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In computing, the kernel is the central component of most computer operating systems (OSs). Its responsibilities include managing the system's resources and the communication between hardware and software components. As a basic component of an operating system, a kernel provides the lowest-level abstraction layer for the resources (especially memory, processors and I/O devices) that applications must control to perform their function. It typically makes these facilities available to application processes through inter-process communication mechanisms and system calls.
These tasks are done differently by different kernels, depending on their design and implementation. While monolithic kernels will try to achieve these goals by executing all the code in the same address space to increase the performance of the system, microkernels run most of their services in user space, aiming to improve maintainability and modularity of the codebase.[Roch 2004) A range of possibilities exists between these two extremes.
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Overview
Most operating systems rely on the kernel concept. The existence of a kernel is a natural consequence of designing a computer system as a series of abstraction layers,[Tanenbaum 79, chapter 1) each relying on the functions of layers beneath itself. The kernel, from this viewpoint, is simply the name given to the lowest level of abstraction that is implemented in software. In order to avoid having a kernel, one would have to design all the software on the system to not use abstraction layers; this would increase the complexity of the design to such a point that only the simplest systems could feasibly be implemented.
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While it is today mostly called the kernel, the same part of the operating system has also in the past been known as the nucleus or core. (Note, however, the term core has also been used to refer to the primary memory of a computer system, typically because the original memory of computers were a type of magnetic "donut" (a "core") connected at the intersection of two wires.)
In most cases, the boot loader starts executing the kernel in supervisor mode,[The highest privilege level has various names throughout different architectures, such as supervisor mode, kernel mode, CPL0, DPL0, Ring 0, etc. See Ring (computer security) for more information.) The kernel then initializes itself and starts the first process. After this, the kernel does not typically execute directly, only in response to external events (e.g. via system calls used by applications to request services from the kernel, or via interrupts used by the hardware to notify the kernel of events). Additionally, the kernel typically provides a loop that is executed whenever no processes are available to run; this is often called the idle process.
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Kernel development is considered one of the most complex and difficult tasks in programming.[[http://osdever.net/bkerndev/index.php?the_id=90 Bona Fide OS Development - Bran's Kernel Development Tutorial], by Brandon Friesen) Its central position in an operating system implies the necessity for good performance, which defines the kernel as a critical piece of software and makes its correct design and implementation difficult. For various reasons, a kernel might not even be able to use the abstraction mechanisms it provides to other software. Such reasons include memory management concerns (for example, a user-mode function might rely on memory being subject to demand paging, but as the kernel itself provides that facility it cannot use it, because then it might not remain in memory to provide that facility) and lack of reentrancy, thus making its development even more difficult for software engineers.
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A kernel will usually provide features for low-level scheduling[for low level scheduling see Deitel 82, chap.10 pp.249-268) of processes (dispatching), Inter-process communication, process synchronization, context switch, manipulation of process control blocks, interrupt handling, process creation and destruction, process suspension and resumption (see process states).]
Kernel basic responsibilities
The kernel's primary purpose is to manage the computer's resources and allow other programs to run and use these resources. Typically, the resources consist of:
* The CPU (frequently called the processor). This is the most central part of a computer system, responsible for running or executing programs on it. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors (each of which can usually run only one program at once)
* The computer's memory. Memory is used to store both program instructions and data. Typically, both need to be present in memory in order for a program to execute. Often multiple programs will want access to memory, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when not enough is available.
* Any Input/Output (I/O) devices present in the computer, such as disk drives, printers, displays, etc. The kernel allocates requests from applications to perform I/O to an appropriate device (or subsection of a device, in the case of files on a disk or windows on a display) and provides convenient methods for using the device (typically abstracted to the point where the application does not need to know implementation details of the device)
Kernels also usually provide methods for synchronization and communication between processes (called inter-process communication or IPC).
A kernel may implement these features itself, or rely on some of the processes it runs to provide the facilities to other processes, although in this case it must provide some means of IPC to allow processes to access the facilities provided by each other.
Finally, a kernel must provide running programs with a method to make requests to access these facilities.
Process management
The main task of a kernel is to allow the execution of applications and support them with features such as hardware abstractions. To run an application, a kernel typically sets up an address space for the application, loads the file containing the application's code into memory (perhaps via demand paging), sets up a stack for the program and branches to a given location inside the program, thus starting its execution.[Silberschatz 1990)
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Multi-tasking kernels are able to give the user the illusion that the number of processes being run simultaneously on the computer is higher than the maximum number of processes the computer is physically able to run simultaneously. Typically, the number of processes a system may run simultaneously is equal to the number of CPUs installed (however this may not be the case if the processors support simultaneous multithreading).
In a pre-emptive multitasking system, the kernel will give every program a slice of time and switch from process to process so quickly that it will appear to the user as if these processes were being executed simultaneously. The kernel uses scheduling algorithms to determine which process is running next and how much time it will be given. The algorithm chosen may allow for some processes to have higher priority than others. The kernel generally also provides these processes a way to communicate; this is known as inter-process communication (IPC) and the main approaches are shared memory, message passing and remote procedure calls (see concurrent computing).
Other systems (particularly on smaller, less powerful computers) may provide co-operative multitasking, where each process is allowed to run uninterrupted until it makes a special request that tells the kernel it may switch to another process. Such requests are known as "yielding", and typically occur in response to requests for interprocess communication, or for waiting for an event to occur. Older versions of Windows and Mac OS both used co-operative multitasking but switched to pre-emptive schemes as the power of the computers to which they were targeted grew.
The operating system might also support multiprocessing (SMP or Non-Uniform Memory Access); in that case, different programs and threads may run on different processors. A kernel for such a system must be designed to be re-entrant, meaning that it may safely run two different parts of its code simultaneously. This typically means providing synchronization mechanisms (such as spinlocks) to ensure that no two processors attempt to modify the same data at the same time.
Memory management
The kernel has full access to the system's memory and must allow processes to access this memory safely as they require it. Often the first step in doing this is virtual addressing, usually achieved by paging and/or segmentation. Virtual addressing allows the kernel to make a given physical address appear to be another address, the virtual address. Virtual address spaces may be different for different processes; the memory that one process accesses at a particular (virtual) address may be different memory from what another process accesses at the same address. This allows every program to behave as if it is the only one (apart from the kernel) running and thus prevents applications from crashing each other.
On many systems, a program's virtual address may refer to data which is not currently in memory. The layer of indirection provided by virtual addressing allows the operating system to use other data stores, like a hard drive, to store what would otherwise have to remain in main memory (RAM). As a result, operating systems can allow programs to use more memory than the system has physically available. When a program needs data which is not currently in RAM, the CPU signals to the kernel that this has happened, and the kernel responds by writing the contents of an inactive memory block to disk (if necessary) and replacing it with the data requested by the program. The program can then be resumed from the point where it was stopped. This scheme is generally known as demand paging.
Virtual addressing also allows creation of virtual partitions of memory in two disjointed areas, one being reserved for the kernel (kernel space) and the other for the applications (user space). The applications are not permitted by the processor to address kernel memory, thus preventing an application from damaging the running kernel. This fundamental partition of memory space has contributed much to current designs of actual general-purpose kernels and is almost universal in such systems, although some research kernels (e.g. Singularity) take other approaches.
Device management
To perform useful functions, processes need access to the peripherals connected to the computer, which are controlled by the kernel through device drivers. For example, to show the user something on the screen, an application would make a request to the kernel, which would forward the request to its display driver, which is then responsible for actually plotting the character/pixel.
A kernel must maintain a list of available devices. This list may be known in advance (e.g. on an embedded system where the kernel will be rewritten if the available hardware changes), configured by the user (typical on older PCs and on systems that are not designed for personal use) or detected by the operating system at run time (normally called Plug and Play).
In a plug and play system, a device manager first performs a scan on different hardware buses, such as Peripheral Component Interconnect (PCI) or Universal Serial Bus (USB), to detect installed devices, then searches for the appropriate drivers.
As device management is a very OS-specific topic, these drivers are handled differently by each kind of kernel design, but in every case, the kernel has to provide the I/O to allow drivers to physically access their devices through some port or memory location. Very important decisions have to be made when designing the device management system, as in some designs accesses may involve context switches, making the operation very CPU-intensive and easily causing a significant performance overhead.
System calls
To actually perform useful work, a process must be able to access the services provided by the kernel. This is implemented differently by each kernel, but most provide a C library or an API, which in turn invoke the related kernel functions.
The method of invoking the kernel function varies from kernel to kernel. If memory isolation is in use, it is impossible for a user process to call the kernel directly, because that would be a violation of the processor's access control rules. A few possibilities are:
* Using a software-simulated interrupt. This method is available on most hardware, and is therefore very common.
* Using a call gate. A call gate is a special address which the kernel has added to a list stored in kernel memory and which the processor knows the location of. When the processor detects a call to that location, it instead redirects to the target location without causing an access violation. Requires hardware support, but the hardware for it is quite common.
* Using a special system call instruction. This technique requires special hardware support, which common architectures (notably, x86) may lack. System call instructions have been added to recent models of x86 processors, however, and some (but not all) operating systems for PCs make use of them when available.
* Using a memory-based queue. An application that makes large numbers of requests but does not need to wait for the result of each may add details of requests to an area of memory that the kernel periodically scans to find requests.
Kernel design decisions
Fault tolerance
An important consideration in the design of a kernel is fault tolerance; specifically, in cases where multiple programs are running on a single computer, it is usually important to prevent a fault in one of the programs from negatively affecting the other. Extended to malicious design rather than a fault, this also applies to security, and is necessary to prevent processes from accessing information without being granted permission.
Two main approaches to the protection of sensitive information are assigning privileges to hierarchical protection domains, for example by using a processor's supervisor mode, or distributing privileges differently for each process and resource, for example by using capabilities or access control lists.
Hierarchical protection domains are much less flexible, as it is not possible to assign different privileges to processes that are at the same privileged level, and can't therefore satisfy Denning's four principles for fault tolerance (particularly the Principle of least privilege). Hierarchical protection domains also have a major performance drawback, since interaction between different levels of protection, when a process has to manipulate a data structure both in 'user mode' and 'supervisor mode', always requires message copying (transmission by value).[Hansen 73, section 7.3 p.233 "interactions between different levels of protection require transmission of messages by value") A kernel based on capabilities, however, is more flexible in assigning privileges, can satisfy Denning's fault tolerance principles,][Linden 76) and typically doesn't suffer from the performance issues of copy by value.
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Both approaches typically require some hardware or firmware support to be operable and efficient. The hardware support for hierarchical protection domains[Schroeder 72) is typically that of "CPU modes". An efficient and simple way to provide hardware support of capabilities is to delegate the MMU the responsibility of checking access-rights for every memory access, a mechanism called capability-based addressing.]
An alternative approach is to simulate capabilities using commonly-support hierarchical domains; in this approach, each protected object must reside in an address space that the application does not have access to; the kernel also maintains a list of capabilities in such memory. When an application needs to access an object protected by a capability, it performs a system call and the kernel performs the access for it. The performance cost of address space switching limits the practicality of this approach in systems with complex interactions between objects, but it is used in current operating systems for objects that are not accessed frequently or which are not expected to perform quickly.[Stephane Eranian & David Mosberger, [http://www.phptr.com/articles/article.asp?p=29961&seqNum=1&rl=1 Virtual Memory in the IA-64 Linux Kernel], Prentice Hall PTR, 2002)(Silberschatz & Galvin, Operating System Concepts, 4th ed, pp445 & 446)
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Approaches where protection mechanism are not firmware supported but are instead simulated at higher levels (e.g. simulating capabilities by manipulating page tables on hardware that does not have direct support), are possible, but there are performance implications.[ Lack of hardware support may not be an issue, however, for systems that choose to use language-based protection.][[http://www.cs.cmu.edu/~rwh/papers/langsec/dagstuhl.pdf A Language-Based Approach to Security], Schneider F., Morrissett G. (Cornell University) and Harper R. (Carnegie Mellon University))
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Security
An important kernel design decision is the choice of the abstraction levels where the security mechanisms and policies should be implemented. One approach is to use firmware and kernel support for fault tolerance (see above), and build the security policy for malicious behavior on top of that (adding features such as cryptography mechanisms where necessary), delegating some responsibility to the compiler. Approaches that delegate enforcement of security policy to the compiler and/or the application level are often called language-based security.
Hardware-based protection or language-based protection
Typical computer systems today use hardware-enforced rules about what programs are allowed to access what data. The processor monitors the execution and stops a program that violates a rule (e.g., a user process that is about to read or write to kernel memory, and so on). In systems that lack support for capabilities, processes are isolated from each other by using separate address spaces.
An alternative approach is to use language-based protection. In a language-based protection system, the kernel will only allow code to execute that has been produced by a trusted language compiler. The language may then be designed such that it is impossible for the programmer to instruct it to do something that will violate a security requirement.
Advantages of this approach include:
* Lack of need for separate address spaces. Switching between address spaces is a slow operation that causes a great deal of overhead, and a lot of optimisation work is currently performed in order to prevent unnecessary switches in current operating systems. Switching is completely unnecessary in a language-based protection system, as all code can safely operate in the same address space.
* Flexibility. Any protection scheme that can be designed to be expressed via a programming language can be implemented using this method. Changes to the protection scheme (e.g. from a hierarchical system to a capability-based one) do not require new hardware.
Disadvantages include:
* Longer application start up time. Applications must be verified when they are started to ensure they have been compiled by the correct compiler, or may need recompiling either from source code or from bytecode.
* Inflexible type systems. On traditional systems, applications frequently perform operations that are not type safe. Such operations cannot be permitted in a language-based protection system, which means that applications may need to be rewritten and may, in some cases, lose performance.
Examples of systems with language-based protection include Microsoft's Singularity.
Process cooperation
Edsger Dijkstra proved that from a logical point of view, atomic lock and unlock operations operating on binary semaphores are sufficient primitives to express any functionality of process cooperation.[Dijkstra, E. W. Cooperating Sequential Processes. Math. Dep., Technological U., Eindhoven, Sept. 1965.) However this approach is generally held to be lacking in terms of safety and efficiency, whereas a message passing approach is more flexible.]
I/O devices management
The idea of a kernel where I/O devices are handled uniformly with other processes, as parallel co-operating processes, was first proposed and implemented by Brinch Hansen (although similar ideas were suggested in 1967). In Hansen's description of this, the "common" processes are called internal processes, while the I/O devices are called external processes.
Kernel-wide design approaches
Naturally, the above listed tasks and features can be provided in many ways that differ from each other in design and implementation. While monolithic kernels execute all of their code in the same address space (kernel space) to increase the performance of the system, microkernels try to run most of their services in user space, aiming to improve maintainability and modularity of the codebase.
The principle of separation of mechanism and policy[Hansen 2001 (os), p.18)][Levin 75) is the substantial difference between the philosophy of micro and monolithic kernels. Here a mechanism is the support that allows to implement many different policies, while a policy is a particular "mode of operation". In minimal microkernel just some very basic policies are included,]
Monolithic kernels
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 maintain that monolithic systems are easier to design and implement than other solutions,(One notable example is UNIX developer Ken Thompson; see links to Torvalds v Tanenbaum debate below) and are extremely efficient if well-written. The main disadvantages of monolithic kernels are the dependencies between system components - a bug in a device driver might crash the entire system - and the fact that large kernels can become very difficult to maintain.
Microkernels
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.
Microkernels generally underperform traditional designs, sometimes dramatically. This is due in large part to the overhead of moving in and out of the kernel, a context switch, to move data between the various applications and servers. By the mid-1990s, most researchers had abandoned the belief that careful tuning could reduce this overhead dramatically, but recently, newer microkernels, optimized for performance, have addressed these problems.[[http://os.inf.tu-dresden.de/L4/overview.html The L4 microkernel family - Overview -->
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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.
Monolithic kernels vs microkernels
As the computer kernel grows, a number of problems become evident. One of the most obvious is that the memory footprint increases. This is mitigated to some degree by perfecting the virtual memory system, but not all computer architectures have virtual memory support.(Virtual addressing is most commonly achieved through a built-in memory management unit.) To reduce the kernel's footprint, extensive editing has to be performed to carefully remove unneeded code, which can be very difficult with non-obvious interdependencies between parts of a kernel with millions of lines of code.
Due to the problems that monolithic kernels pose, they were considered obsolete by the early 1990s. As a result, the design of Linux using a monolithic kernel rather than a microkernel was the topic of a famous flame war between Linus Torvalds and Andrew Tanenbaum.[Recordings of the debate between Torvalds and Tanenbaum can be found at [http://www.dina.dk/~abraham/Linus_vs_Tanenbaum.html dina.dk], [http://groups.google.com/group/comp.os.minix/browse_thread/thread/c25870d7a41696d2/f447530d082cd95d?tvc=2#f447530d082cd95d groups.google.com], [http://www.oreilly.com/catalog/opensources/book/appa.html oreilly.com] and [http://www.cs.vu.nl/~ast/reliable-os/ Andrew Tanenbaum's website --> There is merit on both sides of the argument presented in the Tanenbaum/Torvalds debate.
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Some, including early UNIX developer Ken Thompson, argued that while microkernel designs were more aesthetically appealing, monolithic kernels were easier to implement. However, a bug in a monolithic system usually crashes the entire system, while this doesn't happen in a microkernel with servers running apart from the main thread. Monolithic kernel proponents reason that incorrect code doesn't belong in a kernel, and that microkernels offer little advantage over correct code. Microkernels are often used in embedded robotic or medical computers where crash tolerance is important and most of the OS components reside in their own private, protected memory space. This is impossible with monolithic kernels, even with modern module-loading ones. However, the monolithic model tends to be more efficient through the use of shared kernel memory, rather than the slower IPC system of microkernel designs, which is typically based on message passing.
Hybrid kernels
Hybrid kernels are essentially a compromise between the monolithic kernel approach and the microkernel system. 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.
Nanokernels
A nanokernel delegates virtually all services - including even the most basic ones like interrupt controllers or the timer - to device drivers to make the kernel memory requirement even smaller than a traditional microkernel.
Exokernels
An exokernel is a type of kernel that does not abstract hardware into theoretical models. Instead it allocates physical hardware resources, such as processor time, memory pages, and disk blocks, to different programs. A program running on an exokernel can link to a library operating system that uses the exokernel to simulate the abstractions of a well-known OS, or it can develop application-specific abstractions for better performance.
History of kernel development
Early operating system kernels
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 1950s and early 1960s, 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.
Time-sharing operating systems
In the decade preceding Unix, computers had grown enormously in power - to the point where computer operators were looking for new ways to get people to use the spare time on their machines. 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.
The development of time-sharing systems led to a number of problems. 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 1965. For instance, printers were represented as a "file" at a known location - when data was copied to the file, it printed out. Other systems, to provide a similar functionality, tended to virtualize devices at a lower level - that is, both devices and files would be instances of some lower level concept. 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.
In the Unix model, the Operating System consists of two parts; one the huge collection of utility programs that drive most operations, the other the kernel that runs the programs.
Over the years the computing model changed, and Unix's treatment of everything as a file no longer seemed to be as universally applicable as it was before. Although a terminal could be treated as a file or a stream, which is printed to or read from, the same did not seem to be true for a graphical user interface. Networking posed another problem. Even if network communication can be compared to file access, the low-level packet-oriented architecture dealt with discrete chunks of data and not with whole files. As the capability of computers grew, Unix became increasingly cluttered with code. While kernels might have had 100,000 lines of code in the seventies and eighties, kernels of modern Unix successors like Linux have more than 4.5 million lines.
Windows
Microsoft Windows was first released in 1985 as an add-on to DOS. Similarly to Mac OS, it also lacked important features at first but eventually acquired them in later releases. This product line would continue until the release of the Windows 9x series and end with Windows Me. At the same time, Microsoft has been developing Windows NT since 1993, an operating system intended for the high-end and business user. This line started with the release of Windows NT 3.1 and replaced the main product line with the release of the NT-based Windows 2000.
The highly successful Windows XP brought these two product lines together, combining the stability of the NT line and the visual appeal of the 9x series. It uses the NT kernel, which is generally considered a hybrid kernel because the kernel itself contains tasks such as the Window Manager and the IPC Manager, but several subsystems run in user mode.
Development of microkernels
Although Mach, developed at Carnegie Mellon University from 1985 to 1994 is the best-known general-purpose microkernel, other microkernels have been developed with more specific aims. The L4 microkernel family (mainly the L3 and the L4 kernel) was created to demonstrate that microkernels are not necessarily slow.
QNX is a real-time operating system with a minimalistic microkernel design that has been developed since 1982, having been far more successful than Mach in achieving the goals of the microkernel paradigm. It is principally used in embedded systems and in situations where software is not allowed to fail, such as the robotic arms on the space shuttle and machines that control grinding of glass to extremely fine tolerances, where a tiny mistake may cost hundreds of thousands of dollars, as in the case of the mirror of the Hubble Space Telescope.