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ICS 143: Midterm Review
Operating System Concepts – 9th Edition. Silberschatz, Galvin and Gagne ©2013. Modified by Dmitri V. Kalashnikov and Nalini Venkatasubramanian.
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What is an Operating System?
 OS is a program that acts as an intermediary between a user of a
computer and the computer hardware
 Operating system goals:
1.
Execute user programs and make solving user problems easier
2.
Make the computer system convenient to use
3.
Use the computer hardware in an efficient manner
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Four Components of a Computer System
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Computer System Organization
 Computer-system operation

One or more CPUs and device controllers connect through a
common bus providing access to shared memory

I/O and CPU can execute concurrently
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Operating System Views
 OS is a resource allocator

Manages all resources

Decides between conflicting requests for efficient and
fair resource use
 OS is a control program

Controls execution of programs to prevent errors and
improper use of the computer
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Operating System Definition
 No universally accepted definition of OS
 “Everything a vendor ships when you order an operating
system” is a good approximation

But varies wildly
 “The one program running at all times on the computer” is
the kernel.
 Everything else is either

a system program (ships with the operating system) , or

an application program.
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Operating System: Broad Spectrum
 Special purpose and embedded systems
 Real-time systems
 Batch Systems and Multiprogramming
 Timesharing Systems

workstations, servers, minicomputers, timeframes
 Transaction systems

software/hardware combination that supports transaction processing
 Parallel and Distributed Systems

symmetric vs. asymmetric multiprocessing
 Real-time systems

Hard vs. soft realtime
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A View of Operating System Services
 An operating system provides an environment for the execution of
programs.
 It provides certain services to programs and to the users of those
programs.
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Operating System Services
 Services that provide user-interfaces to OS

Program Execution - load program into memory and run it

I/O Operations - since users cannot execute I/O operations directly

File System Manipulation - read, write, create, delete files

Communication - interprocess and intersystem

Error Detection - in hardware, I/O devices, user programs
 Services for providing efficient system operation

Resource Allocation - for simultaneously executing jobs

Accounting - for account billing and usage statistics

Protection - ensure access to system resources is controlled
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System Calls
 Systems calls -- programming interface to the services provided by the OS
 Typically written in a high-level language (C or C++)
 Mostly accessed by programs via a high-level Application Programming
Interface (API) rather than direct system call use
 Three most common APIs are
 Win32 API for Windows,
 POSIX API for POSIX-based systems
including virtually all versions of UNIX, Linux, and Mac OS X,
 Java API for the Java virtual machine (JVM)

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System Call Implementation
 Typically, a number associated with each system call

System-call interface maintains a table indexed according to
these numbers
 The system call interface invokes the intended system call in OS
kernel and returns status of the system call and any return values
 The caller just needs to obey API

Does not need to know how the system call is implemented
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API – System Call – OS Relationship
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Operating System Structure
 General-purpose OS is a very large program

How to implement and structure it?
 Can apply many ideas from software engineering

Software engineering - a separate area in CS

Studies design, development, and maintenance of software
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Operating System Structure (Modules)
 A common approach is to partition OS into modules/components

Each modules is responsible for one (or several) aspect of the
desired functionality

Each module has carefully defined interfaces
 Advantages:

Traditional advantages of modular programming:

Simplifying development and maintenance of computer
programs, etc

Modules can be developed independently from each other
 Disadvantages:

Efficiency can decrease (vs monolithic approach)
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Operating System Structure (contd)
 In general, various ways are used to structure OSes
 Many OS’es don’t have well-defined structures

Not one pure model: Hybrid systems

Combine multiple approaches to address performance, security,
usability needs
 Examples of OS structures:

Simple structure – MS-DOS

More complex structure - UNIX

Layered OS’es

Microkernel OS’es

OS’es with loadable kernel modules
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Process Management
 Process - fundamental concept in OS

Process is a program in execution

Process needs resources - CPU time, memory, files/data and I/O
devices
 OS is responsible for the following process management activities

Process creation and deletion

Process suspension and resumption

Process synchronization and interprocess communication

Process interactions - deadlock detection, avoidance and correction
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Diagram of Process State
 As a process executes, it changes state

new: The process is being created

ready: The process is waiting to be assigned to a processor

running: Instructions are being executed

waiting: The process is waiting for some event to occur

terminated: The process has finished execution
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Process Control Block (PCB)
Each process is represented in OS by PCB
 PCB - info associated with the process
 Also called task control block
 Process state – running, waiting, etc
 Program counter – location of
instruction to next execute
 CPU registers – contents of all process-
centric registers
 CPU scheduling information- priorities,
scheduling queue pointers
 Memory-management information –
memory allocated to the process
 Accounting information – CPU used,
clock time elapsed since start, time
limits
 I/O status information – I/O devices
allocated to process, list of open files
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CPU Switch From Process to Process
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Process Scheduling
 System maintains scheduling queues of processes

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

Processes migrate among the various queues
 Scheduler – component that decides how processes are
selected from these queues for scheduling purposes
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Schedulers
 Long-term scheduler (or job scheduler)

selects which processes should be brought into the ready queue.

invoked very infrequently (seconds, minutes); may be slow.

controls the degree of multiprogramming
 Short-term scheduler (or CPU scheduler)

selects which process should execute next and allocates CPU.

invoked very frequently (milliseconds) - must be very fast
 Medium-term scheduler

swaps out process temporarily

balances load for better throughput
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Process Creation
 Processes are created and deleted dynamically
 Process which creates another process is called a parent process;

the created process is called a child process.
 Result is a tree of processes

e.g. UNIX - processes have dependencies and form a hierarchy.
 Resources required when creating process

CPU time, files, memory, I/O devices etc.
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Process Termination
 Process executes last statement and asks the operating system to delete it
(exit).

Outputs data from child to parent (via wait).

Process’ resources are deallocated by operating system.
 Parent may terminate execution of child processes

Child has exceeded allocated resources

Task assigned to child is no longer required

Parent is exiting

OS does not allow child to continue if parent terminates

Cascading termination
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Threads
 Processes do not share resources well

high context switching overhead
 A thread (or lightweight process)

basic unit of CPU utilization; it consists of:


program counter, register set and stack space
A thread shares the following with peer threads:
–
code section, data section and OS resources (e.g., open files)
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Single and Multithreaded Processes
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Producer-Consumer Problem
 Paradigm for cooperating processes;

producer process produces information that is consumed by a
consumer process.
 Uses a buffer of items that can be filled by producer and emptied by
consumer


Unbounded-buffer places no practical limit on the size of the buffer.
Consumer may need to wait, producer never waits.

Bounded-buffer assumes that there is a fixed buffer size. Consumer
waits for new item, producer waits if buffer is full.
Producer and Consumer must synchronize.
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Interprocess Communication (IPC)
 Mechanism for processes to communicate and synchronize their actions

Via Shared memory

Via Messaging system - processes communicate without resorting to
shared variables.
 Messaging system and shared memory not mutually exclusive 
can be used simultaneously within a single OS or a single process.
 Messaging IPC facility provides two operations.

send(message) - message size can be fixed or variable

receive(message)
 Direct vs. Indirect communication (mailbox).
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CPU Scheduling
 Scheduling Objectives
 Scheduling Criteria
 Scheduling Algorithms
 Multiple Processor Scheduling
 Real-time Scheduling
 Algorithm Evaluation
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Basic Concepts
 Maximum CPU utilization obtained with
multiprogramming

waiting for I/O is wasteful

1 thread will utilize only 1 core
 CPU–I/O Burst Cycle

Process execution consists of:

a cycle of CPU execution

and I/O wait
 CPU burst followed by I/O burst
 CPU burst distribution is of main concern
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CPU Scheduler
 Short-term scheduler

Selects 1 process from the ready queue


then allocates the CPU to it
Queue may be ordered in various ways
 CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4.
Terminates
 Scheduling under 1 and 4 is called nonpreemptive (=cooperative)
 All other scheduling is called preemptive

Process can be interrupted and must release the CPU
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Scheduling Policies
 FCFS (First Come First Serve)

Process that requests the CPU FIRST is allocated the CPU FIRST.
 SJF (Shortest Job First)

Associate with each process the length of its next CPU burst. Use these
lengths to schedule the process with the shortest time.
 Priority
 A priority value (integer) is associated with each process. CPU allocated
to process with highest priority.
 Round Robin

Each process gets a small unit of CPU time
 Multilevel Queue
 Ready queue partitioned into separate queues
 Variation: Multilevel Feedback queues.
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Process Synchronization
 The Critical Section Problem
 Synchronization Hardware
 Semaphores
 Classical Problems of Synchronization
 Monitors
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Background
 Processes can execute concurrently

May be interrupted at any time, partially completing execution

Might need to read and write some shared data
 Concurrent access to shared data may result in data inconsistency

Reading and writing to shared data concurrently can result in
unpredictable/unintended results!
 How to deal with this?

Use synchronization mechanisms to ensure the orderly execution
of cooperating processes
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Diagram of Critical Section Problem
Shared Data
P0
• Each process Pi has its own critical section
• Accesses to shared data only in CS
• Lock – will consider later
• In shared data, accessible by all
• Often 1 lock for all n processes
• Guards access to all critical sections
• Ensures mutual exclusivity
P1
Pn-1
...
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The Critical Section Problem
 Requirements to the critical section problem
1.
Mutual Exclusion
2.
Progress
3.
Bounded Waiting
 Solutions to the critical section problems

Software based

Hardware based
 Petersen’s solution

to the 2 process case
 Lamport’s Bakery Algorithm

Solution to the n process case

Before entering its critical section, process receives a number. Holder
of the smallest number enters critical section.
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Synchronization Hardware
 Modern machines provide special atomic hardware instructions
for synchronization
 Atomic = non-interruptible
 test_and_set instruction
test memory word and set value
 compare_and_swap instruction
 swap contents of two memory words
 All solutions below based on idea of locking


Protecting critical regions via locks
while (true) {
acquire lock
critical section
release lock
remainder section
}
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test_and_set Instruction
Definition:
boolean test_and_set (boolean *target)
{
boolean rv = *target;
*target = true;
return rv;
}
1. Executed atomically – cannot be interrupted in the middle
2. Returns the original value of passed parameter
3. Set the new value of passed parameter to true.
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Solution using test_and_set()
 Shared Boolean variable lock

initialized lock = false
 unlocked initially
 Solution:
while (true) {
while (test_and_set(&lock))
; // wait until lock becomes equal to false
// here lock=true
/* critical section */
lock = false;
/* remainder section */
}
 Bounded waiting is not satisfied

Other processes might keep acquiring the lock first
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compare_and_swap Instruction
Definition:
int compare_and_swap(int *value, int expected, int new_value)
{
int temp = *value;
if (*value == expected)
*value = new_value;
return temp;
}
1. Executed atomically
2. Returns the original value of passed parameter “value”
3. Set the variable value to new_value but only if value ==expected

The swap takes place only under this condition
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Solution using compare_and_swap
 Shared integer lock

initialized to lock=0; // unlocked
 Solution:
while (true) {
while (compare_and_swap(&lock, 0, 1) != 0)
; // wait for lock to be 0
// here lock = 1
/* critical section */
lock = 0;
/* remainder section */
}

Bounded waiting is not satisfied
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Mutex Locks
 OS designers build convenient software tools to solve critical
section problem
 Usually implemented via hardware atomic instructions
 Simplest is
mutex lock
 Protect a critical section by:

first acquire() a lock

then release() the lock


Boolean variable indicating if lock is available or not
Calls to acquire() and release() are atomic
 Implementations:

Busy waiting -- such a lock called a spinlock
 No busy waiting
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Semaphore

Semaphore – a synchronization tool

Provides more sophisticated ways (than Mutex locks) for processes to
synchronize their activities.

Semaphore S – integer variable

Can only be accessed via two indivisible (atomic) operations


wait() and signal()

Originally called P() and V()
Definition of the wait() operation
wait(S) {
while (S <= 0)
; // Notice, busy waits, spending CPU cycles, not (always) good
S--;
}

Definition of the signal() operation
signal(S) {
S++;
}
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Semaphore Usage


Counting semaphore – integer value can range over an unrestricted domain

Can control access to resource that has a finite number of instances

Initialized to the number of resources available
Binary semaphore – integer value can range only between 0 and 1

Same as a mutex lock

Can solve various synchronization problems

Consider P1 and P2 that require S1 to happen before S2
Create a semaphore “synch” initialized to 0 (locked)
P1:
S1;
signal(synch);
P2:
wait(synch);
S2;
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Classical Problems of Synchronization
 Classical problems used to test newly-proposed synchronization
schemes

Bounded-Buffer Problem

Readers and Writers Problem

Dining-Philosophers Problem
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The Readers-Writers Problem
 A data set is shared among a number of concurrent processes:

Readers – only read the data set


they do not perform any updates
Writers – can both read and write
 Problem

Allow multiple readers to read (shared data) at the same time

Only one single writer can access shared data at the same time

Readers cannot read during this time

Other writers cannot write during this time
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Readers-Writers Problem (Cont.)
 Shared Data:

Data set
semaphore rw_mutex = 1; // mutual exclusion for writers
semaphore mutex = 1;
// guards read_count
int read_count = 0;
// #processes currently reading the object
 The structure of a writer process
while (true) {
wait(rw_mutex);
...
/* writing is performed */
...
signal(rw_mutex);
}
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Readers-Writers Problem (Cont.)
 The structure of a reader process
while (true) {
wait(mutex);
read_count ++;
if (read_count == 1)
wait(rw_mutex);
signal(mutex);
...
/* reading is performed */
...
wait(mutex);
read_count--;
if (read_count == 0)
signal(rw_mutex);
signal(mutex);
}
 Writers can starve in this solution
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Deadlocks
… just have studied them
System Model
Deadlock Characterization
Methods for Handling Deadlocks
Deadlock Prevention
Deadlock Avoidance
Deadlock Detection
Recovery from Deadlock
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End of Midterm Overview
Operating System Concepts – 9th Edition. Silberschatz, Galvin and Gagne ©2013. Modified by Dmitri V. Kalashnikov and Nalini Venkatasubramanian.