What is difference between monolithic and micro kernel?
Monolithic kernel design is much older than the microkernel idea, which appeared at the end of the 1980's.
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.
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.
Monolithic kernels are big in size, while microkernels are small in size - they usually fit into the processor's L1 cache (first generation microkernels).
In monolithic kernels, the device drivers reside in the kernel space while in the microkernels the device drivers are user-space.
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.
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.
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
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.