In modern computers many processes run at once. Active processes are placed in an array called a run queue, or runqueue. The run queue may contain priority values for each process, which will be used by the

scheduler A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible task (project management), tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order ...

to determine which process to run next. To ensure each program has a fair share of resources, each one is run for some time period (quantum) before it is paused and placed back into the run queue. When a program is stopped to let another run, the program with the highest priority in the run queue is then allowed to execute. Processes are also removed from the run queue when they ask to ''sleep'', are waiting on a resource to become available, or have been terminated. In the

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...

operating system (prior to kernel 2.6.23), each CPU in the system is given a run queue, which maintains both an active and expired array of processes. Each array contains 140 (one for each priority level) pointers to

doubly linked list In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked record (computer science), records called node (computer science), nodes. Each node contains three field (computer science), fields: ...

s, which in turn reference all processes with the given priority. The scheduler selects the next process from the active array with highest priority. When a process' quantum expires, it is placed into the expired array with some priority. When the active array contains no more processes, the scheduler swaps the active and expired arrays, hence the name O(1) scheduler. In

UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...

, the

sar SAR or Sar may refer to: Places * Sar (river), Galicia, Spain * Sar, Bahrain, a residential district * Sar, Iran (disambiguation), several places in Iran * Sar, Tibet, Tibet Autonomous Region of China * Šar Mountains, in southeastern Europe ...

command is used to check the run queue. The

vmstat vmstat (''virtual memory statistics'') is a computer system monitoring tool that collects and displays summary information about operating system memory, processes, interrupts, paging and block I/O. Users of vmstat can specify a sampling interv ...

command can also be used to determine the number of processes that are queued to run or waiting to run. These appear in the 'r' column. There are two models for Run queues: one that assigns a Run Queue to each physical processor, and the other has only one Run Queue in the system

References

* Tanenbaum AS (2008) ''Modern Operating Systems'', 3rd ed., p. 753-4. Pearson Education, Inc. * Silberschatz, Galvin, Gange (2012) ''Operating System Concepts'', 9th ed.. Wiley, {{DEFAULTSORT:Run Queue Scheduling (computing) Linux kernel

See also

References