I also speculate if its actually any faster, since I need to change every single byte in the memory region (I'm decrypting).
My understanding of these OS features are weak, but my mental model of it is that ...
I'm by no means an expert on this either, but my understanding is that using memory mapped files is faster especially when you need random access to the file and it allows you to have a logical view of the file greater than what you could normally fit into memory. Even sequential read access may be quicker as it should prevent any double buffering. In the normal case, I would expect that memory writes would be slower since it has to either write the data to the file or cache it for later writes. However, I think for what you're doing you may be better off reading the file into normal memory and manipulating it there. Even with copy on write, you're essentially forcing the OS (through a write protected memory exception) to copy the file into memory anyway. -- Bill