ln-os-ch05 CPU Scheduling

8/7/2019 ln-os-ch05 CPU Scheduling

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ECE/IUPUI RTOS & APPS 1

CPU Scheduling

Electrical and Computer Engineering

Stephen Kim ([email protected])


CPU Schedulingn Basic Concepts

n Scheduling Criteria

n Scheduling Algorithms

n Multiple-Processor Scheduling

n Real-Time Scheduling

n Algorithm Evaluation


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Basic Concepts

n Maximum CPU utilization obtained withmultiprogramming by making some process running at alltimes.

n CPU Burst vs. I/O Burst Process execution consists of a

cycle of CPU execution and I/O wait.

n CPU burst distribution


Alternating Sequence of CPU And

I/O Bursts

CPU Burst IO Burst

IO Request

IO Complete


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Histogram of CPU-burst Times

n exponential or hyperexponential

n IO bound program many very short CPU bursteg.)

n CPU bound program a few very long CPU burst


CPU Scheduler

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

n CPU scheduling decisions may take place when a process:

1. Switches from running to waiting state.

2. Switches from running to ready state.

3. Switches from waiting to ready.

4. Terminates.

n Scheduling under 1 and 4 is nonpreemptive. A new ready

process must be selected for execution.

n All other scheduling ispreemptive.


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Nonpreemptive vs. Preemptive

n Nonpreemptive

u A process keeps the CPU until it release the CPU either by

terminating, or by switching to the waiting state.

u eg) MS Windows 3.1, Apple Macintosh

t limited by hardware support.

n Preemptive

u A process can be interrupted any time.

u Possible Inconsistency

t A process can be interrupted while it modifies some data, which other

process tries to modify or read.

t Frequent between a system call and a user process.

t A simple solution is to disallow to interrupt in a kernel mode.

t The solution is not adequate for real-time systems.


Dispatcher

n Dispatcher module gives control of the CPU to theprocess selected by the short-term scheduler; thisinvolves:

u switching context

u switching to user mode

u jumping to the proper location in the user program to

restart that program

n Dispatch latency time it takes for the dispatcher

to stop one process and start another running.


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Scheduling Criteria

n CPU utilization keep the CPU as busy as possible. Areasonable range is from 40% to 90%.

n Throughput # of processes that complete their execution

per time unit.

n Turnaround time amount of time to execute a particular

process from submission to completion.

n Waiting time amount of time a process has been waiting

in the ready queue.

n Response time amount of time it takes from when a

request was submitted until the first response is produced,not output (for time-sharing environment).


Optimization Criteria

n What we want to

u Maximize CPU utilization

u Maximize throughput

u Minimize turnaround time

u Minimize waiting time

u Minimize response time

n In most cases

u optimize the average measure

n In the following, we will use the average waiting

time


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First-come, First-served (FCFS)Scheduling

Process Burst Time

P1 24

P2 3

P3 3

n Suppose that the processes arrive in the order: P1 , P2 , P3The Gantt Chart for the schedule is:

n Waiting time for P1

= 0; P2

= 24; P3

= 27

n 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 .

n The Gantt chart for the schedule is:

n Waiting time for P1= 6; P

2= 0

;P

3= 3

n Average waiting time: (6 + 0 + 3)/3 = 3

n Much better than previous case.

n Convoy effect: short processes behind long process

P1

P3

P2

63 300


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Shortest-Job-First (SJR)

Schedulingn Associate with each process the length of its next CPU

burst. Use these lengths to schedule the process with theshortest time.

n Two schemes:

u nonpreemptive once CPU given to the process, it cannot be

preempted until completes its CPU burst.

u preemptive if a new process arrives with CPU burst length less

than remaining time of current executing process, preempt.

n 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

n SJF (non-preemptive)

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

Example of Non-Preemptive SJF

P1

P3

P2

73 160

P4

8 12


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Example of Preemptive SJFProcess Arrival Time Burst Time

P1 0.0 7

P2 2.0 4

P3 4.0 1

P4 5.0 4

n SJF (preemptive)

n Average waiting time = (9 + 1 + 0 +2)/4 = 3n Even the SJF is optimal, it cannot be implemented because

we cannot see future.

P1P2P3P4

t


Predicting Length of Next CPU

Burstn Can only estimate the length.

n Can be done by using the length of previous CPU

bursts, using exponential averaging.

1. tn = actual length of nth CPU burst

2. n+1 = predicted value for the next CPU burst

3. 014. Define:

n+1 = tn+ (1-) n


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Predicting Length of Next CPU Burst,cont


Examples of Exponential

Averagingn =0

u n+1 = n

u Recent history does not count.

n =1

u n+1 = tn

u Only the actual last CPU burst counts.

n If we expand the formula, we get:

n+1 = tn+(1 - ) tn -1 +

+(1 - )j tn -1 +

+(1 - )n=1 tn 0

n Since both and (1 - ) are less than or equal to 1, eachsuccessive term has less weight than its predecessor.


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Implementation Consideration

n The coefficient of exponential average is a floating number

n The computation of floating number is burden in designing

a kernel.

n Even worse, many microprocessor used for real-time

operating systems do not have floating point unit (FPU).

n What we can do for that type of processors?


Priority Scheduling

n A priority number (integer) is associated with each process

u UNIX: -20 (highest) to 19 (lowest), default is 10

u microC/OS-II: 0 (highest) to 63 (lowest)

n The CPU is allocated to the process with the highest

priority

u Preemptive

u nonpreemptive

n SJF is a priority scheduling where priority is the predicted

next CPU burst time.

n Starvation low priority processes may never execute.u Solution: Aging as time progresses increase the priority of the

process.


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Round Robin (RR)

n Each process gets a small unit of CPU time (time quantumor time slice), usually 10-100 milliseconds. After this timehas elapsed, the process is preempted and added to the endof the ready queue.

n 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.

n Performance

u q large FIFO

u q small q must be large with respect to context switch,otherwise overhead is too high.


Example of RR with Time Quantum =4

Process Burst Time

P1 24

P2 3

P3 3

The Gantt chart is:

n Average waiting time of RR = (6+4+7)/3=5.66

n Average waiting time of SJF = (6+0+3)/3=3n The average waiting time of RR is often quite long.

P1 P2 P3 P1 P1 P1 P1 P1

P1P2 P3 P1 P1 P1 P1 P1

RR

SJF


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Time Quantum and Context SwitchTime

n The average turnaround of RR is longer than SJFbecause CPU bursts of all processes interleave.

n But, better response.


Turnaround Time Varies With The Time

Quantum


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Multilevel Queue

n Ready queue is partitioned into separate queues:u foreground (interactive)

u background (batch)

n Each queue has its own scheduling algorithm,

u foreground RR

u background FCFS

n Scheduling must be done between the queues.

u Fixed priority scheduling; (i.e., serve all from foreground then

from background). Possibility of starvation.

u Time slice each queue gets a certain amount of CPU time which

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

t 20% to background in FCFS

Is this true?


Multilevel Queue Scheduling


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Multilevel Feedback Queue

Schedulingn A process can move between the various queues

u If a process uses to much CPU time, i t will be moved to a lower-

priority queue, but leaves IO-bound and interactive processes in

the higher priority queue.

u Aging can be implemented with the same way.

n Multilevel-feedback-queue scheduler defined by the

following parameters:

u number of queues

u scheduling algorithms for each queue

u method used to determine when to upgrade a process

u method used to determine when to demote a processu method used to determine which queue a process will enter when

that process needs service


Example of Multilevel Feedback

Queuen Three queues:u Q0 time quantum 8 milliseconds

u Q1 time quantum 16 milliseconds

u Q2 FCFS

n Scheduling

u A new job enters queue Q0 which is served FCFS. When it gains

CPU, job receives 8 milliseconds. If it does not finish in 8

milliseconds, job is moved to queue Q1.

u At Q1job is again served FCFS and receives 16 additional

milliseconds. If it still does not complete, it is preempted and

moved to queue Q2.


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Multilevel Feedback Queues


Example, Scheduling

n For the given processes, answer the following question forFCFS, SJF, RR(q=1), RR(q=4), 3Level Feedback(q=1),and 3Level Feedback(q=1&2).

n What is the average waiting times for each scheduling?

n What is the turnaround time for each scheduling?

512E

59D

23C

51B

30A

Processing TimeArrival TimePrcess Name


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Real-time Scheduling

n Hard real-time systems required to complete a criticaltask within a guaranteed amount of time

n Soft real-time computing requires that critical processes

receive priority over less fortunate ones

n Priority inversion

u When a higher-priority process needs to access kernel data

currently being accessed by another lower-priority process. The

higher-priority process must wait for the lower-priority process to

finish

u Solution: priority inheritance scheme

all processes accessing resources that a higher-priority processneeds, inherit the high priority until they are done with the

resources. When they are completed the resource access, their

priority reverts to its original value


Dispatch Latency

n The conflict phase consists of

Preemption of any process running in the kernelRelease by low-priority processes resources needed by the high-priority

process.

n If the preemption is not allowed, the dispatch latency is quite long, somost of real-time OSs support preemption.


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Rate Monotonic Scheduling

(RMS)n In an RT system, how can we assign priorities to

critical tasks?

n Rate monotonic scheduling

u Task priorities are assigned based on how often tasksexecutes

u Tasks with the highest rate of execution are given thehighest priority

u Assumption

t All tasks are periodic

t Tasks do not synchronize with one another, share resouces, orexchange data

t OS supports preemption


RMS Theorem

n Given n tasks, all task hard real-time deadlines arealways met if the inequality hold.

)12(/1

n

i

i nT

E

whereEi is the maximum execution time of task i, and Ti isthe execution period of taski. Note thatEi is always smallerthan Ti andEi/Ti corresponds to the fraction of CPU timerequired to execute task i.

eg) 2(21/2-1)=0.828, 100(21/100-1)=0.695If the system has 100 tasks, the total CPU utilization must beless than 69.5% to meet the real-time deadline.


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ln-os-ch05 CPU Scheduling