To turn this question on its head, why use multiple processes? That's not just rhetorical, I don't understand why you'd ask that question. The main advantages of separate processes are (1) any global variables are not shared between them, which is sometimes what you want (like Python's GIL), and (2) there is memory protection. Neither of those seem relevant to a Rust data processing program.

To actually answer your question a bit: Threads allow all the benefits of processes (except the two I just mentioned), with the added benefit of being lower overhead and allow sharing information more easily (but you can still use full-blown IPC to communicate between them if you wish).

Excellent question! I think VoltDB, for example, uses the same application-level data partitioning approach, but with processes instead of threads. One advantage of the "thread-per-core" approach is that it allows fast communication between shards because the underlying threads share the same address space. In contrast, processes need to use a more heavy-weight approach of inter-process communication (IPC). Of course, process-based systems will have a reliability advantage, because processes cannot easily mess with each others state.

for a more constructive reply than a downvote, using multiple threads allows you to post closures between executors which is a very flexible and powerful message passing model. I haven't looked deep enough into scipio, but as it is inspired on seastar I assume it allows that as well.

Multiple processes works better if you have little or no sharing or communication between the threads.