Concepts & Types of Linux Kernel
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What is Linux Kernel?
The Linux® kernel is the main component of a Linux operating system (OS) and is the core interface between a computer’s hardware and its processes. It communicates between the 2, managing resources as efficiently as possible.
The kernel is so named because—like a seed inside a hard shell—it exists within the OS and controls all the major functions of the hardware, whether it’s a phone, laptop, server, or any other kind of computer.
What the kernel does?
The kernel has 4 jobs:
Memory management: Keep track of how much memory is used to store what, and where
Process management: Determine which processes can use the central processing unit (CPU), when, and for how long
Device drivers: Act as mediator/interpreter between the hardware and processes
Where the kernel fits within the OS?
To put the kernel in context, you can think of a Linux machine as having 3 layers:
The hardware: The physical machine—the bottom or base of the system, made up of memory (RAM) and the processor or central processing unit (CPU), as well as input/output (I/O) devices such as storage, networking, and graphics. The CPU performs computations and reads from, and writes to, memory.
The Linux kernel: The core of the OS. (See? It’s right in the middle.) It’s software residing in memory that tells the CPU what to do.
User processes: These are the running programs that the kernel manages. User processes are what collectively make up user space. User processes are also known as just processes. The kernel also allows these processes and servers to communicate with each other (known as inter-process communication, or IPC).
Is LINUX A Kernel Or An Operating System?
Well, there is a difference between kernel and OS. Kernel as described above is the heart of OS which manages the core features of an OS while if some useful applications and utilities are added over the kernel, then the complete package becomes an OS. So, it can easily be said that an operating system consists of a kernel space and a user space.
So, we can say that Linux is a kernel as it does not include applications like file-system utilities, windowing systems and graphical desktops, system administrator commands, text editors, compilers etc. So, various companies add these kind of applications over linux kernel and provide their operating system like ubuntu, suse, centOS, redHat etc.
Kernels may be classified mainly in two categories
Monolithic
Micro Kernel
1 Monolithic Kernels
Earlier in this type of kernel architecture, all the basic system services like process and memory management, interrupt handling etc were packaged into a single module in kernel space. This type of architecture led to some serious drawbacks like 1) Size of kernel, which was huge. 2)Poor maintainability, which means bug fixing or addition of new features resulted in recompilation of the whole kernel which could consume hours
In a modern day approach to monolithic architecture, the kernel consists of different modules which can be dynamically loaded and un-loaded. This modular approach allows easy extension of OS's capabilities. With this approach, maintainability of kernel became very easy as only the concerned module needs to be loaded and unloaded every time there is a change or bug fix in a particular module. So, there is no need to bring down and recompile the whole kernel for a smallest bit of change. Also, stripping of kernel for various platforms (say for embedded devices etc) became very easy as we can easily unload the module that we do not want.
Linux follows the monolithic modular approach
2 Microkernels
This architecture majorly caters to the problem of ever growing size of kernel code which we could not control in the monolithic approach. This architecture allows some basic services like device driver management, protocol stack, file system etc to run in user space. This reduces the kernel code size and also increases the security and stability of OS as we have the bare minimum code running in kernel. So, if suppose a basic service like network service crashes due to buffer overflow, then only the networking service's memory would be corrupted, leaving the rest of the system still functional.