Processes, Services and Systemd

Process and Thread

Why Process Abstraction

Goals of an Operating System

• Referee
  • Manages access to shared resources (memory, CPU, etc.)
  • Isolates & protects workloads from each other
• Illusionist
  • Provide abstractions of physical resources
  • Each program thinks it has a dedicated machine
• Glue
  • Provide common interfaces
  • File system, networking, window management, etc.

How Process Achieve these

• User code must run in a process
  • Referee → Restricted execution environment (privilege levels, only allowed to access what it owns)
  • Illusionist → Own virtual address space (~infinite memory from point of view of the program)
  • Glue → I/O (APIs for files, sockets, etc.)
• Each running program has its own process

Multithreaded Process

• A process may have one or more threads.
• Threads encapsulate concurrency
  • Share the same address space, resources, etc.
• Process are for isolation
  • Communication between processes much harder than between threads

Summary

• A process is a single instance of a program
• Processes are isolated from one another and have their own memory, threads, etc.
• The OS provides common interfaces for processes to use for I/O, etc.

What Creates Processes

The answer is … Process!
By fork - join pattern

• fork: creates a copy of the current process
• exec: restart current process with a different executable
• wait: wait for a process to finish, retrieves the exit status
  • OS needs to keep exit status somewhere, even if the parent doesn’t wait
  • If parent never waits, this causes a resource leak! (Zombie process)

Construct a tree structure:

Use htop to monitor processes!

But… How is the first process created?

Kernel

Kernel Code does not run in a process, but kernel creates the init process during boot

Init Process

• Started by kernel during boot

Manage Processes

• If parent exits first, orphan processes are re-parented to the init process
• Init waits on all processes
  • Zombie processes can still occur if the parent never exits!

Manage Services

• We need a way to access the computer! Init will start some critical services
• Critical services include
  • getty
  • Display manager
  • Ssh

What is service?

• A process that needs to be kept track of
• Often dameons, non-interactive background processes
  • getty, sddm, sshd, httpd, nginx, postfix, etc.
• Need to be always running!
• If something is not running, we might not be able to access our computer

Systemd

What is Systemd

• An init system
  • Most commonly used init system(the 1st process) on modern Linux systems
• Provides tools for users to manage services
  • systemctl - start, stop, check status, and more
  • journalctl - check systemd journal
• Also manages a lot more!
  • logind, networkd, resolved, tmpfilesd, homed, …
• Service behavior defined by systemd unit files
  • How should this service start up?
  • How should it respond to various systemctl management commands?
  • When and how should this service stop, restart, reload and so on

Some Command

• systemctl start [name] - starts a service
• systemctl stop [name] - stops a service
• systemctl restart [name] - restarts a service
• systemctl reload [name] - reload a service’s configuration
• systemctl enable [name] - sets a service to start on boot
• systemctl disable [name] - opposite of enable

Behavior for these commands may depend on the service, as defined in the unit file!