Chapter 6: CPU Scheduling

Silberschatz, Galvin and Gagne 20026.1Operating System Concepts

Chapter 6: CPU Scheduling

Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation


Basic Concepts

Maximum CPU utilization obtained with multiprogramming

CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.

CPU burst distribution


Alternating Sequence of CPU And I/O Bursts

Process execution begins with a CPU burst. That is followed by an I\O burst, then another CPU burst, then another I/O burst, and so on.

Eventually, the last CPU burst will end with a system request to terminate execution, rather than with another I/O burst


Histogram of CPU-burst Times

many short CPU bursts, and a few long CPU bursts. An I/O-bound program would typically have many very short CPU bursts. A

CPU-bound program might have a few very long CPU bursts.


CPU Scheduler

Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.

CPU scheduling decisions may take place when a process:1.Switches from running to waiting state. (for example, I/O request,

or invocation of wait for the termination of one of the child processes)

2. Switches from running to ready state. (for example, when an interrupt occurs)

3. Switches from waiting to ready. (for example, completion of I/O)

4. Terminates. When scheduling takes place only under circumstances 1 and 4, we say the

scheduling scheme is nonpreemptive; otherwise, the scheduling scheme is preemptive.

Under nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state.


Dispatcher

Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart that

program

Dispatch latency – time it takes for the dispatcher to stop one process and start another running.


Scheduling Criteria

CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular process Waiting time – amount of time a process has been waiting in the ready

queue Response time – amount of time it takes from when a request was

submitted until the first response is produced. (a process can produce some output fairly early, and can continue computing new results while previous results are being output to the user.)


Optimization Criteria

Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time


First-Come, First-Served (FCFS) Scheduling

Process Burst Time

P1 24

P2 3

P3 3

Suppose that the processes arrive in the order: P1 , P2 , P3

The Gantt Chart for the schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300


FCFS Scheduling (Cont.)

Suppose that the processes arrive in the order

P2 , P3 , P1 .

The Gantt chart for the schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3

Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. The effect short process behind long process

P1P3P2

63 300


Shortest-Job-First (SJR) Scheduling

Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time.

Two schemes: nonpreemptive – once CPU given to the process it cannot be

preempted until completes its CPU burst. preemptive – if a new process arrives with CPU burst length less

than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF).

SJF is optimal – gives minimum average waiting time for a given set of processes.


Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 =4

Example of Non-Preemptive SJF

P1 P3 P2

73 160

P4

8 12


Example of Preemptive SJF

Process Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 - 3

P1 P3P2

42 110

P4

5 7

P2 P1

16


Priority Scheduling

A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority

(smallest integer highest priority). Equal-priority processes are scheduled in FCFS order. Preemptive nonpreemptive

SJF is a priority scheduling where priority is the predicted next CPU burst time. (The larger the CPU burst, the lower

the priority, and vice versa.) Problem Starvation – low priority processes may never

execute. Solution Aging – as time progresses increase the priority of

the process.


Round Robin (RR)

Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.

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 at once. No process waits more than (n-1)q time units.

Performance q large FIFO q small q must be large with respect to context switch,

otherwise overhead is too high.


Example of RR with Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24

The Gantt chart is:

Typically, higher average turnaround than SJF, but better response.

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162


Time Quantum and Context Switch Time


Turnaround Time Varies With The Time Quantum


Multilevel Queue

Ready queue is partitioned into separate queues:foreground (interactive)background (batch)

Each queue has its own scheduling algorithm, foreground – RRbackground – FCFS

Scheduling must be done between the queues. Fixed priority scheduling; (i.e., serve all from foreground then from

background). Possibility of starvation. Time slice – each queue gets a certain amount of CPU time which

it can schedule amongst its processes; i.e., 80% to foreground in RR 20% to background in FCFS


Let us look at an example of a multilevel queue-scheduling algorithm with five queues: 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. Solaris 2 uses a form of this algorithm.


Multilevel Queue Scheduling


Multilevel Feedback Queue

A process can move between the various queues; aging can be implemented this way.

Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when

that process needs service


Example of Multilevel Feedback Queue

For example, consider a multilevel feedback queue scheduler with three queues, numbered from 0 to 2 (Figure 6.7). 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 be executed only if queues 0 and 1 are empty. A process that arrives for queue 1 will preempt a process in queue 2. A process that arrives for queue 0 will, in turn, preempt a process in queue 1.

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. 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, only when queues 0 and 1 are empty.


Example of Multilevel Feedback Queue

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 16, 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.


Figure 6.7


Multiple-Processor Scheduling

CPU scheduling more complex when multiple CPUs are available. Homogeneous processors within a multiprocessor: We concentrate on

systems where the processors are identical (or homogeneous) in terms of their functionality; any available processor can then be used to run any processes in the queue.

Load sharing : If several identical processors are available, then load sharing can occur. It would be possible to provide a separate queue for each processor. In this case, however, one processor could be idle, with an empty queue, while another processor was very busy. To prevent this situation, we use a common ready queue. All processes go into one queue and are scheduled onto any available processor.

Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing. having all scheduling decisions, I/O processing, and other system activities handled by one single processor-the master server. The other processors only execute user code.


Real-Time Scheduling

Hard real-time systems – required to complete a critical task within a guaranteed amount of time.

Soft real-time computing – requires that critical processes receive priority over less fortunate( حظا .ones (أقل


Dispatch Latency

Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

The conflict phase of dispatch latency has two components: 1. Preemption of any process running in the kernel 2. Release by low-priority processes resources needed by the high-priority process


Algorithm Evaluation

How do we select a CPU-scheduling algorithm for a particular system? . The first problem is defining the criteria to be used in selecting an algorithm.

Criteria are often defined in terms of CPU utilization, response time, or throughput.

To select an algorithm, we must first define the relative importance of these measures. Our criteria may include several measures, such as: Maximize CPU utilization under the constraint that the maximum response time is

1 second. Maximize throughput such that turnaround time is (on average) linearly

proportional to total execution time. Once the selection criteria have been defined, we want to evaluate the

various algorithms under consideration.

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Chapter 6: CPU Scheduling