How does Spring boot your Rails app instantly, or Puma route requests across many processes? How can you fine-tune your app for memory efficiency, or simply run Ruby code in parallel? Find out with a deep dive on that staple Unix utensil, the fork() system call! After an operating systems primer on children, zombies, processes and pipes, we'll dig into how exactly Spring and Puma use fork() to power Rails in development and production. We'll finish by sampling techniques for measuring and maximizing copy-on-write efficiency, including a new trick that can reduce memory usage up to 80%.