What is difference between monolithic and micro kernel?

Could anyone please explain with examples difference between monolithic and micro kernel? Also other classifications of the kernel?

Monolithic kernel is a single large process running entirely in a single address space. It is a single static binary file. All kernel services exist and execute in the kernel address space. The kernel can invoke functions directly. Examples of monolithic kernel based OSs: Unix, Linux.

In microkernels, the kernel is broken down into separate processes, known as servers. Some of the servers run in kernel space and some run in user-space. All servers are kept separate and run in different address spaces. Servers invoke "services" from each other by sending messages via IPC (Interprocess Communication). This separation has the advantage that if one server fails, other servers can still work efficiently. Examples of microkernel based OSs: Mac OS X and Windows NT.

1. Monolithic kernel design is much older than the microkernel idea, which appeared at the end of the 1980's.

2. Unix and Linux kernels are monolithic, while QNX, L4 and Hurd are microkernels. Mach was initially a microkernel (not Mac OS X), but later converted into a hybrid kernel. Minix (before version 3) wasn't a pure microkernel because device drivers were compiled as part of the kernel.

3. Monolithic kernels are usually faster than microkernels. The first microkernel Mach was 50% slower than most monolithic kernels, while later ones like L4 were only 2% or 4% slower than the monolithic designs.

4. Monolithic kernels are big in size, while microkernels are small in size - they usually fit into the processor's L1 cache (first generation microkernels).

5. In monolithic kernels, the device drivers reside in the kernel space while in the microkernels the device drivers are user-space.

6. Since monolithic kernels' device drivers reside in the kernel space, monolithic kernels are less secure than microkernels, and failures (exceptions) in the drivers may lead to crashes (displayed as BSODs in Windows). Microkernels are more secure than monolithic kernels, hence more often used in military devices.

7. Monolithic kernels use signals and sockets to implement inter-process communication (IPC), microkernels use message queues. 1st gen microkernels didn't implement IPC well and were slow on context switches - that's what caused their poor performance.

8. Adding a new feature to a monolithic system means recompiling the whole kernel or the corresponding kernel module (for modular monolithic kernels), whereas with microkernels you can add new features or patches without recompiling.

Monolithic kernel

All the parts of a kernel like the Scheduler, File System, Memory Management, Networking Stacks, Device Drivers, etc., are maintained in one unit within the kernel in Monolithic Kernel

Advantages

•Faster processing

Disadvantages

•Crash Insecure •Porting Inflexibility •Kernel Size explosion

Examples •MS-DOS, Unix, Linux

Micro kernel

Only the very important parts like IPC(Inter process Communication), basic scheduler, basic memory handling, basic I/O primitives etc., are put into the kernel. Communication happen via message passing. Others are maintained as server processes in User Space

Advantages

•Crash Resistant, Portable, Smaller Size

Disadvantages

•Slower Processing due to additional Message Passing

Examples •Windows NT

1.Monolithic Kernel (Pure Monolithic) :all

• All Kernel Services From single component

(-) addition/removal is not possible, less/Zero flexible

(+) inter Component Communication is better

e.g. :- Traditional Unix

2.Micro Kernel :few

• few services(Memory management ,CPU management,IPC etc) from core kernel, other services(File management,I/O management. etc.) from different layers/component

• Split Approach [Some services is in privileged(kernel) mode and some are in Normal(user) mode]

(+)flexible for changes/up-gradations

(-)communication overhead

e.g.:- QNX etc.

3.Modular kernel(Modular Monolithic) :most

• Combination of Micro and Monolithic kernel

• Collection of Modules -- modules can be --> Static + Dynamic

• Drivers come in the form of Modules

e.g. :- Linux Modern OS

In the spectrum of kernel designs the two extreme points are monolithic kernels and microkernels.

The (classical) Linux kernel for instance is a monolithic kernel (and so is every commercial OS to date as well - though they might claim otherwise);

In that its code is a single C file giving rise to a single process that implements all of the above services.
To exemplify the encapsulation of the Linux kernel we remark that the Linux kernel does not even have access to any of the standard C libraries. Indeed the Linux kernel cannot use rudimentary C library functions such as printf. Instead it implements its own printing function (called prints).

This seclusion of the Linux kernel and self-containment provide Linux kernel with its main advantage: the kernel resides in a single address space1 enabling all features to communicate in the fastest way possible without resorting to any type of message passing. In particular, a monolithic kernel implements all of the device drivers of the system.

This however is the main drawback of a monolithic kernel: introduction of any new unsupported hardware requires a rewrite of the kernel (in the relevant parts), recompilation of it, and re-installing the entire OS.
More importantly, if any device driver crashes the entire kernel suffers as a result. This un-modular approach to hardware additions and hardware crashes is the main argument for supporting the other extreme design approach for kernels. A microkernel is in a sense a minimalistic kernel that houses only the very basic of OS services (like process management and file system management). In a microkernel the device drivers lie outside of the kernel allowing for addition and removal of device drivers while the OS is running and require no alternations of the kernel.

Microkernel:

Moves as much from the kernel into “user” space.

Communication takes place between user modules using message passing.

Benefits:

1-Easier to extend a microkernel

2-Easier to port the operating system to new architectures

3-More reliable (less code is running in kernel mode)

4-More secure

Detriments:

1-Performance overhead of user space to kernel space communication

Monolithic kernel has all kernel services along with kernel core part, thus are heavy and has negative impact on speed and performance. On the other hand micro kernel is lightweight causing increase in performance and speed.
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