operating systems and computer networks process scheduling … · operating systems and computer...

Operating Systems and Computer Networks

Process Scheduling Part 2

Prof. Dr.-Ing. Axel HungerAlexander Maxeiner, M.Sc.

Institute of Computer EngineeringFaculty of Engineering

University Duisburg-Essen

Dr.-Ing. Pascal A. Klein

[email protected]

Dr.-Ing. Pascal A. KleinUniversity Duisburg-Essen

2Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering OSCN – Process Scheduling

Process Management|

Time Sharing|

Synchronization of Processes

|

Scheduling Classification

|

Scheduling Algorithms

Agenda



A program

• resides in local memory

• partially or entirely transferred to main memory upon execution

The program can be comprised of:

• Programs, sub-routines

• Data

• Instruction pointers

• Stack pointers

• CPU states

• Register contents

Program Execution



Several processes on a single processor

OS needs

• to monitor free resources

• to open tasks

• to allocate CPU time

• change between tasks

What happens if

• another process waits for data (not uses processor power idle)

Process Management



Process Management



Processes are split up into segments

Each segment is given a specific amount of time at the CPU

How much (amount of time) and when (it is given the time) depends on the implemented algorithm

Multitasking: time-multiplexing, only one CPU (or Core) is occupied with several tasks Processor power is allocated within a certain time frame

Time Sharing



Problem 1: How to change between processes?

Problem 2: How to optimize efficiency in the use of CPU?

Time Sharing



Difficult to calculate depends on the situation

• E.g.: 5 processes need 20% of CPU – perfect?

• Who waits, when?

Probabilistic view of CPU Utilization:

CPU utilization = 1 – pn

n – number of processes

pn – probability that all processes wait for I/O (CPU idle time)

Careful: independancy of processes assumed!

CPU Utilization



Time Sharing – Time Wheel Model

P8 P1

P2

P3

P4

P5P6

P7

Context change T

t

P1: Terminal IO (Keyboard or mouse)P2: Account P3: PrintP4..P7

User 1, 2, 3, 4



Time needed to switch can be between μs and ns

Address range of an old process has to be saved and updatedby complex memory management

Process change

tProcess1 OS Process2 OS Process1 OS etc.



When a process/task changes: stack pointer, registers, etc. must be changed

Two methods:

1. Complete set of user data / process data is stored in a private memory space

2. Context change by pointers only

• Program state

• Pointers point to start of memory slot

Process change



Entry in process table

Data structure with information to manage scheduling

Needed for correct and efficient process management

Contains:

• Process identification data (unique identifier)

o e.g.: process id, parent process, user identifier, etc.

• Process state data (status of a process)

o e.g.: content of CPU general-purpose registers, CPU process status, stack and frame pointers, etc.

• Process control data (to manage process itself)

o e.g.: process scheduling state, Process structuring information, Interprocess communication information, Process Privileges, Process Number (PID), Program Counter (PC), CPU Registers, Memory Management Information, etc.

Process Control Block - PCB



Implementation of Time Sharing

Scheduler

I/O-readlines

CPU

Reentrant

Code Computer / OS

PC2

PC1

M2

M1

Peripheral memory

Virtual Machine

(virtual parallel)

Task 1

Task 2



Implementation of Time Sharing

Save PCBof process 1

Load PCBof process 2

Save PCBof process 2

Load PCBof process 1 RUNNING

RUNNING Scheduler or Interrupt

Process 1

READY

Process 2

RUNNING

READY

SCHEDULER OR INTERRUPT



A process queue example

Start

end

PCBi

Register,

etc.

PCBk

Register,

etc.

PCBa

Register,

etc.

CPU-Queue

NIL

Pointer in the Queue

Start

end

Disk QueuePCBb

Register,

etc.

PCBx

Register,

etc.

NIL



When to change between processes?

a) by cyclic flow control (polling), i.e. processes call for data, program-controlled

b) time-controlled in certain given intervals with a real-time clock

c) by request (interrupt) via the technical process event-controlled.

Synchronization of Processes



Drawback: CPU is always busy with polling, though it is often idle due to I/O operations.

Cyclic Flow Control (Polling)

e.g.:

5 processes:

4 idle, 1 busy.

PA

PB

C/D2

C

D

Px : Process looking forData polling



If time intervals too short number of data swapping increases until a process is finished decrease of overall efficiency

If time intervals too long finished process will leave the CPU with idle time

Time-controlled

idle time?Time A

PA

Px : Process execution

Time A

PB



Process requests (triggered by the technical process):

• can be announced at any time

• have high priority (importance) by

opriority in execution and/or

oblocking other requests

also leads to context change

Request (Interrupt)

Interrupt?

Process Scheduling

PA

ExecuteInterrupt

Px : Process execution



A background process is interrupted by two interrupts

Request (Interrupt) – Example

BGP

IRS1

IRS2

IRS1

BGP

Interrupt

BGP: Backgroundprocess

IRSi: Interrupt Subroutine i

Ii: Interrupt i

Priority

Time

I1 I2



Problem: Multiple processes or threads need CPU resources at the same time

OS chooses the next steps for the upcoming queued processes (to be processed)

Three different types of environments

• Batch

• Interactive

• Real time

Why Scheduling?



The scheduler needs to be planned with respect to the following criteria:

• Use/efficiency of CPU-Utilization (40 % - 90 %)

• Throughput (completed processes / time unit)

• Cycle time (time from input to output of the results)

• Waiting time (Waiting in the CPU Waiting Queue)

• Response time (In interactive systems: Time to reaction)

• Turnaround time (The time until a job is finished)

Planning Criteria



Non-Preemptive scheduling:

• process is picked and started until it blocks (either on I/O or waiting state) or voluntarily releases the CPU

• process will not be forcibly suspended until a higher priority interrupt occurs

Preemptive scheduling:

• process starts and runs for a fixed amount of time

• If process still runs at end of time cycle, it is suspended

• requires a clock interrupt occuring at the end of every time interval to give control of CPU back to scheduler

Non-Preemptive vs preemptive Scheduling



Big systems working on steady bulky jobs

Non-preemptive is fine

no users waiting for quick results

Long processing acceptable

Reducing of process switches

Planning in batch systems

Main Memory

CPU

Disk

• Throughput• Turnaround Time• CPU Utilization



PC, Webserver, ATM ...

Preemptive is needed

More than one process running all the time

All users are in big hurry...

Planning in Interactive Systems

• Response Time• Proportionality – meets

users‘ expectations



Systems which keep in real time constraints

immediate action needed

Preemptive handling not needed

• processes need to be securely finished

• processes will not block for long

Only specialized software used

Application runs well-known and specialized programmesstrongly compatible with the application

Real Time Systems

• Meeting Deadlines• Predictability



Scheduling Goals

• Meeting Deadlines• Predictability

• Response Time• Proportionality – meets

users‘ expectations

• Throughput• Turnaround Time• CPU Utilization

• Fairness(fair amount of share

• Policy enforcement(strategy enforcement)

• Balance(all parts of system shouldbe busy)



Depend on certain events:

• scheduler selects next process queue of ready processes from the OS

Scheduling decisions can be made:

• in transition from running to blocked state (waiting for I/O – 4)

• in transition from running to ready state (Interrupt – 2)

• in transition from blocked to ready state (I/O end – 5)

• Upon termination of a process

Scheduler

BlockedRunning

Ready

4

23

5

Existent

1

6When?



Fairness

• Each process needs fair share of CPU time

• comparable processes should get comparable service

Utilization of resources

• keep all parts of the system busy

Algorithm must be executed efficiently

• No wasting of resources

• Minimized overhead

Requirements of Scheduling Algorithms



Static (a priori) Scheduling Algorithms Scheduling decisions take place at a fixed time interval

Process coordination:

• Scheduling is planned before the programs run

• Input of a scheduling algorithm is a set of processes which will be considered for scheduling

• Processes arriving the scheduler during the runtime of a process, stored for next cycle

• Scheduled processes run until all of them are done and/or until the given time is over.

Conditions for static algorithms:

• No dynamic process creation during the program

• Event-driven processes can be incorporated only if the time conditions are schedulable

Classsifications of Scheduling Algorithms



Dynamic Scheduling Algorithms

Coordination of processes takes place while the processes are running

Times of updates:

• at fixed time intervals

• as soon as a new process is created

• as soon as a process ends

Advantages and disadvantages

• event-driven processes must be coordinated

• decision making during scheduler operation costs time

• Lack of efficiency: heuristics are required!




Algorithms for complex process models

Complex process models:

• Interruptible, non-interruptible processes

interruptible process at any or only at certain times?

• Cyclic, non-cyclic processes

• Switching to another processor




Scheduler Algorithms



First process that arrives the scheduler will be the next in line no Interrupts

Example:

• P1: τ1 = 300 ms waiting period 0 ms



Scheduler: First Come, First Serve

Average Waiting Time:(0 + 300 + 340) / 3

= 640 / 3= 213.3 ms

Min Average Waiting Time:P3, P2, P1 (0 + 32 + 72) / 3 = 104 / 3= 34.67 ms

0

P1 (300ms)

P2(40ms)

300 340 372

P3(32ms)

P1 (300ms)

P2(40ms)

0 3727232

P3(32ms)



First process that arrives the scheduler will be the next in line no Interrupts

Example:




Scheduler: First Come, First Serve

Min Average Turnaround Time:P3, P2, P1 (32 + 72 + 372) / 3 = 476 / 3= 158.67 ms

0

P1 (300ms)

P2(40ms)

300 340 372

P3(32ms)

0

P1 (300ms)

P2(40ms)

3727232

P3(32ms)

Average Turnaround Time:(300 + 340 + 372) / 3

= 1012 / 3= 337.3 ms



At Scheduler Decision (which process is next): Choose the job that needs the least amount of time Interruption possible when shorter job arrives

Provides optimum in terms of waiting time!

In Short-term planning:

• Not applicable, since next block computation time is not known!

Critical process with a long runtime? Stalled until faster processes are computed

Scheduler: Shortest Job First




Assumption: Time required for next block is known

P1: τ1 = 8 ms, arrival time: 0ms




Average Waiting Time:[0 + 0 + (5-3) + (10-1) + (17-2)] / 4

= 26 / 4= 6.5 ms

0 5 10 15 20 25 30

Arrival:

0

1

2

3

8

(8)

5

(4)

11

(9)

8

(5)

P1 (1ms) P2 (4ms) P1 (7ms)P4 (5ms) P3 (9ms)

P1

P2

P3

P4




Assumption: Time required for next block is known





Average Waiting Time:[0 + 0 + (5-3) + (10-1) + (17-2)] / 4

= 26 / 4= 6.5 ms

Without interruption:SJF: (0 + 7 + 9 + 15) / 4

= 7.75 ms

FCFS: (0 + 7 + 10 + 18) / 4= 8.75 ms

0 5 10 15 20 25 30

Arrival:

0

1

2

3

8

(8)

5

(4)

11

(9)

8

(5)

P1

P2

P3

P4

P1 (8ms) P2 (4ms) P4 (5ms)P3 (9ms)FCFS

P1 (8ms) P2 (4ms) P4 (5ms) P3 (9ms)SJF



Chooses process with highest priority

If another process with higher priority arrives Sending Interrupt active task is replaced by the new one

Problem: Low-priority processes will wait a long time

Solution: Increasing priority of waiting task

Scheduler: Priority Scheduling



Switches between tasks after fixed interval of time New processes are added complete ones are removed Guarantees fair amount of time

Problems: time waste due to change of processes Completion time of long runtime processes (more active tasks, higher completion time)

Scheduler: Round-robin Scheduler (RR)

0 5 10 15 20 25 30

Arrival:

0

1

2

3

8

(8)

5

(4)

11

(9)

8

(5)

P1

P2

P3

P4

P1

P2

P1

P2

P1

P3

P2

P1

P3P4

P2

P1

P3

P4

P1

P3

P4

P1

P3

P4

P1

P3

P4

P3 P3 P3



Time sharing as background job (RR) plus Interrupt on demand based on priorities

Within a priority level, the allocation shall be made according to RR

After an appropriate waiting time processes can move up in priority – Aging

Scheduler: Combination of

several allocation levels



Two Processors: I & II

5 processes have to be handled

Scheduling: Priority Scheduling A running process will be interrupted if another process with higher priority arrives

If one of the CPUs is unoccupied and a higher priority task needs to be scheduled Free Core will be assigned to the task

Priorities and Arrival times for P1..P5:

• Priority 1: P2 (t=0, τ = 7 ms)

• Priority 2: P1 (t=0 , τ = 3 ms), P3 (t=1 , τ = 10 ms)

• Priority 3: P4 (t=5 , τ = 6 ms), P5 (t=6 , τ = 9 ms)

Example



At t=0ms P1, P2 arrive:

• Priority of P1 is higher, both processors are available => Assign P1 -> Processor I

• Assign P2 -> Processor II

Example

Priorities and Arrival times for P1..P5: •Priority 1: P2 (t=0, τ = 7 ms)•Priority 2: P1 (t=0 , τ = 3 ms), P3 (t=1 , τ = 10 ms)•Priority 3: P4 (t=5 , τ = 6 ms), P5 (t=6 , τ = 9 ms)

0 5 10 15 20 25 30

P1

P2

P3

P4

P5

Processor IP1

Processor IIP2




• Priority of P1 is higher, both processors are available => Assign P1 -> Processor I


At t=1ms P3 arrives:

• Priority P3 > P2

• P2 will be interrupted by P3 (on Processor II)

Example


0 5 10 15 20 25 30

P1

P2

P3

P4

P5

Processor IP1

Processor IIP2P3




• Priority of P1 is higher, both Processors are available => Assign P1 -> Processor I





Example


0 5 10 15 20 25 30

P1

P2

P3

P4

P5

Processor IP1

Processor IIP2P3









• P2 continues (on Processor I) after P1 finished

Example


0 5 10 15 20 25 30

P1

P2

P3

P4

P5

Processor IP1P2

Processor IIP2P3









• P2 continues (on Processor I) after P1 finished

Example

0 5 10 15 20 25 30

P4

P5


P1

P2

P3

Processor IP1P2

Processor IIP2P3



At t=5ms P4 arrives

• Priority P4 > P1 > P2


• P2 will be interrupted by P4 (on Processor I)

Example

0 5 10 15 20 25 30

P4

P5


P1

P2

P3

Processor IP1P2P4

Processor IIP2P3



At t=6ms P5 arrives

• P4, P5 > P1

• P3 > P2

• P1 > P2

• P3 will interrupted by P5 (on Processor II)

Example

0 5 10 15 20 25 30

P4

P5


P1

P2

P3

Processor IP1P2P4

Processor IIP2P3P5



At t=6ms P5 arrives

• P4, P5 > P1

• P3 > P2

• P1 > P2


Example

0 5 10 15 20 25 30

P4

P5


P1

P2

P3

Processor IP1P2P4

Processor IIP2P3P5



After P4 finished, P3 will be continued on Processor I

Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3

Processor IIP2P3P5




Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3

Processor IIP2P3P5




After P5 finished, P2 will be continued on Processor II

Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3

Processor IIP2P3P5P2





P3 finished, Processor I unoccupied

Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3







Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3







P2 finished, Processor II unoccupied

Example

0 5 10 15 20 25 30

P1

P2

P3

P4

P5


Processor IP1P2P4P3




Questions?



Tanenbaum, Andrew S., “Modern Operating Systems”, 3rd edition, Pearson Education Inc, Amsterdam, Netherlands, 2008.

Tanenbaum, Andrew S., “Moderne Bertriebssysteme”, 3rd edition, Pearson Education Inc, Amsterdam, Netherlands, 2009.

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