1 Operating Systems Chapter 6
Mar 28, 2015
1
Operating Systems
Chapter 6
2
What is an operating system? A program that runs on the hardware and supports
Resource Abstraction Resource Sharing
Abstracts and standardises the interface to the user across different type of hardware
Virtual machine hides the messy details witch must be performed.
Manages the hardware resources Each program gets time with the resource Each program gets space on the resource
3
Introduction
The aims of an operating system are: User convenience System performance
Number of requests serviced per unit time, etc
4
Introduction Fundamental tasks of an operation system
Management of Programs Organize their execution by sharing the CPU Ensure good user service and efficient use
Management of Resources Efficient allocation/de-allocation without constraining user
programs Security and Protection
Ensure absence of interference with programs and resources by entities within and outside the operating system
5
Operating Systems Application programs needs to access the
devices connected to a computer. Operation System: (system program – slide-19, chapter
2). is a software layer between the hardware and the
user. provides a consistent application program interface
(API). first program that runs when the computer boots up. is a program that is always running when the
machine is on.
6
Main functions of an operating system
1. User/computer interface:• Provides an interface between the
user and the computer
2. Resource manager: • manages all computer’s resources.
• Process manager• Memory manager• Device manager• File manager, etc.
7
Operation System
User command interface
Resource management
Process Manager
Memory manager
Device Manager
File manager
Network manager
A model of an operation System
8
Operating system as a user/computer interface A user command such as open, save or
print would correspond a sequence of machine-code instructions.
The user does not need to provide these sequences of instructions.
Operating system translates these commands to a machine-code instructions.
9
Operating system as a resource manager
Resource management
Process Manager:• Next program to be executed? • Time to be given to each program?
Memory manager:• Best use of the memory to run as many programs as possible
I/O Device (e.g.printer) Manager:• Which program should use a particular I/O
device?Network manager:
• which computer should execute a particular program?
10
Type of operating systems Multi-programming
Operating system can handle several programs at once.
Time-sharing Operating system allows many user to
share the same computer and interact with it.
Or, in case of a single-user computer (e.g. PC), the user can work on several programs at the same time.
11
How the operating system get started? Main memory has a small section of
permanent read only memory (ROM)
ROM contains a program, bootstrap.
At the start the CPU runs bootstrap. Which directs the CPU to load the operation system from disk and transfer control to it.
12
OperatingSystem
Main memory
Bootstrapprogram
Main memory
BootstrapProgram
Operating System
Disk storageROM
ROM
RAM
RAM
13
Operating system as a process manager Coordinates the occupation of the main
memory by different processes and their data.
At any time the operation system may be dealing with many processes.
e.g. a process may be executed or allowed to wait in main memory, or swapped out of the main memory.
14
Processes
Definition of a process Process Scheduling Operations on Processes Cooperating Processes
15
What is a process
Process – a program in execution; process execution must progress in sequential fashion.
A process includes: program counter stack data section heap
16
Process State
As a process executes, it changes state
new: The process is being created.
running: Instructions are being executed.
waiting: The process is waiting for some event to occur.
ready: The process is waiting to be assigned to a process.
terminated: The process has finished execution.
17
Process Control Block (PCB)
Information associated with each process. Identifier Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information
18
CPU Switch From Process to Process
The PCB is saved when a process is removed from the CPU and another process takes its place (context switch).
19
Process Scheduling Queues
Job queue – set of all processes in the system.
Ready queue – set of all processes residing in main memory, ready and waiting to execute.
Device queues – set of processes waiting for an I/O device.
Process migration between the various queues.
20
Schedulers
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.
21
Medium Term Scheduling
Time sharing Operating systems may introduce a medium term scheduler
Removes processes from memory (and thus CPU contention) to reduce the degree of multiprogramming – swapping
Swapping may be needed to improve the process mix or to free up memory if it has become overcommitted
22
Intermediate queue
Job queue
CPU
I/O
I/O
I/O
I/O
ReadyqueueProcess
request
End
23
Scheduling Criteria
CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their
execution per time unit Turnaround time – amount of time to execute a
particular process waiting to get into memory + waiting in the ready queue
+ executing on the CPU + I/O Waiting time – amount of time a process has been
waiting in the ready queue Response time – amount of time it takes from when
a request was submitted until the first response is produced,
24
Optimization Criteria
Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time In most cases we optimize the average
measure
25
Scheduling AlgorithmsFirst-Come, First-Served (FCFS)
Process Burst TimeP1 24
P2 3 P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 300
CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.
26
FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order P2 , P3 , P1 The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. Average waiting time is generally not minimal and may
vary substantially if the process CPU-burst times vary greatly
P1P3P2
63 300
27
FCFS Scheduling (Cont.)
FCFS is non-preemptive Not good for time sharing systems where where each
user needs to get a share of the CPU at regular intervals
Short process(I/O bound) wait for one long CPU-bound process to complete a CPU burst before they get a turn lowers CPU and device utilization
1. I/O bound processes complete their burst and enter ready queue – I/O devices idle and I/O bound processes waiting
2. CPU bound process completes CPU burst and moves to I/O device
3. I/O bound processes all quickly complete their CPU bursts and enter I/O queue – now CPU is idle
4. CPU bound completes I/O and executes on CPU; back to step 1
28
Shortest-Job-First (SJR) Scheduling
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time (on a tie use FCFS)
Two schemes: nonpreemptive – once CPU given to the process it cannot be
preempted until completes its CPU burst. preemptive – if a new process arrives with CPU burst length
less than remaining time of current executing process, preempt. This scheme is know as the shortest-Remaining-Time-First
(SRTF).
SJF is optimal – gives minimum average waiting time for a given set of processes.
29
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4 SJF (non-preemptive)
Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Example of Non-Preemptive SJF
P1 P3 P2
73 160
P4
8 12
30
Example of Preemptive SJFProcess Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (preemptive)
Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3P2
42 110
P4
5 7
P2 P1
16
31
Priority Scheduling A priority number (integer) is associated with each
process The CPU is allocated to the process with the highest
priority (smallest integer highest priority). Can be preemptive (compares priority of process
that has arrived at the ready queue with priority of currently running process) or non-preemptive (put at the head of the ready queue)
SJF is a priority scheduling where priority is the predicted next CPU burst time.
Problem Starvation – low priority processes may never execute.
Solution Aging – as time progresses increase the priority of the process.
32
Round Robin (RR) Each process gets a small unit of CPU time
(time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.
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.
33
Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24 The Gantt chart is:
Typically, higher average turnaround than SJF, but better response.
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162
34
Memory Management When a process is executed it has to be
in main memory as the main memory can be accessed quicker.
An efficient use of the main memory is an important task of the operation system.
Different memory management techniques are used for this purpose.
35
Memory partition
How processes are arranged in the main memory before been executed?
Fixed-sized partitions Variable-sized partitions
36
Fixed-sized partitions OS 8M
8M
8M
8M
8M
37
Variable-sized partitions OS 8M
2M
4M
8M
18M
38
Swapping I/O operations are slow If a running process requires an I/O
operation. The CPU will move to another process in the main memory.
Suppose the main memory is full of processes waiting on I/O.
CPU becomes idle To solve this problem Swapping
technique is used.
39
Operation System
Operation System
disk
Long-termqueue
Long-termqueue
Medium-term
Completedprocesses
Completedprocesses
No Swapping
With Swapping
Main memory
Main memory
40
os os
P1
os
P1
p2
os
P1
p2
p3
os
P1
P3
os
P1
P4
P3
os
P4
p3
os
p2
P4
p3
adcb
ehgf
41
Fragmentation Memory is divided into partitions Each partition has a different size Processes are allocated space and later freed After a while memory will be full of small holes!
No free space large enough for a new process even though there is enough free memory in total
If we allow free space within a partition we have internal fragmentation
Fragmentation: External fragmentation = unused space between
partitions Internal fragmentation = unused space within partitions
42
Problems with swapping Swapped process are I/O output
processes. I/O processes are slower. The swapping process is slow as
well. Solution:
Reduce the amount of codes that needs to be swapped.
Paging
43
Paging
A program is divided into small fixed-sized chunks(pages).
Main memory is divided into small fixed-sized chunks (frames).
A page is stored in one frame.
A program is stored in a set of frames. These frames do not need to be continuous.
44
disk
Process A
page 0page 1page 2page 3
In use
In use
In use
disk
Process A
page 0page 1page 2page 3
In use
In use
In use page 3
of A
page 2 of A
page 1 of A
page 0 of A
141518
13
A- page table
13
14
1516
17
18
19
20
13
14
1516
17
18
19
20
45
Logical and physical address
disk
Process A
page 0page 1page 2page 3
In use
In use
In use page 3
of A
page 2 of A
page 1 of A
page 0 of A
141518
13
A- page table
1314
1516
17
18
19
20
Page 1
I...
J(30)
Logical address(J)1:30
Physical address(J)
14:30
46
simple paging is not efficient Better than fixed and variable-sized
partitions. OS - loads all pages of a process in the
main memory. However, not all pages of a process
need to be in the main memory in order to be executed.
OS - can still execute a process if only some of the pages are loaded
Demand paging.
47
Demand paging Operating system – loads a page only when it
is required
No swapping in or out of unused pages is needed.
Better use of memory.
CPU can access only a number of pages of a process at one time.
Then asks for more pages to be loaded.
48
Virtual memory Demand paging gives rise the concept of
virtual memory. Only a small part of a process needs to be
in main memory at one time. Programs which require bigger memory
that main memory can still be executed. Impression of a bigger computer memory. This concept of the main memory is called
virtual memory. Demand paging and virtual memory are
widely used in today’s operation systems (wind-2000, XP).
49
InterruptsDefinition of ‘Interrupt’
Event that disrupts the normal execution of a program and causes the execution of special instructions
50
Interrupts
Interrupt
Program
time t
51
Interrupts
Program
time t
52
Interrupts
Program
Interrupt Service Routine
Interrupt
Program
time t
53
Interrupts
Interrupt
Program
time t
mov R1, cent mul R1, 9 div R1, 5 add R1, 32 mov fahr, R1
fahr= (cent * ) +329
5
54
Interrupts
Program
Interrupt Service Routine
Interrupt
Program
time t
mov R1, cent mul R1, 9
55
Interrupts
ProgramSave
Context Interrupt Service Routine
Restore Context
Interrupt
Program
time t
mov R1, cent mul R1, 9
56
Interrupts
ProgramSave
Context Interrupt Service Routine
Restore Context
Interrupt
Program
time t
mov R1, cent mul R1, 9
eg push R1 eg pop R1
57
I/O devices Called peripherals:
Keyboard Mouse Speakers Monitor scanner Printer Disk drive CD-drive.
OS – manages all I/O operations and devices
58
OS - I/O management
There are four main I/O operations. Control: tell the system to perform
some action (e.g. rewind tape). Test: check the status of the device Read: read data from the device Write write data to the device.
59
I/O modules
System bus
CPUMain
memory
I/O module I/O module
I/Odevice
I/Odevice
60
Advantages of I/O modules They allow the CPU to view a wide range of
devices in a simple-minded format CPU does not need to know details of timing,
format, or electronic mechanics. CPU only needs to function in terms of a
simple read and write commands. They help the CPU to work more efficiently They are 3 ways in which I/O modules can
work Programmed I/O Interrupt-driven I/O Direct memory access.
61
Programmed I/O The CPU controls I/O device directly Via the
I/O modules.
The CPU sends an I/O command the I/O module.
And waits until the I/O operation is completed before sending another I/O command.
The performance is poor as the CPU spends too much time waiting the I/O device.
62
Programmed I/OIssue Read to
I/O module
Check status
Read word from I/O module
Write word To memory
done
yes
NO
Next instruction
Ready
63
Interrupt-driven I/O
The CPU issues a command to the I/O module and then gets on with executing other instructions.
The I/O module interrupts the CPU when it is ready to exchange data with the CPU.
The CPU then executes the data transfer.
Most computer have interrupt lines to detect and record the arrival of an interrupt request.
64
Interrupt-driven I/OIssue Read to
I/O module
Check status
Read word from I/O module
Write word To memory
done
yes
NO
Next instruction
Ready
CPU goes to do Other things
When the statusIs ready the I/O module sendsAn interrupt-signal
65
How does I/O module send an interrupt to the CPU? I/O module is linked to the control bus.
I/O module reads a word from the I/O device.
Puts the word in the data register which is linked to data bus.
Sends a interrupt signal to the CPU via control bus.
66
How does CPU know Interrupt-signal?
The CPU executes an instruction cycle. An interrupt stage is added at the end of the
cycle. At the end of an instruction cycle the CPU checks
for interrupts. The CPU hardware has a wire, interrupt-request
line that the CPU can sense. If no interrupt the CPU carries on executing next
instruction. Otherwise, it updates the process control block,
save it. Then process the interrupt. Resume the execution of the interrupted process.
67
How does CPU process interrupts? Interrupt detection. CPU executes Interrupt-handler program. Interrupt-handler program makes use of
the process control block save earlier. Interrupt-handler decides what to do with
interrupt. Then asks the CPU to resume the
execution interrupted.
68
Disadvantages of Interrupt-driven I/O CPU is responsible for managing I/O
data transfer. Every transferred word must go
through the CPU. Devices with large transfer, e.g. disk
drive, the CPU wastes time dealing with data transfer.
Solution: Direct-memory-access(DMA).
69
Direct-memory-access - DMA Special-purpose processor. Handles data transfer. CPU issues to the DMA:
starting address in main memory to read/write to. Starting address in the I/O device to read/write to. The number of words to be transferred.
DMA transfers data without intervention from the CPU.
DMA sends interrupt to the CPU when transfer is completed.
70
DMA/CPU - bus system DMA take care data transfer. CPU free to do other jobs. However, they can not use the bus
at the same time. DMA can use the bus only when
the CPU is not using it. Some times it has to force to CPU
to free the bus, cycles stealing.
71
DMA/CPU
System bus
CPUMain
memory
I/O module
I/Odevice
DMA
72
Summery OS- memory manager
Fixed-sized partition: waist of memory. Variable-sized partition: fragmentation. Swapping. Time wasted in swapping the whole
process Simple paging: process divided into pages and
loaded into main memory(divided into frames). Demand paging: only the required pages are loaded to
main memory. OS- I/O manager
Programmed I/O: CPU waste waiting for I/O operation. Interrupt-driven I/O: CPU responsible for data transfer. DMA: takes care of data transfer instead the CPU.