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Processes in Linux

  • A process (or task) is an instance of a running program/application in the machine.
  • Can be composed of threads (lightweight)
  • Is not a command or program. One of these, can trigger multiple processes
  • Uses system resources (Memory, CPU, NW cards, Hard Drives, etc)
  • The OS (The kernel, scheduler) orchestrates the processes so they get access to the resources

Process Types

Process Type Description Example
Interactive Processes Need to be started by a user, either at a command line or through a graphical interface bash, firefox, top
Batch Processes Automatic processes which are scheduled from and then disconnected from the terminal (Run in background). These tasks are queued and work on a FIFO (First-In, First-Out) basis. updatedb, ldconfig
Daemons Server processes that run continuously. Many are launched during system startup and then wait for a user or system request indicating that their service is required. httpd, sshd, libvirtd, docker
Threads Lightweight processes. These are tasks that run under the umbrella of a main process, sharing memory and other resources, but are scheduled and run by the system on an individual basis. An individual thread can end without terminating the whole process and a process can create new threads at any time. Many non-trivial programs are multi-threaded. firefox, gnome-terminal-server
Kernel Threads Kernel tasks that users neither start nor terminate and have little control over. These may perform actions like moving a thread from one CPU to another, or making sure input/output operations to disk are completed. kthreadd, migration, ksoftirqd

Scheduling

States

A process can be in one of:

  • RUNNING: Currently executing or waiting to be granted a share of time in the run queue (1 for each processor)
  • SLEEP: Waiting for something (external) to happen.
  • Others: F.Ex ZOMBIE, when a child process finished but parent has not checked yet, so it appears alive.

Priorities

  • To determine access to the resources, the OS prioritizes the processes
  • In Linux, the niceness value is used. (Less nice, more priority)
  • Goes from -20 (top priority) to 19
  • Can be checked using ps lf
  • Can be changed using renice (To give more, super user is required): 
      renice +5 3077 #Lowers the priority (niceness = 5) of PID 3077
  sudo renice -5 3077 #Gives more priority(niceness = -5) for PID 3077

Identification

PID

  • Unique ID given by the OS to the process
  • ALlows to track resource usage and to manage
  • Starts with 1 (init process)
  • A process can have:
    • PID
    • PPID (Parent process ID, the one who started it)
    • TID (Thread Unique ID if the process is a thread, inside a main process)

User and Group ID

  • Different users (parts from different groups) can start processes
  • For users, the RUID (Real User Id) is used to signal who started, together with EUID to see who granted access. In practice, UID is shown.
  • For groups, the logic is the same. In practice, is identified with GID

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