Mastering Linux System Administration. Richard Blum

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FIGURE 1.2 The Linux system memory map

      The memory locations are grouped into blocks called pages. The kernel locates each page of memory either in the physical memory or in the swap space. The kernel then maintains a table of the memory pages that indicates which pages are in physical memory and which pages are swapped out to disk.

      The kernel keeps track of which memory pages are in use and automatically copies memory pages that have not been accessed for a period of time to the swap space area (called swapping out), even if there's other memory available. When a program wants to access a memory page that has been swapped out, the kernel must make room for it in physical memory by swapping out a different memory page and swapping in the required page from the swap space. Obviously, this process takes time and can slow down a running process. The process of swapping out memory pages for running applications continues for as long as the Linux system is running.

       Real World Scenario

      LOOKING AT MEMORY

      There are a couple of simple commands you can use to get an idea of just how your Linux system is managing memory. While we'll be exploring these commands in more detail later in the book, here's a quick exercise for you to get started exploring your Linux system:

      SOFTWARE PROGRAM MANAGEMENT

      The Linux operating system calls a running program a process. A process can run in the foreground, displaying output on a display, or it can run in background, behind the scenes. The kernel controls how the Linux system manages all the processes running on the system.

      The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.

There are a few different types of init process implementations available in Linux, but these days, the two most popular are as follows:

The SysVinit initialization method used a concept called runlevels to determine what processes to start. The runlevel defines the state of the running Linux system and what processes should run in each state. Table 1.1 shows the different runlevels associated with the SysVinit initialization method.

TABLE 1.1: The SysVinit Runlevels

      The /etc/inittab file defines the default runlevel for a system. The processes that start for specific runlevels are defined in subdirectories of the /etc/rc.d directory. You can view the current runlevel at any time using the runlevel command, as shown here:

      $ runlevel N 5 $

      The systemd initialization method became popular because it has the ability to start processes based on different events such as these:

The systemd method determines what processes to run by linking events to unit files. Each unit file defines the programs to СКАЧАТЬ