Scheduling - Intranet DEIBhome.deib.polimi.it/brandole/pc/17-scheduling.pdfLevels of scheduling...

SchedulingScheduling

Levels of schedulingLevels of scheduling

AlgorithmsAlgorithms

C.Brandolese - 2 -

IntroductionIntroduction

Scheduling

Decides which process to execute at a certain

moment

Is implemented by a specific component

The scheduler

Schedulers differ according to their goal

Maximize throughput

Minimize latency

Maximize processor usage

Avoid starvation

Complete a process within a predefined time

Real time scheduling

C.Brandolese - 3 -

Levels ofLevels of schedulingscheduling

Complex OS may have different schedulers

Long-term scheduler

Admits processes

New → Ready, New → Ready-Suspend

Determines the number of processes in a system

Medium-term scheduler

Determines which processes may compete for CPU

Ready ↔ Ready-Suspend, Blocked ↔ Blocked-Suspend

Reacts to medium-term processor load fluctuations

Short-term scheduler

Assigns processes to the processor

Ready ↔ Running

Manages priorities

C.Brandolese - 4 -


long-term

medium-term

short-term

New

Exit

Admit

Admit

Release

long-term

medium-term

short-term

C.Brandolese - 5 -


batchjobs

interactive users

DispatchCPU

Release

Ready queue

Event waitEvent occurs

Timeout

Ready suspend queue

Blocked suspend queue

Blocked queue

long-term

short-term

medium-term

C.Brandolese - 6 -

DispatcherDispatcher

Short term scheduler

Called dispatcher

Executed very frequently

Invoked on specific events

I/O and clock interrupt

System call

Signal (software interrupt)

Scheduling criteria

User-oriented

Minimizes the response time

System-oriented

Efficient usage of the processor

C.Brandolese - 7 -

PreemptionPreemption

Scheduling policies can be

Preemptive

Nonpreemptive

Preemptive scheduling

Processes can be interrupted

Tend to rdeuce the response time

Useful in highly interactive systems

Nonpreemptive scheduling

Processes run to completion

A process can block other processes for an

undefinitely long time

C.Brandolese - 8 -

PriorityPriority

Static priority

Fixed when the process is created

Easy to implement

Low overhead

System cannot react to chaning environment

Dynamic priority

Can change at run-time

Reacts to changes of the environment

Improves interactivity

Higher overhead

Higher complexity

C.Brandolese - 9 -

PriorityPriority

CPU

Ready queue 0

Ready queue 1

Admit

Ready queue N

Blocked queue

DispatchRelease

Event wait

Preemption

Eventoccurs

C.Brandolese - 10 -

FIFO or FCFSFIFO or FCFS

Simplest nonpreemptive algorithm

Processes are dispatched based on their arrival time

Waiting times: P1=0, P2=1, P3=5, P4=7, P5=10

Average waiting time: (0+1+5+7+10)/5=4.6

28P5

56P4

44P3

62P2

30P1

Service (run) timeArrival timeProcesso

P1

0 5 10 15 20

P2

P3

P4

P5

C.Brandolese - 11 -


Algorithm

A new process is inserted in the ready queue

When a process terminates, the oldest process in the

queue is dispatched

Limitations

A short process may be delayed by a long one

Favors CPU-bound processes

I/O-bound processes must wait for CPU/bound

processes to complete

C.Brandolese - 12 -


Processi

Case 1: Arrival order is P1, P2, P3

Waiting times: P1=0, P2=24, P3=27

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

30P3

30P2

240P1


P1

0

P2

P3

24 27 30

C.Brandolese - 13 -

FIFO o FCFS FIFO o FCFS –– FirstFirst--Come, FirstCome, First--

Serve Serve

30P3

30P2

240P1


P1

0

P2

P3

3 6 30

Processi

Case 1: Arrival order is P2, P3, P1

Waiting times: P1=6, P2=0, P3=3

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

C.Brandolese - 14 -

RR RR –– Round RobinRound Robin

Preemptive algorithms

Processes are served FIFO

Process can run for a fixed amount of time

Timeslice

If a process does not terminate within its timeslice

The system

Preempts it

Re-queues the process at the end of the queue

The first process in queue is dispatched

C.Brandolese - 15 -

RR RR –– Round RobinRound Robin

Processes

Timeslice is 3 “units”

Waiting times: P1=0+3+3, P2=1, P3=4+3, P4=3+2

Tempo medio di attesa: (6+1+7+5)/4=4.75

510P4

55P3

32P2

70P1


P1

P2

P3

P4

0 5 10 15 20

P1 P1

P3

P4

C.Brandolese - 16 -

SRR SRR –– Selfish Round RobinSelfish Round Robin

Variation of RR

Each process has an initial priority

Process get old and increase their priority

Often implemented with an additional queue

A process enters the queue of new process

It gets old, and when its priority equals that of ready

processes is dispatched

CPU

Ready queue

New processes queue

Admit

DispatchRelease

When process old enough andPriority equat to that in the ready queue

C.Brandolese - 17 -

RR/SRR and TimesliceRR/SRR and Timeslice

What is the optimal duration of the timeslice?

Short or long?

Fixed or variable?

The same for all processes?

Long timeslice

Probably sufficient to run processes to completion

RR algorithm becomes equivalent to FIFO

Short timeslice

Context switch overhead becomes significant

Typical timeslices

Desktop computing: 20ms – 100ms

Real-time computing: 1ms

C.Brandolese - 18 -

SJF SJF –– Shortest Job FirstShortest Job First

Each process

Is characterized by the duration of the next CPU burst

Is scheduled if it has the shortest burst

Two schemes

Nonpreemptive

Waits the end of current CPU-burst

Preemptive

A process with a burst shorter than the remaining time

of the current process preempts it and gets dispatched

Known as SRTF o Shortest-Remaining-Time-First

C.Brandolese - 19 -


Requires estimating the duration of the next burst

Adaptive approach

Estimations are performed for each processes

Computationally light

Exponntial average estimation is often used

τn+1 = αtn + (1- α)τn

Where

tn Actual burst duration at time n (current)

τn+1 Estimated burst duration at time n+1

α Coefficient between 0 and 1

C.Brandolese - 20 -


The behaviour strongly dependes on α

α = 0

τn+1 = τnThe history of the system is unimportant

α = 1

τn+1 = tnOnly the last burst considered

0 ≤ α ≤ 1

The importance of older estimates (τn) tend to

exponentially decrease as (1-α)n

C.Brandolese - 21 -


Processi

Nonpreemptive

Waiting times: P1=0, P2=6, P3=3, P4=7

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

45P4

14P3

42P2

70P1

Burst timeArrival timeProcesso

P1

0 5 10 15 20

P3

P2

P4

C.Brandolese - 22 -


Processi

Preemptive

Waiting times: P1=0+9, P2=0+1, P3=0, P4=2

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

45P4

14P3

42P2

70P1

Burst timeArrival timeProcesso

P1

0 5 10 15 20

P2

P4

P1

P3

P2

C.Brandolese - 23 -

SPN SPN –– Shortest Process NextShortest Process Next

Nonpreemptive scheme

The ready process with shortes estimated run time is selected

Waiting times: P1=0, P2=1, P3=7, P4=9, P5=1


28P5

56P4

44P3

62P2

30P1


P1

0 5 10 15 20

P2

P3

P4

P5

C.Brandolese - 24 -

SPN SPN –– Shortest Process NextShortest Process Next

Advantages

Shorter process are served rapidly

Problems

Long process can be daleyed for long time

Potential starvation

Reduces process completion predictability

Strongly depends from the accuracy of the estimated

completion times

Wrong estimates might require terminating a process

C.Brandolese - 25 -

HRRN HRRN –– Highest Response Ratio NextHighest Response Ratio Next

Overcomes the limitations of SJF/SPN algorithms

Nonpreeptive algorithm

Re-calcultaes the priority of each process

Selects the process with highest priority

Priority is evaluated as:

Considerations

Short processes are favored since execution time

appears at the denominator

Process waiting since long time are favored since

waiting time is on the numerator

(Tattesa + Texec)

Texec

C.Brandolese - 26 -

SRT SRT –– Shortest Remaining TimeShortest Remaining Time

Preemptive version of SPN

Requires remaining execution time estimation

Waiting times: P1=0, P2=1+6, P3=0, P4=9, P5=0


28P5

56P4

44P3

62P2

30P1


P1

0 5 10 15 20

P2

P3

P4

P5

P2

C.Brandolese - 27 -

FSS FSS –– Fair Share SchedulingFair Share Scheduling

Processi in a system sistema

Are often related to each other

Can be grouped according to some criterion

Same user or group

Same application

…

Groups can have different importance

FSS

Prevents that certain groups consume all resources

Distributes resources over groups

According to some criterion

C.Brandolese - 28 -

FSS FSS –– Fair Share SchedulingFair Share Scheduling

Traditional approach

Single scheduler

100% resource equally

shared by all processes

FSS

Two scheduing levels

Fair share

Processes

Resource are assigned

to grops according to

a predefined scheme

Scheduling - Intranet DEIBhome.deib.polimi.it/brandole/pc/17-scheduling.pdfLevels of scheduling...

Documents

Transcript of Scheduling - Intranet DEIBhome.deib.polimi.it/brandole/pc/17-scheduling.pdfLevels of scheduling...