qualche link magari di gente che ne sa un'anticchia più di me:

http://stackoverflow.com/questions/3...e-the-next-pid

http://superuser.com/questions/13500...pids-generated

da cui un estratto:

Under Unix, process IDs are usually allocated on a sequential basis, beginning at 0 and rising to a maximum value which varies from system to system. Once this limit is reached, allocation restarts at zero and again increases. However, for this and subsequent passes any PIDs still assigned to processes are skipped.
per questo i pid non ti sembrano mai molto elevati