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While monolithic kernels execute all of their code in the same address space kernel space , microkernels try to run most of their services in user space, aiming to improve maintainability and modularity of the codebase.
These are called hybrid kernels. More exotic designs such as nanokernels and exokernels are available, but are seldom used for production systems. The Xen hypervisor, for example, is an exokernel.
Main article: Monolithic kernel Diagram of a monolithic kernel In a monolithic kernel, all OS services run along with the main kernel thread, thus also residing in the same memory area.
This approach provides rich and powerful hardware access. Some developers, such as UNIX developer Ken Thompson , maintain that it is "easier to implement a monolithic kernel"  than microkernels. Monolithic kernels, which have traditionally been used by Unix-like operating systems, contain all the operating system core functions and the device drivers.
This is the traditional design of UNIX systems. A monolithic kernel is one single program that contains all of the code necessary to perform every kernel related task. Every part which is to be accessed by most programs which cannot be put in a library is in the kernel space: Device drivers, Scheduler, Memory handling, File systems, Network stacks. Many system calls are provided to applications, to allow them to access all those services.
A monolithic kernel, while initially loaded with subsystems that may not be needed, can be tuned to a point where it is as fast as or faster than the one that was specifically designed for the hardware, although more relevant in a general sense. Modern monolithic kernels, such as those of Linux and FreeBSD , both of which fall into the category of Unix-like operating systems, feature the ability to load modules at runtime, thereby allowing easy extension of the kernel's capabilities as required, while helping to minimize the amount of code running in kernel space.
In the monolithic kernel, some advantages hinge on these points: Since there is less software involved it is faster. As it is one single piece of software it should be smaller both in source and compiled forms. Less code generally means fewer bugs which can translate to fewer security problems. Most work in the monolithic kernel is done via system calls.
These are interfaces, usually kept in a tabular structure, that access some subsystem within the kernel such as disk operations. Essentially calls are made within programs and a checked copy of the request is passed through the system call. Hence, not far to travel at all. The monolithic Linux kernel can be made extremely small not only because of its ability to dynamically load modules but also because of its ease of customization. In fact, there are some versions that are small enough to fit together with a large number of utilities and other programs on a single floppy disk and still provide a fully functional operating system one of the most popular of which is muLinux.
This ability to miniaturize its kernel has also led to a rapid growth in the use of Linux in embedded systems. These types of kernels consist of the core functions of the operating system and the device drivers with the ability to load modules at runtime.
They provide rich and powerful abstractions of the underlying hardware. They provide a small set of simple hardware abstractions and use applications called servers to provide more functionality.
This particular approach defines a high-level virtual interface over the hardware, with a set of system calls to implement operating system services such as process management, concurrency and memory management in several modules that run in supervisor mode.
This design has several flaws and limitations: Coding in kernel can be challenging, in part because one cannot use common libraries like a full-featured libc , and because one needs to use a source-level debugger like gdb. Rebooting the computer is often required. This is not just a problem of convenience to the developers. When debugging is harder, and as difficulties become stronger, it becomes more likely that code will be "buggier".
Bugs in one part of the kernel have strong side effects; since every function in the kernel has all the privileges, a bug in one function can corrupt data structure of another, totally unrelated part of the kernel, or of any running program. Kernels often become very large and difficult to maintain. Even if the modules servicing these operations are separate from the whole, the code integration is tight and difficult to do correctly. Since the modules run in the same address space , a bug can bring down the entire system.
Monolithic kernels are not portable; therefore, they must be rewritten for each new architecture that the operating system is to be used on. In the microkernel approach, the kernel itself only provides basic functionality that allows the execution of servers , separate programs that assume former kernel functions, such as device drivers, GUI servers, etc. A microkernel that is designed for a specific platform or device is only ever going to have what it needs to operate.
The microkernel approach consists of defining a simple abstraction over the hardware, with a set of primitives or system calls to implement minimal OS services such as memory management , multitasking , and inter-process communication. Other services, including those normally provided by the kernel, such as networking , are implemented in user-space programs, referred to as servers. Microkernels are easier to maintain than monolithic kernels, but the large number of system calls and context switches might slow down the system because they typically generate more overhead than plain function calls.
Many critical parts are now running in user space: The complete scheduler, memory handling, file systems, and network stacks. Micro kernels were invented as a reaction to traditional "monolithic" kernel design, whereby all system functionality was put in a one static program running in a special "system" mode of the processor.
In the microkernel, only the most fundamental of tasks are performed such as being able to access some not necessarily all of the hardware, manage memory and coordinate message passing between the processes. In the case of QNX and Hurd user sessions can be entire snapshots of the system itself or views as it is referred to.
The very essence of the microkernel architecture illustrates some of its advantages: Maintenance is generally easier. Patches can be tested in a separate instance, and then swapped in to take over a production instance.
Rapid development time and new software can be tested without having to reboot the kernel.
More persistence in general, if one instance goes hay-wire, it is often possible to substitute it with an operational mirror. Most micro kernels use a message passing system of some sort to handle requests from one server to another. The message passing system generally operates on a port basis with the microkernel.
As an example, if a request for more memory is sent, a port is opened with the microkernel and the request sent through. Once within the microkernel, the steps are similar to system calls. The rationale was that it would bring modularity in the system architecture, which would entail a cleaner system, easier to debug or dynamically modify, customizable to users' needs, and more performing.
Although micro kernels are very small by themselves, in combination with all their required auxiliary code they are, in fact, often larger than monolithic kernels. Advocates of monolithic kernels also point out that the two-tiered structure of microkernel systems, in which most of the operating system does not interact directly with the hardware, creates a not-insignificant cost in terms of system efficiency.
These types of kernels normally provide only the minimal services such as defining memory address spaces, Inter-process communication IPC and the process management. The other functions such as running the hardware processes are not handled directly by micro kernels. Proponents of micro kernels point out those monolithic kernels have the disadvantage that an error in the kernel can cause the entire system to crash.
However, with a microkernel, if a kernel process crashes, it is still possible to prevent a crash of the system as a whole by merely restarting the service that caused the error.
Other services provided by the kernel such as networking are implemented in user-space programs referred to as servers. Servers allow the operating system to be modified by simply starting and stopping programs. For a machine without networking support, for instance, the networking server is not started.
The task of moving in and out of the kernel to move data between the various applications and servers creates overhead which is detrimental to the efficiency of micro kernels in comparison with monolithic kernels. Disadvantages in the microkernel exist however.
Some are: Larger running memory footprint More software for interfacing is required, there is a potential for performance loss. Messaging bugs can be harder to fix due to the longer trip they have to take versus the one off copy in a monolithic kernel. Process management in general can be very complicated. The disadvantages for micro kernels are extremely context based. As an example, they work well for small single purpose and critical systems because if not many processes need to run, then the complications of process management are effectively mitigated.
A microkernel allows the implementation of the remaining part of the operating system as a normal application program written in a high-level language , and the use of different operating systems on top of the same unchanged kernel.
This is mitigated to some degree by perfecting the virtual memory system, but not all computer architectures have virtual memory support.
By the early s, due to the various shortcomings of monolithic kernels versus microkernels, monolithic kernels were considered obsolete by virtually all operating system researchers. Performance[ edit ] Monolithic kernels are designed to have all of their code in the same address space kernel space , which some developers argue is necessary to increase the performance of the system.
Hybrid or modular kernels[ edit ] Main article: Hybrid kernel Hybrid kernels are used in most commercial operating systems such as Microsoft Windows NT 3. They are similar to micro kernels, except they include some additional code in kernel-space to increase performance.
These kernels represent a compromise that was implemented by some developers before it was demonstrated that pure micro kernels can provide high performance. These types of kernels are extensions of micro kernels with some properties of monolithic kernels. Unlike monolithic kernels, these types of kernels are unable to load modules at runtime on their own. Hybrid kernels are micro kernels that have some "non-essential" code in kernel-space in order for the code to run more quickly than it would were it to be in user-space.
Hybrid kernels are a compromise between the monolithic and microkernel designs. This implies running some services such as the network stack or the filesystem in kernel space to reduce the performance overhead of a traditional microkernel, but still running kernel code such as device drivers as servers in user space.
Many traditionally monolithic kernels are now at least adding if not actively exploiting the module capability. The most well known of these kernels is the Linux kernel. The modular kernel essentially can have parts of it that are built into the core kernel binary or binaries that load into memory on demand. It is important to note that a code tainted module has the potential to destabilize a running kernel.
Many people become confused on this point when discussing micro kernels. It is possible to write a driver for a microkernel in a completely separate memory space and test it before "going" live. When a kernel module is loaded, it accesses the monolithic portion's memory space by adding to it what it needs, therefore, opening the doorway to possible pollution. A few advantages to the modular or Hybrid kernel are: Faster development time for drivers that can operate from within modules.
No reboot required for testing provided the kernel is not destabilized. On demand capability versus spending time recompiling a whole kernel for things like new drivers or subsystems. Faster integration of third party technology related to development but pertinent unto itself nonetheless. Modules, generally, communicate with the kernel using a module interface of some sort. The interface is generalized although particular to a given operating system so it is not always possible to use modules.
Often the device drivers may need more flexibility than the module interface affords. Essentially, it is two system calls and often the safety checks that only have to be done once in the monolithic kernel now may be done twice. Some of the disadvantages of the modular approach are: With more interfaces to pass through, the possibility of increased bugs exists which implies more security holes.
Maintaining modules can be confusing for some administrators when dealing with problems like symbol differences. They differ from the other types of kernels in that their functionality is limited to the protection and multiplexing of the raw hardware, providing no hardware abstractions on top of which to develop applications. This separation of hardware protection from hardware management enables application developers to determine how to make the most efficient use of the available hardware for each specific program.
Exokernels in themselves are extremely small. However, they are accompanied by library operating systems see also unikernel , providing application developers with the functionalities of a conventional operating system. A major advantage of exokernel-based systems is that they can incorporate multiple library operating systems, each exporting a different API , for example one for high level UI development and one for real-time control.
History of kernel development[ edit ] Early operating system kernels[ edit ] Main article: History of operating systems Strictly speaking, an operating system and thus, a kernel is not required to run a computer. Programs can be directly loaded and executed on the "bare metal" machine, provided that the authors of those programs are willing to work without any hardware abstraction or operating system support.
Most early computers operated this way during the s and early s, which were reset and reloaded between the execution of different programs. Eventually, small ancillary programs such as program loaders and debuggers were left in memory between runs, or loaded from ROM. As these were developed, they formed the basis of what became early operating system kernels.
The "bare metal" approach is still used today on some video game consoles and embedded systems ,  but in general, newer computers use modern operating systems and kernels. In , the RC Multiprogramming System introduced the system design philosophy of a small nucleus "upon which operating systems for different purposes could be built in an orderly manner",  what would be called the microkernel approach.
One of the major developments during this era was time-sharing , whereby a number of users would get small slices of computer time, at a rate at which it appeared they were each connected to their own, slower, machine. One was that users, particularly at universities where the systems were being developed, seemed to want to hack the system to get more CPU time. For this reason, security and access control became a major focus of the Multics project in Finally, the systems typically offered a memory hierarchy several layers deep, and partitioning this expensive resource led to major developments in virtual memory systems.
The AmigaOS kernel's executive component, exec. There is no memory protection, and the kernel is almost always running in user mode. Only special actions are executed in kernel mode, and user-mode applications can ask the operating system to execute their code in kernel mode. Virtualizing the system at the file level allowed users to manipulate the entire system using their existing file management utilities and concepts, dramatically simplifying operation. As an extension of the same paradigm, Unix allows programmers to manipulate files using a series of small programs, using the concept of pipes , which allowed users to complete operations in stages, feeding a file through a chain of single-purpose tools.
Although the end result was the same, using smaller programs in this way dramatically increased flexibility as well as ease of development and use, allowing the user to modify their workflow by adding or removing a program from the chain.
Probing beyond superficial features, the authors offer valuable insights to people who want to know how things really work inside their machine. Important Intel-specific features are discussed. Relevant segments of code are dissected line by line. But the book covers more than just the functioning of the code; it explains the theoretical underpinnings of why Linux does things the way it does.
This edition of the book covers Version 2. The book focuses on the following topics:. Understanding the Linux Kernel will acquaint you with all the inner workings of Linux, but it's more than just an academic exercise. You'll learn what conditions bring out Linux's best performance, and you'll see how it meets the challenge of providing good system response during process scheduling, file access, and memory management in a wide variety of environments.
This book will help you make the most of your Linux system. Understanding the Linux Kernel 3rd ed pdf Referentie: O'Reilly Media Waarschuwing: