Fork and Threads in ruby

Question

I am running a program on a machine with a two processors, when I do a fork is the child created as a native thread or it is like a green thread/coroutine. Is the child running concurrently with the parent or it is just parallel?

Answer 1

The working of fork() in general is to generate a new, independent process, duplicate the page table, and mark all pages owned by the process that called fork() as copy-on-write in that process. Then, fork() returns in both processes (the return value lets the respective process know which one it is).

On a system with more than one processor (or processor cores) you can normally (assuming you do have a SMP-enabled system, cpu affinity doesn't prevent it) expect those two processes to use both processors, but you do not strictly have a guarantee.

Threads are generated in the same way on some systems (e.g. Linux) with the exception that the pages owned by the first process are not marked copy-on-write, but are instead owned by both processes afterwards (they use the same page table). On other systems, threads may be implemented differently, e.g. in user land, in which case you will not benefit from multiple cpus with threads.

As a side note, the disadvantage of using fork() and running 2 processes instead of threads is that the processes do not share a common address space, which means that the TLB must be flushed on a context switch.