The allocation of system resources to various tasks, known as job scheduling, is a major assignment of the operating system. The system maintains prioritized queues of jobs waiting for CPU time and must decide which job to take from which queue and how much time to allocate to it, so that all jobs are completed in a fair and timely manner.
Graphical user interfaces
A highly visible aspect of the change in operating systems in recent years is the increasingly prevalent use of graphical user interfaces (GUIs). In the early days of computing, punch cards, written in the Job Control Language (JCL), were used to specify precisely which system resources a job would need and when the operating system should assign them to the job. Later, computer consoles allowed an operator directly to type commands—e.g., to open files, run programs, manipulate data, and print results—that could be executed immediately or at some future time. (Operating system commands stored for later execution are generally referred to as scripts; scripts are still widely used, especially for controlling servers.) With the advent of personal computers and the desire to make them more user-friendly, the operating system interface has become for most users a set of icons and menus so that the user only needs to “point and click” to send a command to the operating system.
Distributed operating systems
With the advent of computer networks, in which many computers are linked together and are able to communicate with one another, distributed computing became feasible. A distributed computation is one that is carried out on more than one machine in a cooperative manner. A group of linked computers working cooperatively on tasks, referred to as a distributed system, often requires a distributed operating system to manage the distributed resources. Distributed operating systems must handle all the usual problems of operating systems, such as deadlock. Distributed deadlock is very difficult to prevent; it is not feasible to number all the resources in a distributed system. Hence, deadlock must be detected by some scheme that incorporates substantial communication among network sites and careful synchronization, lest network delays cause deadlocks to be falsely detected and processes aborted unnecessarily. Interprocess communication must be extended to processes residing on different network hosts, since the loosely coupled architecture of computer networks requires that all communication be done by message passing. Important systems concerns unique to the distributed case are workload sharing, which attempts to take advantage of access to multiple computers to complete jobs faster; task migration, which supports workload sharing by efficiently moving jobs among machines; and automatic task replication at different sites for greater reliability. These concerns, in addition to the overall design of distributed operating systems and their interaction with the operating systems of the component computers, are subjects of current research.
