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or cpu modes the microkernel may be the only software executing at the most privileged level which is generally referred to as supervisor or kernel mode traditional operating system functions such as device drivers protocol stacks and file systems are typically removed from the microkernel and are instead run in user space 1 microkernels often have less source code than monolithic kernels the minix 3 microkernel for example has only approximately 12 000 lines of code 2 history edit microkernels trace their roots back to danish computer pioneer per brinch hansen and his tenure in danish computer company regnecentralen where he led software development efforts for the rc 4000 computer 3 in 1967 regnecentralen was installing a rc 4000 prototype in the zakłady azotowe puławy fertilizer plant in poland the computer used a small real time operating system tailored for the needs of the plant brinch hansen and his team became concerned with the lack of generality and reusability of the rc 4000 system they feared that each installation would require a different operating system so they investigated novel and more general ways of creating software for the rc 4000 4 in 1969 their effort resulted in the completion of the rc 4000 multiprogramming system its nucleus provided inter process communication based on message passing for up to 23 unprivileged processes out of which 8 at a time were protected from one another it further implemented scheduling of time slices of programs executed in parallel initiation and control of program execution at the request of other running programs and initiation of data transfers to or from peripherals besides these elementary mechanisms it had no built in strategy for program execution and resource allocation this strategy was to be implemented by a hierarchy of running programs in which parent processes had complete control over child processes and acted as their operating systems 5 6 following brinch hansen s work microkernels have been developed since the 1970s 7 the term microkernel itself first appeared no later than 1981 8 microkernels were meant as a response to changes in the computer world and to several challenges adapting existing mono kernels to these new systems new device drivers protocol stacks file systems and other low level systems were being developed all the time this code was normally located in the monolithic kernel and thus required considerable work and careful code management to work on microkernels were developed with the idea that all of these services would be implemented as user space programs like any other allowing them to be worked on monolithically and started and stopped like any other program this would not only allow these services to be more easily worked on but also separated the kernel code to allow it to be finely tuned without worrying about unintended side effects moreover it would allow entirely new operating systems to be built up on a common core aiding os research microkernels were a hot topic in the 1980s when the first usable local area networks were being introduced 9 the amigaos exec kernel was an early example introduced in 1986 and used in a pc with relative commercial success the lack of memory protection considered in other respects a flaw allowed this kernel to have high message passing performance because it did not need to copy data while exchanging messages between user space programs 10 the same mechanisms that allowed the kernel to be distributed into user space also allowed the system to be distributed across network links the first microkernels notably mach created by richard rashid proved to have disappointing performance but the inherent advantages appeared so great that it was a major line of research into the late 1990s 11 however during this time the speed of computers grew greatly in relation to networking systems and the disadvantages in performance came to overwhelm the advantages in development terms citation needed many attempts were made to adapt the existing systems to have better performance but the overhead was always considerable and most of these efforts required the user space programs to be moved back into the kernel by 2000 most large scale mach kernel efforts had ended although apple s macos released in 2001 still uses a hybrid kernel called xnu which combines a heavily modified hybrid osf 1 s mach kernel osf mk 7 3 kernel with code from bsd unix 12 13 this kernel is also used in ios tvos and watchos windows nt starting with nt 3 1 and continuing with windows 11 uses a hybrid kernel design as of 2012 update the mach based gnu hurd is also functional and included in testing versions of arch linux and debian although major work on microkernels had largely ended experimenters continued development 14 15 using a more pragmatic approach to the problem including assembly code and relying on the processor to enforce concepts normally supported in software led to a new series of microkernels with dramatically improved performance microkernels are closely related to exokernels 16 they also have much in common with hypervisors 17 but the latter make no claim to minimality and are specialized to supporting virtual machines the l4 microkernel frequently finds use in a hypervisor capacity introduction edit early operating system kernels were rather small partly because computer memory was limited as the capability of computers grew the number of devices the kernel had to control also grew throughout the early history of unix kernels were generally small even though they contained various device drivers and file system implementations when address spaces increased from 16 to 32 bits kernel design was no longer constrained by the hardware architecture and kernels grew larger the berkeley software distribution bsd of unix began the era of larger kernels in addition to operating a basic system consisting of the cpu disks and printers bsd added a complete tcp ip networking system and a number of virtual devices that allowed the existing programs to work invisibly over the network this growth continued for many years resulting in kernels with millions of lines of source code as a result of this growth kernels were prone to bugs and became increasingly difficult to maintain the microkernel was intended to address this growth of kernels and the difficulties that resulted in theory the microkernel design allows for easier management of code due to its division into user space services this also allows for increased security and stability resulting from the reduced amount of code running in kernel mode for example if a networking service crashed due to buffer overflow only the networking service s memory would be corrupted leaving the rest of the system still functional inter process communication edit inter process communication ipc is any mechanism which allows separate processes to communicate with each other usually by sending messages shared memory is strictly defined also an inter process communication mechanism but the abbreviation ipc usually refers to message passing only and it is the latter that is particularly relevant to microkernels ipc allows the operating system to be built from a number of smaller programs called servers which are used by other programs on the system invoked via ipc most or all support for peripheral hardware is handled in this fashion with servers for device drivers network protocol stacks file systems graphics etc ipc can be synchronous or asynchronous asynchronous ipc is analogous to network communication the sender dispatches a message and continues executing the receiver checks polls for the availability of the message or is alerted to it via some notification mechanism asynchronous ipc requires that the kernel maintains buffers and queues for messages and deals with buffer overflows it also requires double copying of messages sender to kernel and kernel to receiver in synchronous ipc the first party sender or receiver blocks until the other party is ready to perform the ipc it does not require buffering or multiple copies but the implicit rendezvous can make programming tricky most programmers prefer asynchronous send and synchronous receive first generation microkernels typically supported synchronous as well as asynchronous ipc and suffered from poor ipc performance jochen liedtke assumed the design and implementation of the ipc mechanisms to be the underlying reason for this poor performance in his l4 microkernel he pioneered methods that lowered ipc costs by an order of magnitude 18 these include an ipc system call that supports a send as well as a receive operation making all ipc synchronous and passing as much data as possible in registers furthermore liedtke introduced the concept of the direct process switch where during an ipc execution an incomplete context switch is performed from the sender directly to the receiver if as in l4 part or all of the message is passed in registers this transfers the in register part of the message without any copying at all furthermore the overhead of invoking the scheduler is avoided this is especially beneficial in the common case where ipc is used in a remote procedure call rpc type fashion by a client invoking a server another optimization called lazy scheduling avoids traversing scheduling queues during ipc by leaving threads that block during ipc in the ready queue once the scheduler is invoked it moves such threads to the appropriate waiting queue as in many cases a thread gets unblocked before the next scheduler invocation this approach saves significant work similar approaches have since been adopted by qnx and minix 3 citation needed in a series of experiments chen and bershad compared memory cycles per instruction mcpi of monolithic ultrix with those of microkernel mach combined with a 4 3bsd unix server running in user space their results explained mach s poorer performance by higher mcpi and demonstrated that ipc alone is not responsible for much of the system overhead suggesting that optimizations focused exclusively on ipc will have a limited effect 19 liedtke later refined chen and bershad s results by making an observation that the bulk of the difference between ultrix and mach mcpi was caused by capacity cache misses and concluding that drastically reducing the cache working set of a microkernel will solve the problem 20 in a client server system most communication is essentially synchronous even if using asynchronous primitives as the typical operation is a client invoking a server and then waiting for a reply as it also lends itself to more efficient implementation most microkernels generally followed l4 s lead and only provided a synchronous ipc primitive asynchronous ipc could be implemented on top by using helper threads however experience has shown that the utility of synchronous ipc is dubious synchronous ipc forces a multi threaded design onto otherwise simple systems with the resulting synchronization complexities moreover an rpc like server invocation sequentializes client and server which should be avoided if they are running on separate cores versions of l4 deployed in commercial products have therefore found it necessary to add an asynchronous notification mechanism to better support asynchronous communication this signal like mechanism does not carry data and therefore does not require buffering by the kernel by having two forms of ipc they have nonetheless violated the principle of minimality other versions of l4 have switched to asynchronous ipc completely 21 as synchronous ipc blocks the first party until the other is ready unrestricted use could easily lead to deadlocks furthermore a client could easily mount a denial of service attack on a server by sending a request and never attempting to receive the reply therefore synchronous ipc must provide a means to prevent indefinite blocking many microkernels provide timeouts on ipc calls which limit the blocking time in practice choosing sensible timeout values is difficult and systems almost inevitably use infinite timeouts for clients and zero timeouts for servers as a consequence the trend is towards not providing arbitrary timeouts but only a flag which indicates that the ipc should fail immediately if the partner is not ready this approach effectively provides a choice of the two timeout values of zero and infinity recent versions of l4 and minix have gone down this path older versions of l4 used timeouts qnx avoids the problem by requiring the client to specify the reply buffer as part of the message send call when the server replies the kernel copies the data to the client s buffer without having to wait for the client to receive the response explicitly 22 servers edit microkernel servers are essentially daemon programs like any others except that the kernel grants some of them privileges to interact with parts of physical memory that are otherwise off limits to most programs this allows some servers particularly device drivers to interact directly with hardware a basic set of servers for a general purpose microkernel includes file system servers device driver servers networking servers display servers and user interface device servers this set of servers drawn from qnx provides roughly the set of services offered by a unix monolithic kernel the necessary servers are started at system startup and provide services such as file network and device access to ordinary application programs with such servers running in the environment of a user application server development is similar to ordinary application development rather than the build and boot process needed for kernel development additionally many crashes can be corrected by simply stopping and restarting the server which would not be feasible if the entire kernel had to reboot however part of the system state is lost with the failing server hence this approach requires applications to cope with failure a good example is a server responsible for tcp ip connections if this server is restarted applications will experience a lost connection a normal occurrence in a networked system for other services failure is less expected and may require changes to application code for qnx restart capability is offered as the qnx high availability toolkit 23 device drivers edit device drivers frequently perform direct memory access dma and therefore can write to arbitrary locations of physical memory including various kernel data structures such drivers must therefore be trusted it is a common misconception that this means that they must be part of the kernel in fact a driver is not inherently more or less trustworthy by being part of the kernel while running a device driver in user space does not necessarily reduce the damage a misbehaving driver can cause in practice it is beneficial for system stability in the presence of buggy rather than malicious drivers memory access violations by the driver code as opposed to the device m...
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