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Processes and Threads

Chapter 2

2.1 Processes2.2 Threads2.3 Interprocess communication2.4 Classical IPC problems2.5 Scheduling

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Agenda

• 2.1 Processes

• 2.2 Threads

• 2.3 Interprocess communication

• 2.4 Classical IPC problems

• 2.5 Scheduling

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Process

• The most central concept in any OS

• An abstraction of a running program

• Modern Computers– Can do more than one thing at the same time

• Can run user programs

• Can read disk and work with user terminal

– In a multiprogramming system• More than one user program can be scheduled

• Each may run for tens of msecs.

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ProcessesThe Process Model

• Multiprogramming of four programs• Conceptual model of 4 independent, sequential processes• Only one program active at any instant

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Process Creation

Principal events that cause process creation

1. System initialization

2. Execution of a process creation system

3. User request to create a new process

4. Initiation of a batch job

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Process Termination

Conditions which terminate processes

1. Normal exit (voluntary)

2. Error exit (voluntary)

3. Fatal error (involuntary)

4. Killed by another process (involuntary)

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Process Hierarchies

• Parent creates a child process, child processes can create its own process

• Forms a hierarchy– UNIX calls this a "process group"

• Windows has no concept of process hierarchy– all processes are created equal

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Process States (1)

• Possible process states– running– blocked– ready

• Transitions between states shown

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Process States (2)

• Lowest layer of process-structured OS– handles interrupts, scheduling

• Above that layer are sequential processes

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Implementation of Processes (1)

Fields of a process table entry

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Implementation of Processes (2)

Skeleton of what lowest level of OS does when an interrupt occurs

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Agenda

• 2.1 Processes

• 2.2 Threads

• 2.3 Interprocess communication

• 2.4 Classical IPC problems

• 2.5 Scheduling

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Threads

• In traditional OS, each process has – An address space– A single tread of control

• In modern OS, each process may have– Multiple threads of control– Same address spare– Running in quasi-parallel

• Thread or a Lightweight Process has – a program counter, registers, stack

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ThreadsThe Thread Model (1)

(a) Three processes each with one thread(b) One process with three threads

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The Thread Model (2)

• Items shared by all threads in a process

• Items private to each thread

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The Thread Model (3)

Each thread has its own stack

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Thread Usage (1)

A word processor with three threads

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Thread Usage (2)

A multithreaded Web server

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Thread Usage (3)

• Rough outline of code for previous slide(a) Dispatcher thread(b) Worker thread

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Thread Usage (4)

Three ways to construct a server

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Implementing Threads in User Space

A user-level threads package

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Implementing Threads in the Kernel

A threads package managed by the kernel

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Hybrid Implementations

Multiplexing user-level threads onto kernel- level threads

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Scheduler Activations

• Goal – mimic functionality of kernel threads– gain performance of user space threads

• Avoids unnecessary user/kernel transitions• Kernel assigns virtual processors to each process

– lets runtime system allocate threads to processors

• Problem:– Fundamental reliance on kernel (lower layer) – Calling procedures in user space (higher layer)

• Called upcall

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Pop-Up Threads

• Creation of a new thread when message arrives(a) before message arrives(b) after message arrives

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Making Single-Threaded Code Multithreaded (1)

Conflicts between threads over the use of a global variable

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Making Single-Threaded Code Multithreaded (2)

Threads can have private global variables

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Agenda

• 2.1 Processes

• 2.2 Threads

• 2.3 Interprocess communication

• 2.4 Classical IPC problems

• 2.5 Scheduling

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Interprocess Communication

• Processes may need to communicate – E.g., output of one goes to input of the other one

• Issues– How to pass information

• different address spaces

– Making critical activities• Two process try to grab the last 1MB of memory

– Proper sequencing• One process is producing data for the other one

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Interprocess CommunicationRace Conditions

Two processes want to access shared memory at same time

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Critical Regions (1)

• Critical Regions or Critical Sections– The part of the program where shared memory is accessed.

• Four conditions to provide mutual exclusion1. No two processes simultaneously in critical region

2. No assumptions made about speeds or numbers of CPUs

3. No process running outside its critical region may block another process

4. No process must wait forever to enter its critical region

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Critical Regions (2)

Mutual exclusion using critical regions

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Mutual Exclusion with Busy Waiting (0)

• Mutual Exclusion– Only one process can be in the critical section.

• Disabling Interrupts (HW Solution)– Dangerous: May lead to the end of the system

• Lock Variables (SW Solution)– Fatal Flow: Similar to Spooler Directory

• Strict Alternation (SW Solution)– Busy Waiting, Violating Condition 3

• Peterson’s Solution (SW Solution)– Busy Waiting

• TSL Instruction (HW/SW Solution)– Busy Waiting, but Faster

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Mutual Exclusion with Busy Waiting (1)Strict Alternation (SW Solution)

Proposed solution to critical region problem(a) Process 0. (b) Process 1.

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Mutual Exclusion with Busy Waiting (2)Peterson’s Solution (SW Solution)

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Mutual Exclusion with Busy Waiting (3)Using TSL Instruction (HW/SW Solution)

Entering and leaving a critical region using the

TSL instruction

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Mutual Exclusion with Sleep and Wakeup (0)Producer-Consumer Problem

• Priority Inversion Problem– busy waiting with priority!

• Solution with Fatal Race Condition– Waking up a consumer that is not asleep yet!

• Semaphore– An integer that counts the number of wake-up calls.

• Mutexes– Binary semaphores, good for mutual exclusion.

• Monitors– Easier to program (Synchronized in Java).

• Message Passing– N messages used for communication coordination.

• Barriers– Synchronization of N processes/threads.

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Mutual Exclusion with Sleep and Wakeup (1)Fatal Race Condition

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Mutual Exclusion with Sleep and Wakeup (2) Using Semaphores

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Mutual Exclusion with Sleep and Wakeup (3) Using Mutexes

Implementation of mutex_lock and mutex_unlock

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Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (1)

Example of a monitor

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• Outline of producer-consumer problem with monitors– only one monitor procedure active at one time– buffer has N slots

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (2)

43Solution to producer-consumer problem in Java (part 1)

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (3)

44Solution to producer-consumer problem in Java (part 2)

Mutual Exclusion with Sleep and Wakeup (4) Using Monitors (4)

45The producer-consumer problem with N messages

Mutual Exclusion with Sleep and Wakeup (5) Message Passing

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Barriers

• Use of a barrier– processes approaching a barrier– all processes but one blocked at barrier– last process arrives, all are let through

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Agenda

• 2.1 Processes

• 2.2 Threads

• 2.3 Interprocess communication

• 2.4 Classical IPC problems

• 2.5 Scheduling

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Dining Philosophers (1)

• Philosophers eat/think• Eating needs 2 forks• Pick one fork at a time • How to prevent deadlock

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Dining Philosophers (2)

A nonsolution to the dining philosophers problem

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Dining Philosophers (3)

Solution to dining philosophers problem (part 1)

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Dining Philosophers (4)

Solution to dining philosophers problem (part 2)

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The Readers and Writers Problem

A solution to the readers and writers problem

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The Sleeping Barber Problem (1)

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The Sleeping Barber Problem (2)

Solution to sleeping barber problem.

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Agenda

• 2.1 Processes

• 2.2 Threads

• 2.3 Interprocess communication

• 2.4 Classical IPC problems

• 2.5 Scheduling

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Scheduling

• Scheduler– The part of the OS that make the choice of

which process to run next.

• Scheduling Algorithm– The algorithm used for scheduling

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SchedulingIntroduction to Scheduling (1)

• Bursts of CPU usage alternate with periods of I/O wait– a CPU-bound process: spends most of its time on computing– an I/O bound process: spends most of its time waiting for I/O

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Introduction to Scheduling (2)System Algorithm Goals

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Scheduling Algorithm Goals

• Throughput– The number of jobs per hour that the system

completes.

• Turnaround time– The statically average time from the moment

that a batch job is submitted until the moment it is completed.

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Scheduling in Batch Systems (1)

• First-Come First-Served

• Shortest Job First (non-preemptive)– An example of shortest job first scheduling

• Shortest Remaining Time First (preemptive)

• Three-Level Scheduling

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Scheduling in Batch Systems (2)

Three level scheduling

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Scheduling in Interactive Systems (1)• Round Robin Scheduling

– list of runnable processes– list of runnable processes after B uses up its quantum

• Priority Scheduling• Multiple Queues• Shortest Process Next• Guaranteed Scheduling• Lottery Scheduling• Fair-Share Scheduling

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Scheduling in Interactive Systems (2)

A scheduling algorithm with four priority classes

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Scheduling in Real-Time Systems

Schedulable real-time system

• Given– m periodic events

– event i occurs within period Pi and requires Ci seconds

• Then the load can only be handled if

1

1m

i

i i

C

P

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Policy versus Mechanism

• Separate what is allowed to be done with how it is done– a process knows which of its children threads

are important and need priority

• Scheduling algorithm parameterized– mechanism in the kernel

• Parameters filled in by user processes– policy set by user process

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Thread Scheduling (1)

Possible scheduling of user-level threads• 50-msec process quantum• threads run 5 msec/CPU burst

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Thread Scheduling (2)

Possible scheduling of kernel-level threads• 50-msec process quantum• threads run 5 msec/CPU burst