Mastering Linux System Administration. Richard Blum
of these four parts has a specific job in the Linux system. Figure 1.1 shows a basic diagram of how the parts fit together on top of the computer hardware to create the overall Linux system.
FIGURE 1.1 The Linux system
This section describes these four main parts in detail and gives you an overview of how they work together to create a complete Linux system.
Looking into the Linux Kernel
The core of the Linux system is the kernel. The kernel controls all of the hardware and software on the computer system, allocating hardware when necessary and executing software when required.
If you've been following the Linux world at all, no doubt you've heard the name Linus Torvalds. Linus is the person responsible for creating the first Linux kernel software while he was a student at the University of Helsinki. He intended it to be a copy of the Unix system, at the time a popular operating system used at many universities.
After developing the Linux kernel, Linus released it to the Internet community and solicited suggestions for improving it. This simple process started a revolution in the world of computer operating systems. Soon Linus was receiving suggestions from students as well as professional programmers from around the world.
Allowing anyone to change programming code in the kernel would result in complete chaos. To simplify things, Linus acted as a central point for all improvement suggestions. It was ultimately Linus's decision whether to incorporate suggested code in the kernel. This same concept is still in place with the Linux kernel code, except that instead of just Linus controlling the kernel code, a team of developers has taken on the task.
The kernel is primarily responsible for these four main functions:
System memory management
Software program management
Hardware management
Filesystem management
The following sections explore each of these functions in more detail.
SYSTEM MEMORY MANAGEMENT
One of the primary functions of the operating system kernel is memory management. Not only does the kernel manage the physical memory available on the server, but it can also create and manage virtual memory, or memory that does not actually exist.
It does this by using space on the hard disk, called the swap space. The kernel swaps the contents of virtual memory locations back and forth from the swap space to the actual physical memory. This allows the system to think there is more memory available than what physically exists (shown in Figure 1.2).
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.
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:
1 Log into your Linux system. (If you don't have a Linux system available yet, you can come back to here after going through either Chapter 2, “Installing an Ubuntu Server,” or Chapter 4, “Installing a Red Hat Server.”)
2 From the command prompt, enter the command free. You should see something similar to this output:$ free total used free shared buff/cache available Mem: 2035504 135668 1449568 1048 450268 1742704 Swap: 2097148 0 2097148The output from the free command shows the total amount of physical memory installed on the system, as well as the amount of swap space currently configured.
3 The free command just provides an overview of the memory for your Linux system. For a more detailed look, enter the command cat /proc/meminfo. You should see a long listing, similar to this:$ cat /proc/meminfo MemTotal: 2035504 kB MemFree: 1449632 kB MemAvailable: 1742352 kB Buffers: 25452 kB Cached: 386028 kB SwapCached: 0 kB Active: 166036 kB Inactive: 290704 kB Active(anon): 51796 kB Inactive(anon): 128 kB Active(file): 114240 kB Inactive(file): 290576 kB Unevictable: 18640 kB Mlocked: 18640 kB SwapTotal: 2097148 kB SwapFree: 2097148 kB Dirty: 156 kB Writeback: 0 kB AnonPages: 63940 kB Mapped: 63344 kB Shmem: 1048 kB KReclaimable: 38664 kB Slab: 74316 kB SReclaimable: 38664 kB SUnreclaim: 35652 kB KernelStack: 2044 kB PageTables: 1268 kB NFS_Unstable: 0 kB Bounce: 0 kB WritebackTmp: 0 kB CommitLimit: 3114900 kB Committed_AS: 376812 kB VmallocTotal: 34359738367 kB VmallocUsed: 27676 kB VmallocChunk: 0 kB Percpu: 516 kB HardwareCorrupted: 0 kB AnonHugePages: 0 kB ShmemHugePages: 0 kB ShmemPmdMapped: 0 kB FileHugePages: 0 kB FilePmdMapped: 0 kB CmaTotal: 0 kB CmaFree: 0 kB HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 HugePages_Surp: 0 Hugepagesize: 2048 kB Hugetlb: 0 kB DirectMap4k: 90048 kB DirectMap2M: 2007040 kB $The kernel continually updates the meminfo file to show exactly what's going on in memory at that moment in time, so the file constantly changes.
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:
SysVinit—The SysVinit (SysV) initialization method was the original method used by Linux and was based on the Unix System V initialization method. Though it is not used by many Linux distributions these days, you still may find it around in older Linux distributions.
Systemd—The systemd initialization method was created in 2010 and has become the most popular initialization and process management system used by Linux distributions.