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I think the fundamentally tricky part is that sometimes you want your concurrent code to have this sort of nondeterministic behavior. I think the Rayon FAQ does

I think the fundamentally tricky part is that sometimes you want your concurrent code to have this sort of nondeterministic behavior. I think the Rayon FAQ does a good job of summarizing the problem: https://github.com/rayon-rs/rayon/blob/master/FAQ.md#but-wait-isnt-rust-supposed-to-free-me-from-this-kind-of-thinking https://github.com/rayon-rs/rayon/blob/master/FAQ.md#but-wai... > Consider for example when you are conducting a search in parallel, say to find the shortest route. To avoid fruitless search, you might want to keep a cell with the shortest route you've found thus far. This way, when you are searching down some path that's already longer than this shortest route, you can just stop and avoid wasted effort...Now in this case, we really WANT to see results from other threads interjected into our execution!

ivanbakel 6y ago
Oh, certainly. You don't want programmers to go around insisting there's a global ordering to their message systems. But the problem I think could be solved is where programmers have a causal dependency, but they don't encode it in the program. The next generation of concurrent languages could have the killer correctness features for solving causality problems.
polyglotfacto 6y ago
Yes, I would say the tricky part is accurately modelling what you want. Also, instead of actual nondeterministic behavior, I think that you rather want different things to be deterministic. The article is an example of "task parallelism", where you have different threads doing different things on different (local) data. Having those threads communicate via message-passing is a nice way to model workflows crossing thread boundaries, and in that setup you usually want some sort of deterministic behavior related to the ordering of operations(for a given workflow). On the other hand, I think the Rayon example is a good case of "data parallelism", where you have workers doing "the same thing" on "the same (shared-)data". In such a case, perhaps locks and shared-data are a better fit than message-passing, and indeed what you're after is not necessarily some sort of sequential ordering of worker operations. However you still probably want the ability to make other deterministic assertions about the behavior of the workers. I wrote another article highlighting some of these "different determinism" at https://medium.com/@polyglot_factotum/rust-concurrency-five-easy-pieces-871f1c62906a?source=friends_link&sk=fa043036d86e078fd7a50c7f109c1163 https://medium.com/@polyglot_factotum/rust-concurrency-five-...

ivanbakel 6y ago

Oh, certainly. You don't want programmers to go around insisting there's a global ordering to their message systems. But the problem I think could be solved is where programmers have a causal dependency, but they don't encode it in the program. The next generation of concurrent languages could have the killer correctness features for solving causality problems.

polyglotfacto 6y ago

Yes, I would say the tricky part is accurately modelling what you want. Also, instead of actual nondeterministic behavior, I think that you rather want different things to be deterministic. The article is an example of "task parallelism", where you have different threads doing different things on different (local) data. Having those threads communicate via message-passing is a nice way to model workflows crossing thread boundaries, and in that setup you usually want some sort of deterministic behavior related to the ordering of operations(for a given workflow). On the other hand, I think the Rayon example is a good case of "data parallelism", where you have workers doing "the same thing" on "the same (shared-)data". In such a case, perhaps locks and shared-data are a better fit than message-passing, and indeed what you're after is not necessarily some sort of sequential ordering of worker operations. However you still probably want the ability to make other deterministic assertions about the behavior of the workers. I wrote another article highlighting some of these "different determinism" at https://medium.com/@polyglot_factotum/rust-concurrency-five-easy-pieces-871f1c62906a?source=friends_link&sk=fa043036d86e078fd7a50c7f109c1163 https://medium.com/@polyglot_factotum/rust-concurrency-five-...