One thing to remember: when fork() was invented, shared-memory threads basically didn't exist. Everything that would be done with threads today, had to be done

One thing to remember: when fork() was invented, shared-memory threads basically didn't exist. Everything that would be done with threads today, had to be done with either fork() or asynchronous event loops, because no mainstream OS had threading. Another thing to remember: there's essentially no difference between a fork() and a thread. On Linux, they're both implemented by the same clone() syscall -- they just specify different flags about what should be shared versus what should be split and copied. Both are extremely cheap, and if you're going to implement threading, you might as well implement fork() because [on hardware with virtual memory] it's almost free.