I read in a paper that the underlying system call to create processes and threads is actually the same, and thus the cost of creating processes over threads is not that great.
- First, I wanna know what is the system call that creates processes/threads (possibly a sample code or a link?)
- Second, is the author correct to assume that creating processes instead of threads is inexpensive?
Replacing pthreads with processes is surprisingly inexpensive, especially on Linux where both pthreads and processes are invoked using the same underlying system call.
Processes are usually created with
fork, threads (lightweight processes) are usually created with
clone nowadays. However, anecdotically, there exist 1:N thread models, too, which don't do either.
clone map to the same kernel function
do_fork internally. This function can create a lightweight process that shares the address space with the old one, or a separate process (and many other options), depending on what flags you feed to it. The
clone syscall is more or less a direct forwarding of that kernel function (and used by the higher level threading libraries) whereas
do_fork into the functionality of the 50 year old traditional Unix function.
The important difference is that
fork guarantees that a complete, separate copy of the address space is made. This, as Basil points out correctly, is done with copy-on-write nowadays and therefore is not nearly as expensive as one would think.
When you create a thread, it just reuses the original address space and the same memory.
However, one should not assume that creating processes is generally "lightweight" on unix-like systems because of copy-on-write. It is somewhat less heavy than for example under Windows, but it's nowhere near free.
One reason is that although the actual pages are not copied, the new process still needs a copy of the page table. This can be several kilobytes to megabytes of memory for processes that use larger amounts of memory. Another reason is that although copy-on-write is invisible and a clever optimization, it is not free, and it cannot do magic. When data is modified by either process, which inevitably happens, the affected pages fault.
Redis is a good example where you can see that
fork is everything but lightweight (it uses
fork to do background saves).