Chapter 02

Processes and Threads Chapter 2 2.1 Processes 2.2 Threads 2.3 Interprocess communication 2.4 Classical IPC problems 2.5 Scheduling

Processes The Process Model Multiprogramming of four programs Conceptual model of 4 independent, sequential processes Only one program active at any instant

Process Creation Principal events that cause process creation System initialization Execution of a process creation system User request to create a new process Initiation of a batch job

Process Termination Conditions which terminate processes Normal exit (voluntary) Error exit (voluntary) Fatal error (involuntary) Killed by another process (involuntary)

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

Process States (1) Possible process states running blocked ready Transitions between states shown

Process States (2) Lowest layer of process-structured OS handles interrupts, scheduling Above that layer are sequential processes

Implementation of Processes (1) Fields of a process table entry

Implementation of Processes (2) Skeleton of what lowest level of OS does when an interrupt occurs

Threads The Thread Model (1) (a) Three processes each with one thread (b) One process with three threads

The Thread Model (2) Items shared by all threads in a process Items private to each thread

The Thread Model (3) Each thread has its own stack

Thread Usage (1) A word processor with three threads

Thread Usage (2) A multithreaded Web server

Thread Usage (3) Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread

Thread Usage (4) Three ways to construct a server

Implementing Threads in User Space A user-level threads package

Implementing Threads in the Kernel A threads package managed by the kernel

Hybrid Implementations Multiplexing user-level threads onto kernel- level threads

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)

Pop-Up Threads Creation of a new thread when message arrives (a) before message arrives (b) after message arrives

Making Single-Threaded Code Multithreaded (1) Conflicts between threads over the use of a global variable

Making Single-Threaded Code Multithreaded (2) Threads can have private global variables

Interprocess Communication Race Conditions Two processes want to access shared memory at same time

Critical Regions (1) Four conditions to provide mutual exclusion No two processes simultaneously in critical region No assumptions made about speeds or numbers of CPUs No process running outside its critical region may block another process No process must wait forever to enter its critical region

Critical Regions (2) Mutual exclusion using critical regions

Mutual Exclusion with Busy Waiting (1) Proposed solution to critical region problem (a) Process 0. (b) Process 1.

Mutual Exclusion with Busy Waiting (2) Peterson's solution for achieving mutual exclusion

Mutual Exclusion with Busy Waiting (3) Entering and leaving a critical region using the TSL instruction

Sleep and Wakeup Producer-consumer problem with fatal race condition

Semaphores The producer-consumer problem using semaphores

Mutexes Implementation of mutex_lock and mutex_unlock

Monitors (1) Example of a monitor

Monitors (2) Outline of producer-consumer problem with monitors only one monitor procedure active at one time buffer has N slots

Monitors (3) Solution to producer-consumer problem in Java (part 1)

Monitors (4) Solution to producer-consumer problem in Java (part 2)

Message Passing The producer-consumer problem with N messages

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

Dining Philosophers (1) Philosophers eat/think Eating needs 2 forks Pick one fork at a time How to prevent deadlock

Dining Philosophers (2) A non solution to the dining philosophers problem

Dining Philosophers (3) Solution to dining philosophers problem (part 1)

Dining Philosophers (4) Solution to dining philosophers problem (part 2)

The Readers and Writers Problem A solution to the readers and writers problem

The Sleeping Barber Problem (1)

The Sleeping Barber Problem (2) Solution to sleeping barber problem.

Scheduling Introduction to Scheduling (1) Bursts of CPU usage alternate with periods of I/O wait a CPU-bound process an I/O bound process

Introduction to Scheduling (2) Scheduling Algorithm Goals

Scheduling in Batch Systems (1) An example of shortest job first scheduling

Scheduling in Batch Systems (2) Three level scheduling

Scheduling in Interactive Systems (1) Round Robin Scheduling list of runnable processes list of runnable processes after B uses up its quantum

Scheduling in Interactive Systems (2) A scheduling algorithm with four priority classes

Scheduling in Real-Time Systems Schedulable real-time system Given m periodic events event i occurs within period P i and requires C i seconds Then the load can only be handled if

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

Thread Scheduling (1) Possible scheduling of user-level threads 50-msec process quantum threads run 5 msec/CPU burst

Thread Scheduling (2) Possible scheduling of kernel-level threads 50-msec process quantum threads run 5 msec/CPU burst

Chapter 02

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Chapter 02