Download Operating Systems

Operating Systems
Definition 1:
OS is the interface between the hardware and the software
environment.
Compilers
Hardware drivers
OS
Applications
OS Kernel
Hardware
Files
Command Language Interpreter
OS is also known as: Kernel, Supervisor, Nucleus, Exec
Outline (1)
I. What is an Operating System?
II. Scheduling
• Process Life Cycle
• Job scheduling
• Process Scheduling
• Turnaround Evaluation
III. Deadlock
• Necessary Conditions
• Prevention
• Avoidance
• Detection & Recovery
Outline (2)
IV. Memory Management
• Addressing
• Management Techniques
V. Synchronization
• Concurrency
• 2- process Synchronization
• Semaphores
• Monitors
OS Characteristics
Desired qualities in an Operating System:
• Reliable
• Efficient
• Provide Protection
• Predictable
• Convenient
Resources
Resource:
Any object which can be allocated within a system:
• Processors
• Files
• Memory
• I/O devices
• Communication network port
Definition 2:
An Operating System manages resources
OS History (1)
0-th generation: (1940 – 1950)
Front panel programming
1-st generation: (1950’s)
Tape contains batches of jobs
2-nd generation: (Early 1960’s)
Multiprogramming
• Time sharing terminals
• Device independent programs
Terms
IO-bound: A job that spends most of its time doing I/O
CPU-bound: A job that spends most of its time computing
on the CPU
Multiprogramming: Rather than executing one job at a
time, computer interleaves execution of multiple jobs
Multiprocessing: Executing of jobs on multiple, rather
than just one, processor
OS History(2)
3-rd generation: (Mid 1960’s - mid 1970’s)
• General purpose OS (Univac, IBM/360, etc.)
• Job control Language (JCL)
• OS is software layer forming a virtual machine
• OS is one big monolithic assembly program
• Supervisor /User modes
• Multiprocessing
OS History(3)
4-rd generation: (Since mid 1970’s)
•OS written in high level languages
•Multiprocessing
•OS = kernel+user processes for maintenance
•OS runs on PC’s Minicomputers
•Hardware platform portable OS’s (Mach, OSF)
•Networking support
•Virtual memory
•GUI supported on bitmapped display
OS History(4)
Future generations:
•
OS constructed using object-oriented paradigm?
•
One OS controls multiple, networked computers?
•
Notion of local vs. remote file vanishes?
•
Increases network access security?
•
Lightweight threads permit efficient concurrent
programming?
•
Software piracy protection?
 Floating network licenses
•
Auto update of OS, application releases over a network?
What pieces Make up an OS?
•Resource managers, for:
• CPU (decides how to share CPU among processes)
•Memory (decides how to share memory)
•Disk (decides how to share disk)
• File system (creates logical file system, addressable by symbolic name,
on top of disk sectors and blocks)
• Interrupt handlers and device drivers
• Command interpreter (or command shell)
• I/O routines
• Network protocols (to communicate with other machine sover network)
What happens with a computer is
powered On? (1)
1. All integrated Circuits receive a reset signal
2. CPU begins fetching instructions at a pre-assigned
physical address in memory. This address is the entry point
of a bootstrap program in Read Only Memory (ROM)
3. Bootstrap program will:
•
Optionally perform self-test (e.g., write, then read all
memory locations)
(Contd…)
What happens with a computer is
powered On? (2)
•
Loads OS from a pre-assigned disk address into
memory
•
Transfer control to a pre-assigned physical address in
memory, which is the entry point of the OS
4. OS now begins execution. On DOS, OS prints a command
prompt. On UNIX, OS (eventually) starts a login process.
(More on this at end of course.)
“Bootstrap” refers to tiny program in ROM telling computer
how to load entire OS.
(What happens on a diskless workstation?)
What Does the CPU Do When It
Has No Programs To Run?
The CPU always executes instructions.
The CPU will execute a tiny loop, possibly containing the
“no-op” instruction, when no useful work needs execution.
An interrupt (e.g., from keyboard, disk, network) will cause
the CPU to be reassigned to execute useful work.
Can a High Level Programming
Language be Used to Write an OS?
Yes, if it:
• Allows processor and I/O space registers to be bound to program
variables
• Allows manipulations of bits within a program variable (AND, OR,
SHIFT)
• Has a facility to embed assembly code within source code
• Example: every machine uses different instructions to input, output data
to disk and console
• Allows separate compilation
C language was an innovation in its time to meet these requirements
Process Life Cycle
Complete
Run
Submit
Hold
Ready
Wait
Job Scheduler
Process Scheduler
Part 2:
Scheduling
Definitions (1)
Concurrent process execution can be:
• Interleaved, or
• Physically simultaneous
Interleaved:
Multiprogramming on uniprocessor
Physically simultaneous:
Uni – or multiprogramming on multiprocessor
(Contd…)
Definitions (2)
Process, thread, or task:
Schedulable unit of computation
Granularity:
Process “size” or computation to communication ratio
• Too small: excessive overhead
• Too large: less concurrency
Scheduling Terms (1)
The OS looks at your program as a process
Program: Executable code stored on disk (static)
Process: Executable code (in memory0 that is or is awaiting
execution (dynamic)
(Contd…)
Scheduling Terms (2)
Stack
Heap
Uninitialized data
Initialized R/W data
Initialized read-only data
Text
Read from disk when
process starts
Process Life Cycle (1)
Process Scheduler
Run
Job Scheduler
Hold
Ready
Blocked
(Contd…)
Process Life Cycle (2)
Job Scheduler: Most popular techniques:
• FIFO
: First in, first out
• SJF
: Shortest job first
Process scheduler:
Most popular technique is Round Robin
Give each process one time slice in turn until complete
Dark square contains fixed, maximum number of processes
Job Scheduling:
SJF: Shortest Job First
Scheduling based on estimated run time.
(Estimating run time is, however, normally impossible!)
• Usually non-preemptive
• Compared to FIFO:
Reduces average waiting time
Increases variance
• Favors short jobs over long jobs
• Tends to:
Reduce number of jobs waiting, but
Increase turnaround time of long jobs
Job and Process Scheduling
Scheduling objectives
• Be fair to all waiting jobs or processes
(Does every job eventually run?)
• Maximize throughput
(# jobs completed/time interval)
• Minimize turnaround time
(delay from when job starts until it finishes execution)
•Balance resource usage
(CPU, memory disk all utilized 100%)
•Give preference to processes holding key resources
•Enforce priorities
Turnaround Time
Let:
N be number of jobs
Ai be arrival time of i-th job
Fi be finish time of i-th job
Ti be turnaround time of i-th job
T be turnaround time, averaged over all N jobs
Turnaround time:
Ti = Fi – Ai
T=  Ti / N
Weighted Turnaround takes into account the run time of each job.
Scheduling Example 1
Assume:
Batch Processing
No I/O or Memory Constraints
Job
Arrives Run Time
1
10.0
2.0
2
10.1
1.0
3
10.25
0.25
When would jobs finish using:
1.
FIFO ?
2.
SJF ?
Example 1 – FIFO Solution
Job
Arrives
Start
Finish
Turnaround
1
10.0
10.0
12.0
2.0
2
10.1
12.0
13.0
2.9
3
10.25
13.0
13.25
3.0
======
7.9
Average Turnaround time T = 2.63
Example 1 – SJF Solution
Job
Arrives
Start
Finish
Turnaround
1
10.0
10.0
12.0
2.0
2
10.1
12.25
13.25
3.15
3
10.25
12.0
12.25
2.0
======
7.15
Average Turnaround time T = 2.38
Weighted Turnaround Time
Weighted Turnaround time (Wi)
Wi =
Ti / Ri
Note that Wi must be equal or exceed 1
Average weighted Turnaround time W
W =  Wi / N
Weighted Turnaround takes into account the run time of each job.
Scheduling Example 2 (1)
Assume:
•
Multiprogramming
•
FIFO Job Scheduling
•
Processor Sharing Process Scheduling
(Contd…)
Scheduling Example 2 (2)
Job
Arrives
Run Time
1
10.0
0.3
2
10.2
0.5
3
10.4
0.1
4
10.5
0.4
5
10.8
0.1
Ready / Running
Example 2 (Cont…)
Time Event
10.0
10.2
1 A,S
2 A,S
10.4
#Jobs
Headway
1
0.2
2
0.1
10.5
1F
3 A,S
4 A,S
2
0.05
10.65
3F
3
0.05
10.8
5 A,S
2
0.075
11.1
5F
3
0.1
11.35
11.40
2F
4F
2
1
0.125
0.05
Time Left
1
1
2
2
3
2
3
4
2
4
2
4
5
2
4
4
0.3
0.1
0.5
0.4
0.1
0.35
0.05
0.4
0.3
0.35
0.225
0.275
0.1
0.125
0.175
0.05
T and W for Example 2
Job
1
2
3
4
5
Run
0.3
0.5
0.1
0.4
0.1
===
1.4
Arrival
10.0
10.2
10.4
10.5
10.8
Finish
10.4
11.35
10.65
11.4
11.1
Ti
Wi
0.4
1.33
1.15 2.3
0.25 2.5
0.9 2.25
0.3
3.0
===== =====
3.0
11.38
T = 0.6
W = 2.276
Check:
Because CPU was never idle, 1.4 + 10.0 must equal time
of last event (11.4)