Scheduling (computing)

Scheduling (computing)

Scheduling is a key concept in computer multitasking, multiprocessing operating system and real-time operating system designs. Scheduling refers to the way processes are assigned to run on the available CPUs, since there are typically many more processes running than there are available CPUs. This assignment is carried out by a software known as a scheduler and dispatcher.

The scheduler is concerned mainly with:

  • Throughput - number of processes that complete their execution per time unit.
  • Latency, specifically:
    • Turnaround - total time between submission of a process and its completion.
    • Response time - amount of time it takes from when a request was submitted until the first response is produced.
  • Fairness / Waiting Time - Equal CPU time to each process (or more generally appropriate times according to each process' priority).

In practice, these goals often conflict (e.g. throughput versus latency), thus a scheduler will implement a suitable compromise. Preference is given to any one of the above mentioned concerns depending upon the user's needs and objectives.

In real-time environments, such as mobile devices for automatic control in industry (for example robotics), the scheduler also must ensure that processes can meet deadlines; this is crucial for keeping the system stable. Scheduled tasks are sent to mobile devices and managed through an administrative back end.


Types of operating system schedulers

Operating systems may feature up to 3 distinct types of scheduler, a long-term scheduler (also known as an admission scheduler or high-level scheduler), a mid-term or medium-term scheduler and a short-term scheduler . The names suggest the relative frequency with which these functions are performed. The Scheduler is an operating system module that selects the next jobs to be admitted into the system and the next process to run.

Long-term scheduling

The long-term, or admission scheduler, decides which jobs or processes are to be admitted to the ready queue (in the Main Memory); that is, when an attempt is made to execute a program, its admission to the set of currently executing processes is either authorized or delayed by the long-term scheduler. Thus, this scheduler dictates what processes are to run on a system, and the degree of concurrency to be supported at any one time - i.e.: whether a high or low amount of processes are to be executed concurrently, and how the split between IO intensive and CPU intensive processes is to be handled. In modern OS's, this is used to make sure that real time processes get enough CPU time to finish their tasks. Without proper real time scheduling, modern GUI interfaces would seem sluggish. The long term queue exists in the Hard Disk or the "Virtual Memory". [Stallings, 399].

Long-term scheduling is also important in large-scale systems such as batch processing systems, computer clusters, supercomputers and render farms. In these cases, special purpose job scheduler software is typically used to assist these functions, in addition to any underlying admission scheduling support in the operating system.

Medium-term scheduling

The medium-term scheduler temporarily removes processes from main memory and places them on secondary memory (such as a disk drive) or vice versa. This is commonly referred to as "swapping out" or "swapping in" (also incorrectly as "paging out" or "paging in"). The medium-term scheduler may decide to swap out a process which has not been active for some time, or a process which has a low priority, or a process which is page faulting frequently, or a process which is taking up a large amount of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available, or when the process has been unblocked and is no longer waiting for a resource. [Stallings, 396] [Stallings, 370]

In many systems today (those that support mapping virtual address space to secondary storage other than the swap file), the medium-term scheduler may actually perform the role of the long-term scheduler, by treating binaries as "swapped out processes" upon their execution. In this way, when a segment of the binary is required it can be swapped in on demand, or "lazy loaded". [stallings, 394]

Short-term scheduling

The short-term scheduler (also known as the CPU scheduler) decides which of the ready, in-memory processes are to be executed (allocated a CPU) next following a clock interrupt, an IO interrupt, an operating system call or another form of signal. Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers - a scheduling decision will at a minimum have to be made after every time slice, and these are very short. This scheduler can be preemptive, implying that it is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process, or non-preemptive (also known as "voluntary" or "co-operative"), in which case the scheduler is unable to "force" processes off the CPU. [Stallings, 396]. In most cases short-term scheduler is written in assembler because it is a critical part of the operating system.


Another component involved in the CPU-scheduling function is the dispatcher. The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. This function involves the following:

  • Switching context
  • Switching to user mode
  • Jumping to the proper location in the user program to restart that program

The dispatcher should be as fast as possible, since it is invoked during every process switch. The time it takes for the dispatcher to stop one process and start another running is known as the dispatch latency. [Gravin, 155].

Scheduling disciplines

Scheduling disciplines are algorithms used for distributing resources among parties which simultaneously and asynchronously request them. Scheduling disciplines are used in routers (to handle packet traffic) as well as in operating systems (to share CPU time among both threads and processes), disk drives (I/O scheduling), printers (print spooler), most embedded systems, etc.

The main purposes of scheduling algorithms are to minimize resource starvation and to ensure fairness amongst the parties utilizing the resources. Scheduling deals with the problem of deciding which of the outstanding requests is to be allocated resources. There are many different scheduling algorithms. In this section, we introduce several of them.

First in first out

Also known as First­ Come, First ­Served (FCFS), is the simplest scheduling algorithm, FIFO simply queues processes in the order that they arrive in the ready queue.

  • Since context switches only occur upon process termination, and no reorganization of the process queue is required, scheduling overhead is minimal.
  • Throughput can be low, since long processes can hog the CPU
  • Turnaround time, waiting time and response time can be high for the same reasons above
  • No prioritization occurs, thus this system has trouble meeting process deadlines.
  • The lack of prioritization means that as long as every process eventually completes, there is no starvation. In an environment where some processes might not complete, there can be starvation.
  • It is based on Queuing

Shortest remaining time

Similar to Shortest­ Job­ First (SJF). With this strategy the scheduler arranges processes with the least estimated processing time remaining to be next in the queue. This requires advance knowledge or estimations about the time required for a process to complete.

  • If a shorter process arrives during another process' execution, the currently running process may be interrupted (known as preemption), dividing that process into two separate computing blocks. This creates excess overhead through additional context switching. The scheduler must also place each incoming process into a specific place in the queue, creating additional overhead.
  • This algorithm is designed for maximum throughput in most scenarios.
  • Waiting time and response time increase as the process' computational requirements increase. Since turnaround time is based on waiting time plus processing time, longer processes are significantly affected by this. Overall waiting time is smaller than FIFO, however since no process has to wait for the termination of the longest process.
  • No particular attention is given to deadlines, the programmer can only attempt to make processes with deadlines as short as possible.
  • Starvation is possible, especially in a busy system with many small processes being run.

Fixed priority pre-emptive scheduling

The O/S assigns a fixed priority rank to every process, and the scheduler arranges the processes in the ready queue in order of their priority. Lower priority processes get interrupted by incoming higher priority processes.

  • Overhead is not minimal, nor is it significant.
  • FPPS has no particular advantage in terms of throughput over FIFO scheduling.
  • Waiting time and response time depend on the priority of the process. Higher priority processes have smaller waiting and response times.
  • Deadlines can be met by giving processes with deadlines a higher priority.
  • Starvation of lower priority processes is possible with large amounts of high priority processes queuing for CPU time.

Round-robin scheduling

The scheduler assigns a fixed time unit per process, and cycles through them.

  • RR scheduling involves extensive overhead, especially with a small time unit.
  • Balanced throughput between FCFS and SJF, shorter jobs are completed faster than in FCFS and longer processes are completed faster than in SJF.
  • Poor average response time, waiting time is dependent on number of processes, and not average process length.
  • Because of high waiting times, deadlines are rarely met in a pure RR system.
  • Starvation can never occur, since no priority is given. Order of time unit allocation is based upon process arrival time, similar to FCFS.

Multilevel queue scheduling

This is used for situations in which processes are easily divided into different groups. For example, a common division is made between foreground (interactive) processes and background (batch) processes. These two types of processes have different response-time requirements and so may have different scheduling needs. it is very useful for shared memory problem


Scheduling algorithm CPU Overhead Throughput Turnaround time Response time
First In First Out Low Low High Low
Shortest Job First Medium High Medium Medium
Priority based scheduling Medium Low High High
Round-robin scheduling High Medium Medium High
Multilevel Queue scheduling High High Medium Medium

How to choose a scheduling algorithm

When designing an operating system, a programmer must consider which scheduling algorithm will perform best for the use the system is going to see. There is no universal “best” scheduling algorithm, and many operating systems use extended or combinations of the scheduling algorithms above. For example, Windows NT/XP/Vista uses a Multilevel feedback queue, a combination of fixed priority preemptive scheduling, round-robin, and first in first out. In this system, processes can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively. Every priority level is represented by its own queue, with round-robin scheduling amongst the high priority processes and FIFO among the lower ones. In this sense, response time is short for most processes, and short but critical system processes get completed very quickly. Since processes can only use one time unit of the round robin in the highest priority queue, starvation can be a problem for longer high priority processes.

Operating system scheduler implementations


Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs. It relied on the program to end or tell the OS that it didn't need the processor so that it could move on to another process. This is usually called cooperative multitasking. Windows 95 introduced a rudimentary preemptive scheduler; however, for legacy support opted to let 16 bit applications run without preemption.[1]

Windows NT-based operating systems use a multilevel feedback queue. 32 priority levels are defined, 0 through to 31, with priorities 0 through 15 being "normal" priorities and priorities 16 through 31 being soft real-time priorities, requiring privileges to assign. 0 is reserved for the Operating System. Users can select 5 of these priorities to assign to a running application from the Task Manager application, or through thread management APIs. The kernel may change the priority level of a thread depending on its I/O and CPU usage and whether it is interactive (i.e. accepts and responds to input from humans), raising the priority of interactive and I/O bounded processes and lowering that of CPU bound processes, to increase the responsiveness of interactive applications.[2] The scheduler was modified in Windows Vista to use the cycle counter register of modern processors to keep track of exactly how many CPU cycles a thread has executed, rather than just using an interval-timer interrupt routine.[3] Vista also uses a priority scheduler for the I/O queue so that disk defragmenters and other such programs don't interfere with foreground operations.[4]

Mac OS

Mac OS 9 uses cooperative scheduling for threads, where one process controls multiple cooperative threads, and also provides preemptive scheduling for MP tasks. The kernel schedules MP tasks using a preemptive scheduling algorithm. All Process Manager processes run within a special MP task, called the "blue task". Those processes are scheduled cooperatively, using a round-robin scheduling algorithm; a process yields control of the processor to another process by explicitly calling a blocking function such as WaitNextEvent. Each process has its own copy of the Thread Manager that schedules that process's threads cooperatively; a thread yields control of the processor to another thread by calling YieldToAnyThread or YieldToThread.[5]

Mac OS X uses a multilevel feedback queue, with four priority bands for threads - normal, system high priority, kernel mode only, and real-time.[6] Threads are scheduled preemptively; Mac OS X also supports cooperatively-scheduled threads in its implementation of the Thread Manager in Carbon.[5]


In AIX Version 4 there are three possible values for thread scheduling policy :

  • FIFO: Once a thread with this policy is scheduled, it runs to completion unless it is blocked, it voluntarily yields control of the CPU, or a higher-priority thread becomes dispatchable. Only fixed-priority threads can have a FIFO scheduling policy.
  • RR: This is similar to the AIX Version 3 scheduler round-robin scheme based on 10ms time slices. When a RR thread has control at the end of the time slice, it moves to the tail of the queue of dispatchable threads of its priority. Only fixed-priority threads can have a RR scheduling policy.
  • OTHER This policy is defined by POSIX1003.4a as implementation-defined. In AIX Version 4, this policy is defined to be equivalent to RR, except that it applies to threads with non-fixed priority. The recalculation of the running thread's priority value at each clock interrupt means that a thread may lose control because its priority value has risen above that of another dispatchable thread. This is the AIX Version 3 behavior.

Threads are primarily of interest for applications that currently consist of several asynchronous processes. These applications might impose a lighter load on the system if converted to a multithreaded structure.

AIX 5 implements the following scheduling policies: FIFO, round robin, and a fair round robin. The FIFO policy has three different implementations: FIFO, FIFO2, and FIFO3. The round robin policy is named SCHED_RR in AIX, and the fair round robin is called SCHED_OTHER. This link provides additional information on AIX 5 scheduling: .


Linux 2.4

In Linux 2.4, an O(n) scheduler with a multilevel feedback queue with priority levels ranging from 0-140. 0-99 are reserved for real-time tasks and 100-140 are considered nice task levels was used. For real-time tasks, the time quantum for switching processes was approximately 200 ms, and for nice tasks approximately 10 ms. The scheduler ran through the run queue of all ready processes, letting the highest priority processes go first and run through their time slices, after which they will be placed in an expired queue. When the active queue is empty the expired queue will become the active queue and vice versa.

However, some Enterprise Linux distributions such as SUSE Linux Enterprise Server replaced this scheduler with a backport of the O(1) scheduler (which was maintained by Alan Cox in his Linux 2.4-ac Kernel series) to the Linux 2.4 kernel used by the distribution.

Linux 2.6.0 to Linux 2.6.22

From versions 2.6 to 2.6.22, the kernel used an O(1) scheduler developed by Ingo Molnar and many other kernel developers during the Linux 2.5 development. For many kernel in time frame, Con Kolivas developed patch sets which improved interactivity with this scheduler or even replaced it with his own schedulers.

Since Linux 2.6.23

Con Kolivas's work, most significantly his implementation of "fair scheduling" named "Rotating Staircase Deadline", inspired Ingo Molnár to develop the Completely Fair Scheduler as a replacement for the earlier O(1) scheduler, crediting Kolivas in his announcement.[7]

The Completely Fair Scheduler (CFS) uses a well-studied, classic scheduling algorithm called fair queuing riginally invented for packet networks. Fair queuing had been previously applied to CPU scheduling under the name stride scheduling.

The fair queuing CFS scheduler has a scheduling complexity of O(log N), where N is the number of tasks in the runqueue. Choosing a task can be done in constant time, but reinserting a task after it has run requires O(log N) operations, because the run queue is implemented as a red-black tree.

CFS is the first implementation of a fair queuing process scheduler widely used in a general-purpose operating system.[8]


FreeBSD uses a multilevel feedback queue with priorities ranging from 0-255. 0-63 are reserved for interrupts, 64-127 for the top half of the kernel, 128-159 for real-time user threads, 160-223 for time-shared user threads, and 224-255 for idle user threads. Also, like Linux, it uses the active queue setup, but it also has an idle queue.[9]


NetBSD uses a multilevel feedback queue with priorities ranging from 0-223. 0-63 are reserved for time-shared threads (default, SCHED_OTHER policy), 64-95 for user threads which entered kernel space, 96-128 for kernel threads, 128-191 for user real-time threads (SCHED_FIFO and SCHED_RR policies), and 192-223 for software interrupts.


Solaris uses a multilevel feedback queue with priorities ranging from 0-169. 0-59 are reserved for time-shared threads, 60-99 for system threads, 100-159 for real-time threads, and 160-169 for low priority interrupts. Unlike Linux, when a process is done using its time quantum, it's given a new priority and put back in the queue.


Operating System Preemption Algorithm
Windows 3.1x None Cooperative Scheduler
Windows 95, 98, Me Half Preemptive for 32-bit processes, Cooperative Scheduler for 16-bit processes
Windows NT (including 2000, XP, Vista, 7, and Server) Yes Multilevel feedback queue
Mac OS pre-9 None Cooperative Scheduler
Mac OS 9 Some Preemptive for MP tasks, Cooperative Scheduler for processes and threads
Mac OS X Yes Multilevel feedback queue
Linux pre-2.6 Yes Multilevel feedback queue
Linux 2.6-2.6.23 Yes O(1) scheduler
Linux post-2.6.23 Yes Completely Fair Scheduler
Solaris Yes Multilevel feedback queue
NetBSD Yes Multilevel feedback queue
FreeBSD Yes Multilevel feedback queue

Further reading

See also


  • Blazewicz, J., Ecker, K.H., Pesch, E., Schmidt, G. und J. Weglarz, Scheduling Computer and Manufacturing Processes, Berlin (Springer) 2001, ISBN 3-540-41931-4
  • Stallings, William (2004). Operating Systems Internals and Design Principles (fifth international edition). Prentice Hall. ISBN 0-13-147954-7. 
  • Stallings, William (2004). Operating Systems Internals and Design Principles (fourth edition). Prentice Hall. ISBN 0-13-031999-6. 
  • Information on the Linux 2.6 O(1)-scheduler

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