If you need to share a large readonly structure, the best way IMO is that approach. Implement the structure in a low-level language that supports mmap (be very sure to make the whole structure be in the mmap'd block - it is easy to wind up with pointers to random other memory and you don't want that!) and have high performance accessors to use in your code.
Good luck. Another benefit of this strategy is that you optimize that data structure using techniques that aren't available in higher languages. So, for instance, small trees can be set up to have all of the nodes of the tree very close together, improving the odds of a cache hit. You can switch from lots of small strings to having integers that index a lookup table of strings for display only.
The amount of work to do this is insane. Expect 10x what it took to write it in a high level language. But the performance can often be made 10-100x as well. Which is a giant payoff.
Thanks! I've already partly rewritten it in C once, but I misunderstood the access pattern and I ended up having a lot of cache misses. The speedup was measurable, but disappointing, and the prospect of doing another rewrite had put me off. I hadn't put two and two together about this being an effective way to share memory under multiprocessing until reading this thread, so it's worth revisiting now.
If you need to share a large readonly structure, the best way IMO is that approach. Implement the structure in a low-level language that supports mmap (be very sure to make the whole structure be in the mmap'd block - it is easy to wind up with pointers to random other memory and you don't want that!) and have high performance accessors to use in your code.