6_unix_processes.md

title: Unix Processes

Unix Processes

Objective: Understand how process works in GNU/Linux

A Program is a list of intructions to be executed on a computer. These instruction are written in one or many files in a format readable by the computer (binary files), or interpertable by the computer (text file). The interpretable format need to be processed by an interperter who is in binary format.

The execution of a program on the system is represented as one or many processes. The program is the file of instruction while the process is the instructions being read.

mkdir is the program, when you type mkdir my_folder, you launch an mkdir process.

Your shell is a process to manipulate other processes.

In multitasking operating systems, processes (running programs) need a way to create new processes, e.g. to run other programs. Fork and its variants are typically the only way of doing so in Unix-like systems. For a process to start the execution of a different program, it first forks to create a copy of itself. Then, the copy, called the "child process", calls the exec system call to overlay itself with the other program: it ceases execution of its former program in favor of the other.

Process attributes

• PID : the process identifier is an integer, at a given time each PID is unique to a process
• PPID : the parent process identifier is the PID of the process that has stared the current process
• UID : the user identifier, is the identifier of the user that has started the process
• PGID : the process group identifier (like users process have groups)

You can use the command ps to see thes process launched by your user

ps

Like for the command ls you can use the switch -l to have more details

Open another tab in your browser to log again on your VM.

In this new tab, you are going to launch a less process

less .bashrc

Comme back, to your first tab, can you see your less process with the command ps ?

The ps option -u [login] list all the process with UIDthe UIDassociated with [login]

ps -l -u etudiant

Is the number of bash processes consistent with the number of tabs you openned ?

What is the PPID of the less process ? Can you identify in which bash process less is running ?

Did you launch the systemd and (sd-pam) process ?

pam stand for pluggable authentication modules, its a collection of tools that handle identification and resource access restriction. From the PPID of the (sd-pam) can you find which process launched (sd-pam) ? What is the PPID of this process ?

The option -C allow you to filter process by name

ps -l -C systemd

Who launched the first systemd process ?

From PPID to PPID, you can guess that like the file system, processes are organized in a tree. The command pstree can give you a nice representation of this tree.

The following ps command show information on the process with PID 1

ps -l -1

Can you look at the corresponding program (with the command which) ?

Can you look for informations on PID 0 ?

The root of the processes tree is the PID 1.

While ps give you a static snapshot of the processes but processes are dynamic. To see them running you can use the command top. While top is functional, most system have htop with a more accessible interface. You can test top and htop.

Like ps you can use -u etduiant with htop to only display your user process

With the F6 key you can change the column on which to sort your process.

• Which process is consuming the most of CPU ?
• Which process is consuming the most of memory ?

What is the difference between M_SIZE (VIRT column), M_RESIDENT (RES column) and M_SHARE (SHR column) ? To which value, the column MEM% correspond to ?

• M_SIZE : The total amount of virtual memory used by the task. It includes all code, data and shared libraries plus pages that have been swapped out and pages that have been mapped but not used (If an application requests 1 GB of memory but uses only 1 MB, then VIRT will report 1 GB).
• M_RESIDENT : what's currently in the physical memory. This does not include the swapped out memory and some of the memory may be shared with other processes (If a process uses 1 GB of memory and it calls fork(), the result of forking will be two processes whose RES is both 1 GB but only 1 GB will actually be used since Linux uses copy-on-write).
• M_SHARE : The amount of shared memory used by a task. It simply reflects memory that could be potentially shared with other processes.