09 Uniprocessor Scheduling

8/8/2019 09 Uniprocessor Scheduling

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Uniprocessor Scheduling


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types of scheduling in OS


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Long-Term Scheduling

Determines which programs areadmitted to the system forprocessing

Controls the degree ofmultiprogramming

More processes, smaller percentage

of time each process is executed


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Medium-Term Scheduling

i.e. swapping Based on the need to manage the

degree of multiprogramming


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Short-Term (cpu) Scheduling a cpu scheduling policy decides for each cpu...

which process should be executed how long it will be executed

implemented in the dispatcher, a.k.a. scheduler the scheduler executes very frequently

Invoked when an event occurs

Timer interrupts

I/O interrupts traps

Operating system calls


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process states

h d li d t t


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scheduling and process statetransitions


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levels ofscheduling


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short term cpu scheduling


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queue

1

s

w+s = r= residence time

response time

turnaround time

s = service time

(so far, at a given instant: e)w= waiting time

schduling(or dispatching)

policy

(statistically)


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Optimality Criteria

Performance-related Quantitative

Measurable such as response time

and throughput

other

predictability fairness

ecc.


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Optimality Criteria

User-oriented Response Time

Elapsed time from submission to begin of

the service. Normalized Response Time w.r.t.service time, that is (w+s)/s

System-oriented Effective and efficient utilization of the

processor


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optimality criteria


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scheduling criteria


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scheduling queuing diagram


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Priorities

a priority is assigned to each process a ready process queue for each priority Scheduler will always choose a process of

higher priority over one of lower priority Lower-priority may suffer starvation

Allow a process to change its priority based on

its age or execution history


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Decision Mode

Nonpreemptive Once a process is in the running state, it will

continue until it terminates or blocks itself forI/O

Preemptive Currently running process may be interrupted

and moved to the Ready state by theoperating system

Allows for better service since any oneprocess cannot monopolize the processor forvery long


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priority queuing and preemption


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Process Scheduling Example

arrival time: when the process enter theready queue

service time: the process virtual time elapsed

till the next blocking operation


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First-Come-First-Served

(FCFS)

When the current process ceases toexecute, the oldest process in theReady queue is selected


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First-Come-First-Served(FCFS)

A short process may have to wait avery long time before it can execute

Favors CPU-bound processes

I/O processes have to wait until CPU-bound process completes

Fi t C Fi t S d


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First-Come-First-Served(FCFS)

Tr/ Ts(Tr)


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Round-Robin

(RR), q=1

Uses preemption based on a clock An amount of time (quantumq) is

determined that allows each processto use the processor for that length of

time


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Round-Robin

Clock interrupt is generated atperiodic intervals

When an interrupt occurs, the

currently running process is placedin the read queue (preempted) Next ready job is selected

a.k.a. time slicing

ff f i f i i


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effect of size of preemption timequantum

ff t f i f ti ti


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effect of size of preemption timequantum


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unfairness of RR

I/O-bound processes usually release cpubefore expiration of their quantum

cpu-bound processes run for the whole

quantum

RR prefers cpu-bound processes


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virtual round robin

aux queueis preferredover readyqueue

processesfrom auxqueue run

for theirremainingquantum

fraction

Sh t t P


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Shortest ProcessNext (SPN, SJF)

Nonpreemptive policy, optimal w.r.t. min total waiting time

need to know future! approximated. expected processing time to the next blocking i/o operation

process with shortest expected processing time is selectednext


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Shortest Process Next

Predictability of longer processes isreduced

Possibility of starvation for longer

processes exstimation of time length of the

next cpu-bust may be done by

exponential averaging

Sn1=Tn1Sn

( 0,1 ]


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ShortestRemaining

Time (SRT)

Preemptive version of shortestprocess next policy

Must estimate processing time


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Highest ResponseRatio Next (HRRN)

non preemptive choose next process with the greatest ratio

time spent waiting + expected service time

expected service time

HRRNC, D, E=? HRRND, E=?

=ws

s


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Highest ResponseRatio Next (HRRN)

HRRNC=(5+4)/4=2.25

HRRND=(3+5)/5=1.6

HRRNE=(1+2)/2=1.5

HRRND=(5+2)/2=3.5

HRRNE=(7+5)/2=6

ws

s


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a first comparison

FCFS penalize i/o-bound processes

RR

penalize i/o-bound processes

VRR

fair, do not emphasize response time of i/o-bound

processes

SPN, SRT, HRRN

need service time prediction


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feedback

does not needservice timeestimation

preemptive as inRR (q=1 or q=2i)

demotesprocesses at eachexpired timequantum intolower priority

queues


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feedback: varianti

un processo scala di priorita'... sempre quando scade il suo quanto di tempo

oppure

quando scade il quanto e c'e` una altro processonella sua coda

oppure

quando scade il quanto e c'e` almeno una altroprocesso nel sistema (Stallings)


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Feedback

0 1

42 30 1 542 30 142 30 1 5

0 1

0 120 1

20 1 20 10 1


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simulation results

Documents

09 Uniprocessor Scheduling