Scheduling Algorithms: FCFS, SJF, SRTN, Priority based, Round Robin, Multilevel Queue scheduling

CPU scheduling deals with the problem of deciding which of the processes in the ready queue is to be allocated the CPU. 
  • There are many different CPU-scheduling algorithms. In this section, we describe several of them.

2.3.1 First-Come, First-Served Scheduling (FCFS)

  • By far the simplest CPU-scheduling algorithm is the first-come, first-served (FCFS) scheduling algorithm.
  • With this scheme, the process that requests the CPU first is allocated the CPU first.
  • The implementation of the FCFS policy is easily managed with a FIFO queue.
  • When a process enters the ready queue, its PCB is linked onto the tail of the queue.
  • When the CPU is free, it is allocated to the process at the head of the queue.
  • The running process is then removed from the queue.
  • The code for FCFS scheduling is simple to write and understand.
  • On the negative side, the average waiting time under the FCFS policy is often quite long.
  • Consider the following set of processes that arrive at time 0, with the length of the CPU burst given in milliseconds:

Process      Burst Time

      P1             24

      P2              3

      P3              3

  • If the processes arrive in the order P1, P2, P3, and are served in FCFS order, we get the result shown in the following Gantt chart, which is a bar chart that illustrates a particular schedule, including the start and finish times of each of the participating processes:

  • The waiting time is 0 milliseconds for process P1, 24 milliseconds for process P2 , and 27 milliseconds for process P3 .
  • Thus, the average waiting time is (0 + 24 + 27) / 3 = 17 milliseconds
  • If the processes arrive in the order P2, P3 , P1, however, the results will be as shown in the following Gantt chart:
  • The average waiting time is now (6 + 0 + 3) / 3 = 3 milliseconds. This reduction is substantial.

  • Thus, the average waiting time under an FCFS policy is generally not minimal and may vary substantially if the processes CPU burst times vary greatly.
  • In addition, consider the performance of FCFS scheduling in a dynamic situation. Assume we have one CPU-bound process and many I/O-bound processes.
  • As the processes flow around the system, the following scenario may result.
  • The CPU-bound process will get and hold the CPU. During this time, all the other processes will finish their I/O and will move into the ready queue, waiting for the CPU.
  • While the processes wait in the ready queue, the I/O devices are idle. Eventually, the CPU-bound process finishes its CPU burst and moves to an I/O device.
  • All the I/O-bound processes, which have short CPU bursts, execute quickly and move back to the I/O queues.
  • At this point, the CPU sits idle. The CPU-bound process will then move back to the ready queue and be allocated the CPU.
  • Again, all the I/O processes end up waiting in the ready queue until the CPU-bound process is done.
  • There is a convoy effect as all the other processes wait for the one big process to get off the CPU.
  • This effect results in lower CPU and device utilization than might be possible if the shorter processes were allowed to go first.
  • Note also that the FCFS scheduling algorithm is nonpreemptive. Once the CPU has been allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O.
  • The FCFS algorithm is thus particularly troublesome for time-sharing systems, where it is important that each user get a share of the CPU at regular intervals.
  • It would be disastrous to allow one process to keep the CPU for an extended period.

2.3.2 Shortest-Job-First Scheduling (SJF)

  • A different approach to CPU scheduling is the shortest-job-first (SJF) scheduling algorithm.
  • This algorithm associates with each process the length of the process's next CPU burst.
  • When the CPU is available, it is assigned to the process that has the smallest next CPU burst.
  • If the next CPU bursts of two processes are the same, FCFS scheduling is used to break the tie. Note that a more appropriate term for this scheduling method would be the shortest-next-CPU-burst algorithm, because scheduling depends on the length of the next CPU burst of a process, rather than its total length.
  • We use the term SJF because m.ost people and textbooks use this term to refer to this type of scheduling.
  • As an example of SJF scheduling, consider the following set of processes, with the length of the CPU burst given in milliseconds:

Process      Burst Time

      P1              6

      P2              8

      P3              7

      P4              3

  • Using SJF scheduling, we would schedule these processes according to the following Gantt chart:

  • The waiting time is 3 milliseconds for process P1, 16 milliseconds for process P2, 9 milliseconds for process P3, and 0 milliseconds for process P4.
  • Thus, the average waiting time is (3 + 16 + 9 + 0) / 4 = 7 milliseconds.
  • By comparison, if we were using the FCFS scheduling scheme, the average waiting time would be 10.25 milliseconds.
  • The SJF scheduling algorithm is provably optimal, in that it gives the minimum average waiting time for a given set of processes.
  • Moving a short process before long one decreases the waiting time of the short process more than it increases the waiting time of the long process.
  • Consequently, the average waiting time decreases. The real difficulty with the SJF algorithm is knowing the length of the next CPU request.
  • For long-term (job) scheduling in a batch system, we can use as the length the process time limit that a user specifies when he submits the job.
  • Thus, users are motivated to estimate the process time limit accurately, since a lower value may mean faster response. (Too low a value will cause a time-limit-exceeded error and require resubmission.)
  • SJF scheduling is used frequently in long-term scheduling.
  • Although the SJF algorithm is optimal, it cannot be implemented at the level of short-term CPU scheduling.
  • With short-term scheduling, there is no way to know the length of the next CPU burst.
  • One approach is to try to approximate SJF scheduling.
  • We may not know the length of the next CPU burst, but we may be able to predict its value.
  • We expect that the next CPU burst will be similar in length to the previous ones.
  • By computing an approximation of the length of the next CPU burst, we can pick the process with the shortest predicted CPU burst.
  • The next CPU burst is generally predicted as an exponential average of the measured lengths of previous CPU bursts. We can define the exponential average with the following formula.
  • Let tn be the length of the nth CPU burst, and let τn+1 be our predicted value for the next CPU burst.
  • Then, for α, 0 ≤ α ≤  1, define

τn+1= α tn + (1-α) τn

  • The value of tn contains our most recent information; τn stores the past history.
  • The parameter α controls the relative weight of recent and past history in our prediction. If α = 0, then τn+1 = τn, and recent history has no effect (current conditions are assumed to be transient). If α = 1, then then τn+1 = τn, and only the most recent CPU burst matters (history is assumed to be old and irrelevant).
  • More commonly, α = 1/2, so recent history and past history are equally weighted.
  • The initial τ0 can be defined as a constant or as an overall system average.
  • Figure 2.15 shows an exponential average with α= 1/2 and τ0= 10.
  • To understand the behavior of the exponential average, we can expand the formula for τn+l by substituting for τn , to find

τn+1 = α tn + (1-α) α tn-1+……..+ (1 - α) j α tn-j+……. + (1 - α) n+1 τ0

  • Since both a and (1 - α) are less than or equal to 1, each successive term has less weight than its predecessor.

Figure 2.15 Prediction of the length of the next CPU burst.

  • The SJF algorithm can be either preemptive or nonpreemptive.
  • The choice arises when a new process arrives at the ready queue while a previous process is still executing.
  • The next CPU burst of the newly arrived process may be shorter than what is left of the currently executing process.
  • A preemptive SJF algorithm will preempt the currently executing process, whereas a nonpreemptive SJF algorithm will allow the currently running process to finish its CPU burst.
  • Preemptive SJF scheduling is sometimes called shortest-remaining-time-first scheduling.
  • As an example, consider the following four processes, with the length of the CPU burst given in milliseconds:

Process     Arrival Time   Burst Time

      P1              0                      8

      P2              1                      4

      P3              2                      9

      P4              3                      5

  • If the processes arrive at the ready queue at the times shown and need the indicated burst times, then the resulting preemptive SJF schedule is as depicted in the following Gantt chart:

  • Process P1 is started at time 0, since it is the only process in the queue. Process P2 arrives at time 1. The remaining time for process P1 (7 milliseconds) is larger than the time required by process P2 (4 milliseconds), so process P1 is preempted, and process P2 is scheduled.
  • The average waiting time for this example is [(10- 1) + (1 - 1) + (17- 2) + (5-3)]/ 4 = 26/4 = 6.5 milliseconds.
  • Nonpreemptive SJF scheduling would result in an average waiting time of 7.75 milliseconds.

2.3.3 Priority Scheduling

  • The SJF algorithm is a special case of the general priority scheduling algorithm. A priority is associated with each process, and the CPU is allocated to the process with the highest priority.
  • Equal-priority processes are scheduled in FCFS order. An SJF algorithm is simply a priority algorithm where the priority (p) is the inverse of the (predicted) next CPU burst.
  • The larger the CPU burst, the lower the priority, and vice versa.
  • Note that we discuss scheduling in terms of high priority and low priority. Priorities are generally indicated by some fixed range of numbers, such as 0 to 7 or 0 to 4,095.
  • However, there is no general agreement on whether 0 is the highest or lowest priority.
  • Some systems use low numbers to represent low priority; others use low numbers for high priority.
  • This difference can lead to confusion. In this text, we assume that low numbers represent high priority.
  • As an example, consider the following set of processes, assumed to have arrived at time 0 in the order P1, P2, · · ·, P5, with the length of the CPU burst given in milliseconds:

Process     Burst Time        Priority

      P1              10                    3

      P2              1                      1

      P3              2                      4

      P4              1                      5

      P5              5                      2

  • Using priority scheduling, we would schedule these processes according to the following Gantt chart:

  • The average waiting time is 8.2 milliseconds.
  • Priorities can be defined either internally or externally. Internally defined priorities use some measurable quantity or quantities to compute the priority of a process.
  • For example, time limits, memory requirements, the number of open files, and the ratio of average I/O burst to average CPU burst have been used in computing priorities.
  • External priorities are set by criteria outside the operating system, such as the importance of the process, the type and amount of funds being paid for computer use, the department sponsoring the work, and other, often political factors.
  • Priority scheduling can be either preemptive or nonpreemptive. When a process arrives at the ready queue, its priority is compared with the priority of the currently running process. 
  • A preemptive priority scheduling algorithm will preempt the CPU if the priority of the newly arrived process is higher than the priority of the currently running process.
  • A nonpreemptive priority scheduling algorithm will simply put the new process at the head of the ready queue.
  • A rnajor problem with priority scheduling algorithms is indefinite blocking, or starvation. A process that is ready to run but waiting for the CPU can be considered blocked.
  • A priority scheduling algorithm can leave some low priority processes waiting indefinitely. In a heavily loaded computer system, a steady stream of higher-priority processes can prevent a low-priority process from ever getting the CPU.
  • Generally, one of two things will happen. Either the process will eventually be run (at 2 A.M. Sunday, when the system is finally lightly loaded), or the computer system will eventually crash and lose all unfinished low-priority processes.
  • (Rumor has it that when they shut down the IBM 7094 at MIT in 1973, they found a low-priority process that had been submitted in 1967 and had not yet been run.)
  • A solution to the problem of indefinite blockage of low-priority processes is aging.
  • Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time.
  • For example, if priorities range from 127 (low) to 0 (high), we could increase the priority of a waiting process by 1 every 15 minutes. Eventually, even a process with an initial priority of 127 would have the highest priority in the system and would be executed.
  • In fact, it would take no more than 32 hours for a priority-127 process to age to a priority-0 process.

2.3.4 Round-Robin Scheduling

  • The round-robin (RR) scheduling algorithm is designed especially for timesharing systems.
  • It is similar to FCFS scheduling, but preemption is added to enable the system to switch between processes.
  • A small unit of time, called a time quantum or time slice, is defined. A time quantum is generally from 10 to 100 milliseconds in length.
  • The ready queue is treated as a circular queue.
  • The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1 time quantum.
  • To implement RR scheduling, we keep the ready queue as a FIFO queue of processes.
  • New processes are added to the tail of the ready queue. The CPU scheduler picks the first process from the ready queue, sets a timer to interrupt after 1 time quantum, and dispatches the process.
  • One of two things will then happen. The process may have a CPU burst of less than 1 time quantum. In this case, the process itself will release the CPU voluntarily.
  • The scheduler will then proceed to the next process in the ready queue.
  • Otherwise, if the CPU burst of the currently running process is longer than 1 time quantum, the timer will go off and will cause an interrupt to the operating system.
  • A context switch will be executed, and the process will be put at the tail of the ready queue.
  • The CPU scheduler will then select the next process in the ready queue. The average waiting time under the RR policy is often long.
  • Consider the following set of processes that arrive at time 0, with the length of the CPU burst given in milliseconds:

Process      Burst Time

      P1              24

      P2              3

      P3              3

  • If we use a time quantum of 4 milliseconds, then process P1 gets the first 4 milliseconds. Since it requires another 20 milliseconds, it is preempted after the first time quantum, and the CPU is given to the next process in the queue, process P2.
  •  Process P2 does not need 4 milliseconds, so it quits before its time quantum expires. The CPU is then given to the next process, process P3.
  • Once each process has received 1 time quantum, the CPU is returned to process P1 for an additional time quantum. The resulting RR schedule is as follows:

  • Let's calculate the average waiting time for the above schedule. P1 waits for 6 milliseconds (10- 4), P2 waits for 4 milliseconds, and P3 waits for 7 milliseconds.
  • Thus, the average waiting time is 17/3 = 5.66 milliseconds.
  • In the RR scheduling algorithm, no process is allocated the CPU for more than 1 time quantum in a row (unless it is the only runnable process).
  • If a process's CPU burst exceeds 1 time quantum, that process is preempted and is put back in the ready queue.
  • The RR scheduling algorithm is thus preemptive. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units.
  • Each process must wait no longer than (n - 1) x q time units until its next time quantum. For example, with five processes and a time quantum of 20 milliseconds, each process will get up to 20 milliseconds every 100 milliseconds.
  • The performance of the RR algorithm depends heavily on the size of the time quantum.
  • At one extreme, if the time quantum is extremely large, the RR policy is the same as the FCFS policy.
  • In contrast, if the time quantum is extremely small (say, 1 millisecond), the RR approach is called processor sharing and (in theory) creates the appearance that each of n processes has its own processor running at 1 / n the speed of the real processor.
  • This approach was used in Control Data Corporation (CDC) hardware to implement ten peripheral processors with only one set of hardware and ten sets of registers.
  • The hardware executes one instruction for one set of registers, then goes on to the next. This cycle continues, resulting in ten slow processors rather than one fast one. (Actually, since the processor was much faster than memory and each instruction referenced memory, the processors were not much slower than ten real processors would have been.)
  • In software, we need also to consider the effect of context switching on the performance of RR scheduling.
  • Assume, for example, that we have only one process of 10 time units. If the quantum is 12 time units, the process finishes in. less than 1 time quantum, with no overhead. If the quantum is 6 time units, however, the process requires 2 quanta, resulting in a context switch.

Figure 2.16 How a smaller time quantum increases context switches.

  • If the time quantum is 1 time unit, then nine context switches will occur, slowing the execution of the process accordingly (Figure 2.16). Thus, we want the time quantum to be large with respect to the context switch time.
  • If the context-switch time is approximately 10 percent of the time quantum, then about 10 percent of the CPU time will be spent in context switching.
  • In practice, most modern systems have time quanta ranging from 10 to 100 milliseconds.
  • The time required for a context switch is typically less than 10 microseconds; thus, the context-switch time is a small fraction of the time quantum.
  • Turnaround time also depends on the size of the time quantum. As we can see from Figure 2.17, the average turnaround time of a set of processes does not necessarily improve as the time-quantum size increases.
  • In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum.
  • For example, given three processes of 10 time units each and a quantum of 1 time unit, the average turnaround time is 29. If the time quantum is 10, however, the average turnaround time drops to 20.
  • If context-switch time is added in, the average turnaround time increases even more for a smaller time quantum, since more context switches are required.

Figure 2.17 How turnaround time varies with the time quantum.

  • Although the time quantum should be large compared with the context switch time, it should not be too large. If the time quantum is too large, RR scheduling degenerates to an FCFS policy.
  • A rule of thumb is that 80 percent of the CPU bursts should be shorter than the time quantum.

2.3.5 Multilevel Queue Scheduling

  • Another class of scheduling algorithms has been created for situations in which processes are easily classified 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.
  • In addition, foreground processes may have priority (externally defined) over background processes.

Figure 2.18 Multilevel queue scheduling.

  • A multilevel queue scheduling algorithm partitions the ready queue into several separate queues (Figure 2.18).
  • The processes are permanently assigned to one queue, generally based on some property of the process, such as memory size, process priority, or process type.
  • Each queue has its own scheduling algorithm.
  • For example, separate queues might be used for foreground and background processes.
  • The foreground queue might be scheduled by an RR algorithm, while the background queue is scheduled by an FCFS algorithm.
  • In addition, there must be scheduling among the queues, which is commonly implemented as fixed-priority preemptive scheduling.
  • For example, the foreground queue may have absolute priority over the background queue.
  • Let's look at an example of a multilevel queue scheduling algorithm with five queues, listed below in order of priority:

  1. System processes
  2. Interactive processes
  3. Interactive editing processes
  4. Batch processes
  5. Student processes

  • Each queue has absolute priority over lower-priority queues.
  • No process in the batch queue, for example, could run unless the queues for system processes, interactive processes, and interactive editing processes were all empty.
  • If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted.
  • Another possibility is to time-slice among the queues. Here, each queue gets a certain portion of the CPU time, which it can then schedule among its various processes.
  • For instance, in the foreground-background queue example, the foreground queue can be given 80 percent of the CPU time for RR scheduling among its processes, whereas the background queue receives 20 percent of the CPU to give to its processes on an FCFS basis.

2.3.6 Multilevel Feedback Queue Scheduling

  • Normally, when the multilevel queue scheduling algorithm is used, processes are permanently assigned to a queue when they enter the system.
  • If there are separate queues for foreground and background processes, for example, processes do not move from one queue to the other, since processes do not change their foreground or background nature.
  • This setup has the advantage of low scheduling overhead, but it is inflexible. The multilevel feedback queue scheduling algorithm, in contrast, allows a process to move between queues.
  • The idea is to separate processes according to the characteristics of their CPU bursts.
  • If a process uses too much CPU time, it will be moved to a lower-priority queue.
  • This scheme leaves I/O-bound and interactive processes in the higher-priority queues. In addition, a process that waits too long in a lower-priority queue may be moved to a higher-priority queue. This form of aging prevents starvation.
  • For example, consider a multilevel feedback queue scheduler with three queues, numbered from 0 to 2 (Figure 2.19). The scheduler first executes all processes in queue 0.
  • Only when queue 0 is empty will it execute processes in queue 1. Similarly, processes in queue 2 will only be executed if queues 0 and 1 are empty.
  • A process that arrives for queue 1 will preempt a process in queue 2. A process in queue 1 will in turn be preempted by a process arriving for queue 0.
  • A process entering the ready queue is put in queue 0. A process in queue 0 is given a time quantum of 8 milliseconds. If it does not finish within this time, it is moved to the tail of queue 1.

Figure 2.19 Multilevel feedback queues.

  • If queue 0 is empty, the process at the head of queue 1 is given a quantum of 16 milliseconds. If it does not complete, it is preempted and is put into queue 2. Processes in queue 2 are run on an FCFS basis but are run only when queues 0 and 1 are empty.
  • This scheduling algorithm gives highest priority to any process with a CPU burst of 8 milliseconds or less.
  • Such a process will quickly get the CPU, finish its CPU burst, and go off to its next I/O burst. Processes that need more than 8 but less than 24 milliseconds are also served quickly, although with lower priority than shorter processes.
  • Long processes automatically sink to queue 2 and are served in FCFS order with any CPU cycles left over from queues 0 and 1.
  • In general, a multilevel feedback queue scheduler is defined by the following parameters:
    • The number of queues
    • The scheduling algorithm for each queue
    • The method used to determine when to upgrade a process to a higher priority queue
    • The method used to determine when to demote a process to a lower priority queue
    • The method used to determine which queue a process will enter when that process needs service
  • The definition of a multilevel feedback queue scheduler makes it the most general CPU-scheduling algorithm.
  • It can be configured to match a specific system under design.
  • Unfortunately, it is also the most complex algorithm, since defining the best scheduler requires some means by which to select values for all the parameters.


  1. MODERN OPERATING SYSTEMS by Andrew S. Tanenbaum, Second Edition
  2. The Operating System Concepts by Silberschatz, Galvin, Gagne, 8th Edition

Last modified: Sunday, 19 November 2017, 11:42 AM