Running Linux, 5th Edition - Matthias Kalle Dalheimer [14]
It is widely held that networking performance under Linux is superior to other operating systems. We talk more about networking in Chapter 13 and Part IV.
Kernel
The kernel is the guts of the operating system itself; it's the code that controls the interface between user programs and hardware devices, the scheduling of processes to achieve multitasking, and many other aspects of the system. The kernel is not a separate process running on the system. Instead, you can think of the kernel as a set of routines, constantly in memory, to which every process has access. Kernel routines can be called in a number of ways. One direct method to utilize the kernel is for a process to execute a system call, which is a function that causes the kernel to execute some code on behalf of the process. For example, the read system call will read data from a file descriptor. To the programmer, this looks like any other C function, but in actuality the code for read is contained within the kernel.
The Linux kernel is known as a monolithic kernel, in that all core functions and device drivers are part of the kernel proper. Some operating systems employ a microkernel architecture whereby device drivers and other components (such as filesystems and memory management code) are not part of the kernel—rather, they are treated as independent services or regular user applications. There are advantages and disadvantages to both designs: the monolithic architecture is more common among Unix implementations and is the design employed by classic kernel designs, such as System V and BSD. Linux does support loadable device drivers (which can be loaded and unloaded from memory through user commands); this is covered in Chapter 18.
The Linux kernel on Intel platforms is developed to use the special protected-mode features of the Intel x86 processors (starting with the 80386 and moving on up to the current Pentium 4). In particular, Linux makes use of the protected-mode descriptor-based memory management paradigm and many of the other advanced features of these processors. Anyone familiar with x86 protected-mode programming knows that this chip was designed for a multitasking system such as Unix (the x86 was actually inspired by Multics). Linux exploits this functionality.
Like most modern operating systems, Linux is a multiprocessor operating system: it supports systems with more than one CPU on the motherboard. This feature allows different programs to run on different CPUs at the same time (or "in parallel"). Linux also supports threads, a common programming technique that allows a single program to create multiple "threads of control" that share data in memory. Linux supports several kernel-level and user-level thread packages, and Linux's kernel threads run on multiple CPUs, taking advantage of true hardware parallelism. The Linux kernel threads package is compliant with the POSIX 1003.1c standard.
The Linux kernel supports demand-paged loaded executables. That is, only those segments of a program that are actually used are read into memory from disk. Also, if multiple instances of a program are running at once, only one copy of the program code will be in memory. Executables use dynamically linked shared libraries, meaning that executables share common library code in a single library file found on disk. This allows executable files to occupy much less space on disk. This also means that a single copy of the library code is held in memory at one time, thus reducing overall memory usage. There are also statically linked libraries for those who wish to maintain "complete" executables without the need for shared libraries to be in place. Because Linux shared libraries are dynamically linked at runtime, programmers can replace modules of the libraries with their own routines.
In order to make the best use of the system's memory, Linux implements so-called virtual memory with disk paging. That is, a certain amount of swap space [*]