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A monolithic kernel is a kernel where all services (file system, VFS, device drivers, etc) as well as core functionality (scheduling, memory allocation, etc.) are a tight knit group sharing the same space. This directly opposes a microkernel. A microkernel prefers an approach where core functionality is isolated from system services and device drivers (which are basically just system services). For instance, VFS (virtual file system) and block device file systems (i.e. minixfs) are separate processes that run outside of the kernel's space, using IPC to communicate with the kernel, other services and user processes. In short, if it's a module in Linux, it's a service in a microkernel, indicating an isolated process. Do not confuse the term modular kernel to be anything but monolithic. Some monolithic kernels can be compiled to be modular (e.g Linux), what matters is that the module is inserted to and runs from the same space that handles core functionality. The advantage to a microkernel is that any failed service can be easily restarted, for instance, there is no kernel halt if the root file system throws an abort. The disadvantage to a microkernel is that asynchronous IPC messaging can become very difficult to debug, especially if fibrils are implemented. Additionally, just tracking down a FS/write issue means examining the user space process, the block device service, VFS service, file system service and (possibly) the PCI service. If you get a blank on that, its time to look at the IPC service. This is often easier in a monolithic kernel. GNU Hurd suffers from these debugging problems (reference). I'm not even going to go into checkpointing when dealing with complex message queues. Microkernels are not for the faint of heart. The shortest path to a working, stable kernel is the monolithic approach. Either approach can offer a POSIX interface, where the design of the kernel becomes of little interest to someone simply wanting to write code to run on any given design. I use Linux (monolithic) in production. However, most of my learning, hacking or tinkering with kernel development goes into a microkernel, specifically HelenOS. Edit If you got this far through my very long-winded answer, you will probably have some fun reading the 'Great Torvalds-Tanenbaum debate on kernel design'. It's even funnier to read in 2013, more than 20 years after it transpired. The funniest part was Linus' signature in one of the last messages: Obviously, that did not come true any more than Tanenbaum's prediction that x86 would soon be obsolete. NB: When I say "Minix", I do not imply Minix 3. Additionally, when I mention The HURD, I am referencing (mostly) the Mach microkernel. It is not my intent to disparage the recent work of others. | |||||||||||||||||||||
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From wikipedia
Recent versions of Windows on the other hand use a Hybric kernel.
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Monolithic kernel means that the whole operating system runs in kernel mode (i.e. highly privileged by the hardware). That is, no part of the OS runs in user mode (lower privilege). Only applications on top of the OS run in user mode. In non-monolithic kernel operating systems, such as Windows, a large part of the OS itself runs in user mode. In either case, the OS can be highly modular. | |||||||||||||||||||||
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'Monolithic' in this context does not refer to there being a single large executable, and as you say, there Linux supports the dynamic loading of kernel modules at runtime. When talking about kernels, 'monolithic' means that the entire operating system runs in 'privileged' or 'supervisor' mode, as opposed to other types of operating systems that use a type of kernel such as a 'microkernel', where only a minimal set of functionality runs in privileged mode, and most of the operating system runs in user space. Proponents of microkernels say that this is better because smaller code means less bugs, and bugs running in supervisor mode can cause much greater problems than in user space code (such as a greater chance of having security vulnerabilities or total system crashes in the form of a 'kernel panic'). Some microkernels are sufficiently minimal that they can be 'formally verified', which means you can mathematically prove that the kernel is 'correct' according to a specification. L4 is a good example of this. | |||||
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