a HashMap or tokio's select! can introduce non-determinism. We've looked at this before, but its pretty difficult to cover everything out there.

[mcches]

We've got a long standing issue to create some documentation about this: #19. It just hasn't been picked up yet.

[mlowicki]

Thanks! I’m planning to deep dive into how everything works so might be able to tackle #19 at some point.

If you don’t mind I’ll use this thread for additional questions.

Do I understand right that LocalSet contains only software and when running tick() there is additionally sleep future to make progress with time limit? Since we’ve only those in LocalSet then as long as software is deterministic the whole tick() should be deterministic?

[mcches]

Yes, that's correct. We basically say poll tasks in the provided software for the tick duration and because time is paused we achieve determinism here.