A parked car can make a trip, but only once someone starts and drives it. A program is similar: its instructions sit on disk until the operating system runs them. That running instance is a process. The operating system supplies and manages the resources it needs, rather than leaving the program to own the computer outright. section 4.2
A process has an execution context that identifies what it is running and what it can use:
- An address space holds the memory it can access, including its variables. section 4.3
- An executable supplies its instructions, and a process ID (PID) identifies the running instance. section 4.3
- Resource information tracks such things as open files and network connections. section 4.3
Separate processes have separate address spaces. This lets the operating system isolate their work and allocate resources to each one; a child process cannot directly read another process’s data. Isolation has a cost: starting and managing a process is relatively heavy, and exchanging data between processes requires a communication mechanism. Threads within one process share its address space and generally cost less to create, but sharing resources calls for synchronization. section 4.2 section 4.7
Three independent children
A Python parent can start three child processes with multiprocessing.Process(target=run_child).start(). Each child runs the target function and prints its own PID and its parent’s PID. Their execution is independent; starting several processes makes concurrent work possible, but does not by itself guarantee that the hardware runs them in parallel. section 4.5 section 6.12
# Chapter 4/child_processes.py
import os
from multiprocessing import Process
def run_child() -> None:
print("Child: I am the child process")
print(f"Child: Child’s PID: {os.getpid()}")
print(f"Child: Parent’s PID: {os.getppid()}")
def start_parent(num_children: int) -> None:
print("Parent : I am the parent process")
print(f"Parent : Parent’s PID: {os.getpid()}")
for i in range(num_children):
print(f"Starting Process {i}")
Process(target=run_child).start() #1
if __name__ == "__main__":
num_children = 3
start_parent(num_children)
The Python listing shows the parent starting three processes and using PIDs to distinguish their execution. section 4.5
