Operating SystemsDefinition 1:

OS is the interface between the hardware and the software environment.

Compilers

Applications

Command Language Interpreter

Files

Hardware drivers

OS

OS Kernel

Hardware

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

ResourcesResource:

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

Wait

Submit Hold

Complete

Ready

Run

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)

Hold Ready

Run

Blocked

Process Scheduler

Job Scheduler

(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 SchedulingScheduling 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 TimeLet:

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 1Assume:

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 FinishTurnaround

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 FinishTurnaround

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 #Jobs Headway Time Left10.0 1 A,S 1 0.310.2 2 A,S 1 0.2 1 0.1

2 0.510.4 1 F 2 0.1 2 0.4

3 A,S 3 0.110.5 4 A,S 2 0.05 2 0.35

3 0.054 0.4

10.65 3 F 3 0.05 2 0.34 0.35

10.8 5 A,S 2 0.075 2 0.2254 0.2755 0.1

11.1 5 F 3 0.1 2 0.1254 0.175

11.35 2 F 2 0.125 4 0.0511.40 4 F 1 0.05

T and W for Example 2 Job Run Arrival Finish Ti Wi

1 0.3 10.0 10.4 0.4 1.332 0.5 10.2 11.35 1.15 2.33 0.1 10.4 10.65 0.25 2.54 0.4 10.5 11.4 0.9 2.255 0.1 10.8 11.1 0.3 3.0

=== ===== ===== 1.4 3.0 11.38

T = 0.6W = 2.276

Check:Because CPU was never idle, 1.4 + 10.0 must equal time

of last event (11.4)