I think that it is generally not reasonable to convert existing large projects to be multithreaded, in the same way that it is generally not reasonable to fully rewrite existing large projects from scratch.
The alternative that I have seen be successful, is to achieve parallelism by forking separate processes wherever you would have spawned threads, and then communicate through shared memory regions.
It's a lot like having Rust-style unsafe blocks, in that you know that if you are having a thread-safety issue it will definitely be in one of the code sections where you are touching the shared memory region, and not anywhere else.
Obviously there's a higher startup cost for forking, but this makes it possible to gain parallelism without breaking all the thread-unsafe code that is certainly in an existing large project.
The alternative that I have seen be successful, is to achieve parallelism by forking separate processes wherever you would have spawned threads, and then communicate through shared memory regions.
It's a lot like having Rust-style unsafe blocks, in that you know that if you are having a thread-safety issue it will definitely be in one of the code sections where you are touching the shared memory region, and not anywhere else.
Obviously there's a higher startup cost for forking, but this makes it possible to gain parallelism without breaking all the thread-unsafe code that is certainly in an existing large project.