spawn (default), fork or vfork. If the method is spawn
but this cannot be used because it is either not supported by the OS
or the cwd(Dir) option is given fork is used.
The problem is to be understood as follows. The official portable
and safe method to create a process is using the fork() system call.
This call however copies the process page tables and get seriously
slow as the (Prolog) process is multiple giga bytes large.
Alternatively, we may use vfork() which avoids copying the process
space. But, the safe usage as guaranteed by the POSIX standard of
vfork() is insufficient for our purposes. On practical systems your
mileage may vary. Modern posix systems also provide posix_spawn(),
which provides a safe and portable alternative for the fork() and
exec() sequence that may be implemented using fork() or may use a
fast but safe alternative. Unfortunately posix_spawn() doesn't
support the option to specify the working directory for the child
and we cannot use working_directory/2 as the working directory is
shared between threads.
Summarizing, the default is safe and tries to be as fast as
possible. On some scenarios and on some OSes it is possible to do
better. It is generally a good idea to avoid using the cwd(Dir)
option of process_create/3 as without we can use posix_spawn().