Exokernel - AbsoluteAstronomy.com

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems.

The idea behind exokernels is to force as few abstractions

Abstraction (computer science)

In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...

as possible on developers, enabling them to make as many decisions as possible about hardware abstractions. Exokernels are tiny, since functionality is limited to ensuring protection and multiplexing

Multiplexing

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

of resources, which are vastly simpler than conventional microkernel

Microkernel

In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...

s' implementation of message passing and monolithic kernel

Monolithic kernel

A monolithic kernel is an operating system architecture where the entire operating system is working in the kernel space and alone as supervisor mode...

s' implementation of abstractions.

Implemented applications are called library operating systems; they may request specific memory addresses, disk blocks, etc. The kernel only ensures that the requested resource is free, and the application is allowed to access it. This low-level hardware access allows the programmer to implement custom abstractions, and omit unnecessary ones, most commonly to improve a program's performance. It also allows programmers to choose what level of abstraction they want, high, or low.

Exokernels can be seen as an application of the end-to-end principle

End-to-end principle

The end-to-end principle is a classic design principle of computer networking which states that application specific functions ought to reside in the end hosts of a network rather than in intermediary nodes, provided they can be implemented "completely and correctly" in the end hosts...

to operating systems, in that they do not force an application program to layer its abstractions on top of other abstractions that were designed with different requirements in mind. For example, in the MIT Exokernel project, the Cheetah web server

Web server

Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....

stores preformatted Internet Protocol

Internet Protocol

The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

packets on the disk, the kernel provides safe access to the disk by preventing unauthorized reading and writing, but how the disk is abstracted is up to the application or the libraries the application uses.

Motivation

Traditionally kernel designers have sought to make individual hardware resources invisible to application programs by requiring the programs to interact with the hardware via some abstraction model. These models include file system

File system

A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

s for disk storage, virtual address spaces for memory, schedulers

Scheduling (computing)

In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...

for task management, and sockets

Berkeley sockets

The Berkeley sockets application programming interface comprises a library for developing applications in the C programming language that perform inter-process communication, most commonly for communications across a computer network....

for network communication. These abstractions of the hardware make it easier to write programs in general, but limit performance and stifle experimentation in new abstractions. A security-oriented application might need a file system that does not leave old data on the disk, while a reliability-oriented application might need a file system that keeps such data for failure recovery.

One option is to remove the kernel completely and program directly to the hardware, but then the entire machine would be dedicated to the application being written (and, conversely, the entire application codebase would be dedicated to that machine). The exokernel concept is a compromise: let the kernel allocate the basic physical resources of the machine (e.g. disk blocks, memory pages, and processor time) to multiple application programs, and let each program decide what to do with these resources. The program can then link to a support library that implements the abstractions it needs (or it can implement its own).

MIT exokernel

MIT developed two exokernel-based operating systems, using two kernels: Aegis, a proof of concept with limited support for storage, and XOK, which applied the exokernel concept more thoroughly.

An essential idea of the MIT exokernel system is that the operating system should act as an executive for small programs provided by the application software, which are constrained only by the requirement that the exokernel must be able to guarantee that they use the hardware safely.

Processor

The kernel represents the processor resources as a timeline from which programs can allocate intervals of time. A program can yield the rest of its time slice to another designated program. The kernel notifies programs of processor events, such as interrupt

Interrupt

In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

s, hardware exceptions

Exception handling

Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....

, and the beginning or end of a time slice. If a program takes a long time to handle an event, the kernel will penalize it on subsequent time slice allocations; in extreme cases the kernel can abort the program.

Memory

The kernel allocates physical memory pages to programs and controls the translation lookaside buffer

Translation Lookaside Buffer

A translation lookaside buffer is a CPU cache that memory management hardware uses to improve virtual address translation speed. All current desktop and server processors use a TLB to map virtual and physical address spaces, and it is ubiquitous in any hardware which utilizes virtual memory.The...

. A program can share a page with another program by sending it a capability
Capability-based security
Capability-based security is a concept in the design of secure computing systems, one of the existing security models. A capability is a communicable, unforgeable token of authority. It refers to a value that references an object along with an associated set of access rights...

to access that page. The kernel ensures that programs access only pages for which they have a capability.

Disk storage

The kernel identifies disk blocks to the application program by their physical block address, allowing the application to optimize data placement. When the program initializes its use of the disk, it provides the kernel with a function that the kernel can use to determine which blocks the program controls. The kernel uses this callback to verify that when it allocates a new block, the program claims only the block that was allocated in addition to those it already controlled.

Networking

The kernel implements a programmable packet filter, which executes programs in a byte code language designed for easy security-checking by the kernel.

Applications

The available library operating systems for Exokernel include the custom ExOS system and an emulator for BSD

Berkeley Software Distribution

Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...

. In addition to these, the exokernel team created the Cheetah web server

Web server

Web server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....

, which uses the kernel directly.

History

The exokernel concept has been around since at least 1994, but exokernels are still a research effort and have not been used in any major commercial operating systems. A concept operating exokernel system is Nemesis

Nemesis (computing)

Nemesis is an operating system designed by the University of Cambridge, the University of Glasgow, the Swedish Institute of Computer Science and Citrix Systems....

, written by University of Cambridge

University of Cambridge

The University of Cambridge is a public research university located in Cambridge, United Kingdom. It is the second-oldest university in both the United Kingdom and the English-speaking world , and the seventh-oldest globally...

, University of Glasgow

University of Glasgow

The University of Glasgow is the fourth-oldest university in the English-speaking world and one of Scotland's four ancient universities. Located in Glasgow, the university was founded in 1451 and is presently one of seventeen British higher education institutions ranked amongst the top 100 of the...

, Citrix Systems

Citrix Systems

Citrix Systems, Inc. is a multinational corporation founded in 1989, that provides server and desktop virtualization, networking, software-as-a-service , and cloud computing technologies, including Xen open source products....

, and the Swedish Institute of Computer Science

Swedish Institute of Computer Science

The Swedish Institute of Computer Science, SICS, is an independent non-profit research organization with a research focus on applied computer science. The institute carries out research in a number of areas, including networked embedded systems, future Internet technologies, large scale...

. MIT

Massachusetts Institute of Technology

The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

has also built several exokernel based systems, including ExOS.

External links

Microkernels. The extent to which simple, efficient operations are a good choice in a kernel interface design. by Úlfar Erlingsson and Athanasios Kyparlis
MIT Exokernel Operating System Putting the Application in Control
Nemesis at Cambridge An operating system with principles
IntraServices
XOmB

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