rr does use fork() internally to create checkpoints, and checkpoints are created implicitly as the program executes to support time-travel. The COW behaviour of

rr does use fork() internally to create checkpoints, and checkpoints are created implicitly as the program executes to support time-travel. The COW behaviour of fork() is extremely helpful for performance. There's more to checkpoints than just fork(), though; for example, when two processes are sharing memory, checkpointing the process group fork()s both processes and then must explicitly duplicate the shared memory region, otherwise the checkpoint would share memory with the original, which would be disastrous. Unfortunately this means we don't get COW behaviour for shared memory regions; unfortunately the Linux/POSIX APIs are a bit deficient here.