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Chapter 2.5 - Process Management

Learning Objectives​

At the end of this chapter, you will be able to:

  • Understand how Linux manages processes
  • Explain process states
  • Understand scheduling
  • Identify priorities and nice values

What is a Process?​

A process is an instance of a running program. Each process has:

  • A unique PID (Process ID)
  • A PPID (Parent Process ID)
  • Its own memory space
  • Open files
  • System resources

Process States​

A process can be in different states:

States:

  • Running (R): Executing or ready
  • Sleeping (S): Waiting for an event
  • Stopped (T): Suspended (Ctrl+Z)
  • Zombie (Z): Finished but not cleaned up
  • Uninterruptible (D): Waiting for disk I/O

Process Scheduling​

The Linux scheduler (CFS - Completely Fair Scheduler) decides which process to run:

Scheduling Criteria:

  • Priority (nice value)
  • CPU time used
  • Waiting time
  • CPU affinity

Priorities and Nice Values​

A process's priority determines its access to the CPU:

Nice values:

  • -20 to +19 (lower = more priority)
  • 0: Default priority
  • -20: High priority (root only)
  • +19: Very low priority
# View priority
ps -o pid,ni,comm

# Change priority at start
nice -n 10 command

# Change priority of an existing process
renice 10 PID

# Real-time process (root)
chrt -f 50 command

Process Hierarchy​

Processes form a hierarchy (tree):

Characteristics:

  • Parent process can create children (fork)
  • If parent dies, children become orphans (adopted by init)
  • Zombie process if parent does not retrieve status

Management Commands​

# View processes
ps aux
top
htop

# Kill a process
kill PID
kill -9 PID  # Force
killall process_name

# Suspend/Resume
kill -STOP PID
kill -CONT PID

# View process tree
pstree
ps auxf

Inter-Process Communication​

Processes can communicate via:

  1. Signals: kill, SIGTERM, SIGKILL
  2. Pipes: | (command line)
  3. Sockets: Network communication
  4. Shared files: Shared memory
  5. Message queues: IPC System V

Summary​

In this chapter, you learned:

Process: Instance of a running program
States: Running, Sleeping, Stopped, Zombie
Scheduler: CFS schedules process execution
Priorities: Nice values from -20 to +19
Hierarchy: Tree with init as root
Management: ps, top, kill, nice, renice

Next Steps​

Chapter 2.6: Init and systemd Processes
Chapter 2.7: Services and Daemons

Chapter created on: December 2024