Operating Systems and Computer Networks Process Scheduling ... V6... · optimal use of scheduling algorithms. Idle times. Alexander Maxeiner, M.Sc. University Duisburg-Essen 7 Prof

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 & Alexander Maxeiner M.Sc.

[email protected]

Alexander Maxeiner, M.Sc.University 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



Modern software consists of more than one process.

If a program is executed, all processes necessary for a certain task are loaded into main memory.

One processor core can only execute one task at a time.

OS needs

• to monitor free resources (cores, cache, etc)

• to open tasks

• to allocate CPU time

• change between tasks

• To keep track of current active tasks

Process Management



Process Management



Problematic are idle times within the CPU. When an operation waits for I/O Data the CPU does nothing.

Longer idle times decrease throughput of the System leading to performance drop.

In case of idle times the OS needs to switch processes in order to keep the CPU busy.

The perfect Computer system has 0% idle time. In reality this is almost impossible, since context changes between processes are also lost time for computing.

Increased throughput can be achieved by planning ahead and optimal use of scheduling algorithms.

Idle times



Processes are split up into segments.

Each segment is part of the entire process and consists of at least one complete operation within the CPU.

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 shared between tasks to execute all tasks semi-parallel.

Segmenting tasks



Problem 1: How to optimize efficiency in the use of CPU?

Problem 2: How to change between processes?

Time Sharing



Difficult to calculate depends on the situation

• E.g.: 5 processes need 20% of the total CPU power within a certain timeframe. Perfect scenario?

• Which process is first, which one is last?

In reality: Difficult for users to estimate CPU demands of processes, therefore difficult to implement time demands.

Number of segments indicator of actual processing expenditure, but only as an approximation.

OS equipped with algorithms to estimate CPU times of processes. Actual processesing time may vary.

CPU Utilization



Actual predetermination not possible, but approximation

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)

Works only on independent processes. Dependent processes are merged into one independent.

All processes share I/O probability. Can be derived from cache-/ register hit rates.

CPU Utilization



CPU utilization = 1 – pn

Theoretically: Infinite amount of processes simultaneously at the CPU -> CPU utilization = 1

Formula ignores time for context changes. Switch between one process and another not instantaneously.

The more processes are lined up the more time for context changes is wasted.

Cap the amount of tasks processed by the CPU & plan ahead.

CPU Utilization



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

OS has to step in between different processes. Time efficient procedures needed.

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

• Pointer targets the actual program state within memory.

• Pointers point to start of memory slot with data.

Memory mustn’t change or task will be corrupted upon resuming

Process change



A PCB is an entry in the process table

Process table contains all information on currently running tasks.

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, Process Privileges, Process Number (PID), Program Counter (PC), CPU Registers, Memory Management Information, etc.

Process Control Block



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



Interrupt - 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

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



Problems with context changes:

Processes are not finished if interrupted by other processes. May lead to huge delays in relevant, but time consuming processes.

The more processes are on hold, the more context changes will occur leading to huge overly-times, decreasing the system efficiency.

Pointer-based context changes block memory. Memory depletion.

Memory stored context changes lead to increased loading times, decreasing overall efficiency.

Context changes



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



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

Cyclic Flow Control

e.g.:

5 processes:

4 idle, 1 busy.

PA

PB

C/D2

C

D

Px : Process looking forData polling

Cyclic Flow Control (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

High priority tasks can be executed fast.

Interrupts can be activated if higher priority

task finished loading data.

Request (Interrupt)

Interrupt?

Process Scheduling

PA

ExecuteInterrupt

Px : Process execution



A background process is interrupted by two interrupts

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 occurring at the end of every time interval to give control of CPU back to scheduler

Scheduling Methods



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 - batch systems

Main Memory

CPU

Disk

• Throughput• Turnaround Time• CPU Utilization



OS in a PC bundles Jobs in Batches

Usually only done in multi-core systems

Batches send to cores for processing

Batch in PC

Main Memory

Core 1


Core 2

Job Batch



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


• 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!

Classsifications of Scheduling Algorithms

Documents

Operating Systems and Computer Networks Process Scheduling ... V6... · optimal use of scheduling algorithms. Idle times. Alexander Maxeiner, M.Sc. University Duisburg-Essen 7 Prof