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Threads

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Processes versus Threads

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Why Threads?

Processes do not share resources very wellWhy?

Process context switching cost is very highWhy?

Thread: a light-weighted processA sequence of execution

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Threads: Lightweight Processes

Environment (resource)execution

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Real Life Example?

Process “system programming” course Different from “internet engineering”

Thread homework, Reading, Self-assessment quiz Each is a different “execution” But all share

Content Textbook Personnel (TAs, instructors)

Affect each other

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Review: threads vs. processes (created with fork)

PropertyProcesses created with fork

Threads of a process Ordinary function calls

variables get copies of all variables share global variables share global variables

IDs get new process IDsshare the same process ID but have unique thread ID

share the same process ID (and thread ID)

Communication

Must explicitly communicate, e.g.pipesor use small integer return value

May communicate with return valueor shared variablesif done carefully

May communicate with return valueor shared variables(don't have to be careful)

Parallelism (one CPU)

Concurrent Concurrent Sequential

Parallelism (multiple CPUs)

May be executed simultaneously

Kernel threads may be executed simultaneously

Sequential

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Thread components

A thread has its own program counter and stack, but shares a number of resources with its process and other threads of the process: address space: code and global variables open files semaphores signals timers process ID

Thread specific resource: Thread ID Program counter Register set Stack space Signal mask (later…)

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Thread vs. Process

Each thread execute separatelyThreads in the same process share resourcesNo protection among threads!!

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Storage for Threads

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Thread Model : Context Switch

Extensive sharing makes CPU switching among peer threads and creation of threads inexpensive compared to processes

Thread context switch still requires Register set switch But no memory management related work!!!

Why need to switch from one thread (process) to another? Some thread (process) may block for I/O Many threads (processes) share limited #CPUs

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Thread State

Threads states are Ready Blocked Running Terminated

Why these states? Threads share CPU

On single processor machine only one thread can run at a time Threads can block waiting for a system call to be completed

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Creating a Thread

When a new thread is created it runs concurrently with the creating process.

When creating a thread you indicate which function the thread should execute.

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Normal function call

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Threaded function call

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Benefits of Threads

Responsiveness Multi-threading allows applications to run even if

part of it is blocked Resource sharing

Sharing of memory, files and other resources of the process to which the threads belong

Economy Much more costly and time consuming to create

and manage processes than threads Utilization of multiprocessor architectures

Each thread can run in parallel on a different processor

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Thread Creation vs. Process Creation

Time in seconds for 50000 fork or thread creations

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Threaded Function Call Detail

A function that is used as a thread must have a special format.

It takes a single parameter and returns a single parameter.

Can point to a structure, so in effect, the function can use any number of parameters.

http://www.llnl.gov/computing/tutorials/pthreads/

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An Example

The thread function casts and unpacks the first argument:

void * myWorkerFunction(void *arg) {          int fd1 = ((int *)arg) [0];

int fd2 = ((int *)arg) [1];}

    

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Pthread Operations

POSIX function description

pthread_create create a thread

pthread_detach

set thread to release resources

pthread_equal test two thread IDs for equality

pthread_exit exit a thread without exiting process

pthread_kill send a signal to a thread

pthread_join wait for a thread

pthread_self find out own thread ID

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pthread_* return values

Unlike most POSIX functions, they do not set errno but the value returned when an error occurs has the value that errno would have.

None of the POSIX thread functions ever return the error EINTR.

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Example program

#include <phtread.h>#include <thread.h>#include <stdio.h>void *threadex(void *);

int main() {

pthread_t tid; /* stores the new thread ID */ pthread_create(&tid, NULL, threadex, NULL); /*create a new thread*/ pthread_join(tid, NULL); /*main thread waits for other thread to terminate */ return 0; /* main thread exits */ } void *threadex(void *arg) /*thread routine*/ { int i; for (i=0; i<5; i++) fprintf(stderr, `Hello, world! \n''); return NULL; }

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Thread Usage: word processor

What if it is single-threaded?

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Thread Usage: Web Server

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Designing Threaded Programs

Thread candidates?Discrete, independent tasks which can execute concurrently

E.g. if routine1 and routine2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading

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Tasks Suitable for threading

Block for potentially long waits Use many CPU cycles Must respond to asynchronous events Are of lesser or greater importance than other

tasks Are able to be performed in parallel with other

tasks

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Common Multi-thread Software Architectures

Manager/worker a single thread, the manager assigns work to other threads,

the workers. Typically, the manager handles all input and parcels out work

to the other tasks Pipeline:

a task is broken into a series of sub-operations, each of which is handled in series, but concurrently, by a different thread.

An automobile assembly line best describes this model Peer

similar to the manager/worker model, but after the main thread creates other threads, it participates in the work

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A Challenge: Making Single-Threaded Code Multithreaded

Conflicts between threads over the use of a global variable

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A solution: Private Global Variables

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Thread Packages

Kernel thread packages Implemented and supported at kernel level

User-level thread packages Implemented at user level

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User-level Thread

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User Level Threads

User-level threads without direct O/S support Threads are invisible to the kernel Simpler kernel implementation Can only use one processor at a time Implementation dependent

Some threads can block other threads (or) Requires a special library of system calls to

prevent blocking

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User-level Threads

Advantages Fast Context Switching:

User level threads are implemented using user level thread libraries, hence no call to OS and no interrupts to kernel

One key difference with processes: when a thread is finished running for the moment, it can call

thread_yield. This instruction

(a) saves the thread information in the thread table itself, and (b) calls the thread scheduler to pick another thread to run.

The procedure that saves the local thread state and the scheduler are local procedures, hence no trap to kernel, no context switch, no memory switch, and this makes the thread scheduling very fast.

Customized Scheduling

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Kernel-Level Thread

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Kernel-level Threads

Kernel can schedule threads in addition to processes.

Multiple threads of a process can run simultaneously on multiple CPUs.

Synchronization more efficient than for processes (but less than for user-level threads).

Kernel-level threads can make blocking I/O calls without blocking other threads of same process

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Trade-offs (review)?

Kernel thread packages Each thread can make blocking I/O calls Can run concurrently on multiple processors

Threads in User-level Fast context switch Customized scheduling

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Implementing Threads in User Space (old Linux)

A user-level threads package

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Hybrid Implementations (Solaris)

Multiplexing user-level threads onto kernel- level threads

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What’s POSIX Got To Do With It?

Each OS had it’s own thread library and style That made writing multithreaded programs difficult

because: you had to learn a new API with each new OS you had to modify your code with each port to a

new OS POSIX (IEEE 1003.1c-1995) provided a standard

known as Pthreads Unix International (UI) threads (Solaris threads) are

available on Solaris (which also supports POSIX threads)

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Pthreads--- POSIX Threads

It is a standard API Supported by most vendors General concepts applicable to other thread

APIs (java threads, NT threads,etc). Low level functions

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Creating a thread with pthread

A thread is created with

int pthread_create( pthread_t *restrict thread,

const pthread_attr_t *restrict attr,

void *(*start_routine)(void *),

void *restrict arg);

The creating process (or thread) must provide a location for storage of the thread id.

The third parameter is just the name of the function for the thread to run.

The last parameter is a pointer to the arguments.

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Restrict Keyword

One of the new features in the recently approved C standard C99

This qualifier can be applied to a data pointer to indicate that During the scope of that pointer declaration, all data

accessed through it will be accessed only through that pointer but not through any other pointer.

It enables the compiler to perform certain optimizations based on the premise that a given object cannot be changed through another pointer

http://www.cellperformance.com/mike_acton/2006/05/demystifying_the_restrict_keyw.html

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The Thread ID

pthread_t pthread_self(void) Each thread has an id of type pthread_t.

On most systems this is just an integer (like a process ID)

But it does not have to be A thread can get its ID with pthread_self Compare two threads

int pthread_equal(thread_t t1, pthread_t t2)

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Exiting and Cancellation

Question: If a thread calls exit(), what about other threads in

the same process? A process can terminate when:

it calls exit directly one of its threads calls exit it returns from main() it receives a termination signal

In any of these cases, all threads of the process terminate.

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Exiting

When a thread is done, it can return from its first function (the one used by pthread_create) or it can call pthread_exit

void pthread_exit(void *value_ptr);

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Cancel that thread! One thread can request that another exit

with pthread_cancel

int pthread_cancel(pthread_t thread);

The pthread_cancel returns after making the request.

A successful return does not mean that the target thread has terminated or even that it eventually will terminate as a result of the request

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Thread Attributes

Create an attribute object (initialize it with default properties)

Modify the properties of the attribute object Create a thread using the attribute object The attribute object can be changed or reused

without affecting the thread The attribute object affects the thread only at

the time of thread creation

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Attribute Initialization and Deletion

Initialize or destroy an attribute with: int pthread_attr_destroy(pthread_attr_t *attr); int pthread_attr_init(pthread_attr_t *attr);

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Example: Create a detached threadint e, fd; pthread_attr_t tattr; pthread_t tid;

if(e = pthread_attr_init(&tattr)) fprintf(stderr, "Failed to create attribute object: %s\n", strerror(e));

else if(e = pthread_attr_setdetachstate(&tattr, PTHREAD_CREATE_DETACHED)) fprintf(stderr, "Failed to set attribute state to detached: %s\n", strerror(e));

else if(e = pthread_create(&tid, &tattr, data, &fd)) fprintf(stderr, "Failed to create thread: %s\n", strerror(e));

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The thread stack

You can set a location and size for the thread stack.

int pthread_attr_setstack( pthread_attr_t *attr,void *stackaddr,

size_t stacksize )

(there’s a getstack function too)

Some systems allow you to set a guard for the stack so that an overflow into the guard area can generate a SIGSEGV signal.

int pthread_attr_getguardsize(const pthread_attr_t *restrict attr, size_t *restrict guardsize); int pthread_attr_setguardsize(pthread_attr_t *attr, size_t guardsize);

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Settable properties of thread attributes

property function

attribute objects pthread_attr_destroy

pthread_attr_init

detach state pthread_attr_getdetachstate

pthread_attr_setdetachstate

stack pthread_attr_getguardsize

pthread_attr_setguardsize

pthread_attr_getstack

pthread_attr_setstack

scheduling pthread_attr_getinheritsched

pthread_attr_setinheritsched

pthread_attr_getschedparam

pthread_attr_setschedparam

pthread_attr_getschedpolicy

pthread_attr_setschedpolicy

pthread_attr_getscope

pthread_attr_setscope

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Thread Detach & Join

Call pthread_join() or pthread_detach() for every thread that is created joinable so that the system can reclaim all resources

associated with the thread Failure to join or to detach threads memory

and other resource leaks until the process ends

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Detaching a Thread

int pthread_detach(pthread_t threadid); Indicate that system resources for the specified thread

should be reclaimed when the thread ends If the thread is already ended, resources are reclaimed

immediately This routine does not cause the thread to end

Threads are detached after a pthread_detach() call after a pthread_join() call if a thread terminates and the

PTHREAD_CREATE_DETACHED attribute was set on creation

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How to make a thread detached

void *processfd(void *arg);

int error; int fd pthread_t tid;

if (error = pthread_create(&tid, NULL, processfd, &fd)) { fprintf(stderr, "Failed to create thread: %s\n", strerror(error)); }else if (error = pthread_detach(tid)){ fprintf(stderr, "Failed to detach thread: %s\n", strerror(error));}

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How a thread can detach itself

void *runnger(void *arg) { … if (!pthread_detach( pthread_self()) ) return NULL;

…}

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“Wating” on a Thread: pthread_join()

int pthread_join(pthread_t thread, void** retval);

pthread_join() is a blocking call on non-detached threads

It indicates that the caller wishes to block until the thread being joined exits

You cannot join on a detached thread, only non-detached threads

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Pthread_join

int error; int *exitcodep; pthread_t tid;

if (error = pthread_join(tid, &exitcodep)){ fprintf(stderr, "Failed to join thread: %s\n", strerror(error)); }else { fprintf(stderr, "The exit code was %d\n", *exitcodep); }

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Thread scheduling

int pthread_attr_getscope(const pthread_attr_t *restrict attr,

int *restrict contentionscope);

int pthread_attr_setscope(pthread_attr_t *attr, int contentionscope);

The contentionscope can be: PTHREAD_SCOPE_PROCESS PTHREAD_SCOPE_SYSTEM.

The scope determines whether the thread competes with other threads of the process or with other processes in the system.

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Create a thread that contends with other processes

pthread_attr_init(&tattr))

pthread_attr_setscope(&tattr, PTHREAD_SCOPE_SYSTEM))

thread_create(&tid, &tattr, myfunction, &myptr))