Review: Chapters 1 – 7Review: Chapters 1 – 7

1.2

Chapter 1:Chapter 1:

OS is a layer between user and hardware to make life easier for user and use hardware efficiently

Control program or resource allocator

Computer organization

CPU(s), memory, and I/O devices connect to a common bus

Devices request CPU attention through interrupts

Storage hierarchy: speed, cost, volatility

Caching: copy frequently used data to faster storage

Multiprogramming: multiple jobs in memory efficiency

Timesharing: frequently switch between jobs interactive, short response time users get the impression that each has his/her own computer

Dual mode operation: user and kernel modes

Protect OS and users from each other

Privileged instructions executed only in kernel mode

Timer to prevent processes from holding resources forever

1.3

Operating-System OperationsOperating-System Operations

OS is interrupt driven: it sits idle till something happens Interrupts are generated by devices (hardware) Traps (or exceptions) are software-generated interrupts due to

software errors, e.g., divide by zero Request for operating system services (system calls)

Dual-mode operation allows OS to protect itself and other system components User mode and kernel mode Mode bit provided by hardware

Provides ability to distinguish when system is running user code or kernel code

Some instructions designated as privileged, only executable in kernel mode

System call changes mode to kernel, return from call resets it to user

1.4

Transition from User to Kernel ModeTransition from User to Kernel Mode

1.5

Chapter 2: OS Services and StructuresChapter 2: OS Services and Structures

OS provides two sets of services for

user convenience and

efficient use of resources

System calls: programming interface to OS services

Typically used through APIs for portability and ease

OS structures

monolithic

layered

microkernel

modular

1.6

Chapter 3: ProcessesChapter 3: Processes

Process is a program in execution

OS maintains process info in PCB

Process State diagram

Creating and terminating processes (fork)

Process scheduling

Long-, short-, and medium-term schedulers

Scheduling queues

1.7

Scheduling: The Big PictureScheduling: The Big Picture

DiskJobs

Job sched. CPU sched.

Midterm sched.

In most small and interactive systems (UNIX, WinXP, …), only the CPU scheduler exists

1.8

Process Lifetime Process Lifetime

1.9

CPU Switch From Process to ProcessCPU Switch From Process to Process(Context Switch)(Context Switch)

When switching occurs, kernel Saves state of P0 in

PCB0 (in memory)

Loads state of P1 from PCB1 into registers

State = values of the CPU registers, including the program counter, stack pointer

1.10

Interprocess Communications Models Interprocess Communications Models

Message Passing Shared Memory

1.11

Chapter 4: ThreadsChapter 4: Threads

A thread is a basic unit of CPU utilization, a process is composed of one or more threads

Each thread has: Program counter, stack, registers

Threads share: code, data, OS resources (e.g., open files and signals)

1.12

Single and Multithreaded ProcessesSingle and Multithreaded Processes

Shared among threads

1.13

User level threads vs. kernel threadsUser level threads vs. kernel threads

1.14

Chapter 5: CPU SchedulingChapter 5: CPU Scheduling

Process execution: cycle of CPU bursts and I/O bursts

CPU bursts lengths: many short bursts, and few long ones

Scheduler selects one process from ready queue

Dispatcher performs the switching

Scheduling criteria (usually conflicting)

CPU utilization, waiting time, response time, throughput, …

Scheduling Algorithms

FCFS, SJF, Priority, RR, Multilevel Queues, …

1.15

First-Come, First-Served (FCFS) SchedulingFirst-Come, First-Served (FCFS) Scheduling

Process Burst Time

P1 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

1.16

Multilevel Feedback QueuesMultilevel Feedback Queues

1.17

CPU SchedulingCPU Scheduling

Multiprocessor Scheduling

Processor affinity vs. load balancing

Evaluation of Algorithms

Modeling, simulation, implementation

1.18

Chapter 6: SynchronizationChapter 6: Synchronization

Processor Synchronization

Techniques to coordinate access to shared data

Race condition

Multiple processes manipulating shared data and result depends on execution order

Critical section problem

Three requirements: mutual exclusion, progress, bounded waiting

Software solution: Peterson’s Algorithm

Hardware support: TestAndSet(), Swap()

Busy waiting (or spinlocks)

Semaphores:

Not busy waiting

wait(), signal() must be atomic moves the CS problem to kernel

Monitors

High-level (programming language) constructs

1.19

SynchronizationSynchronization

Some classical synchronization problems

Consumer-producer

Dining philosopher

Readers-writers

1.20

Chapter 7: DeadlockChapter 7: Deadlock

A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set

Four necessary (but not sufficient) conditions

Mutual exclusion: only one process at a time can use a resource

Hold and wait: a process holding at least one resource and is waiting to acquire additional resources held by other processes

No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task

Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by

P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0

1.21

Deadlock HandlingDeadlock Handling

Prevention: ensure that at least one of the necessary conditions does not hold

Avoidance: decide for each request whether or not the issuing process should wait to avoid leaving the system in unsafe state

Resource-allocation graph: single instance of a resource type

Banker’s algorithm: multiple instances of a resource type

Detection and Recovery

Detection algorithm

Recovery: process termination or resource preemption

1.22

Good Luck on the Exam!