What is difference between monolithic and micro kernel

Question

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

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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

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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

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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

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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

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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

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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  I answered same question at wordpress site  For the difference between monolithic  microkernel and exokernel in tabular form  you can visit here

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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

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