chapter 4 threads. shandong university 2 contents overview multithreading models threading issues...
TRANSCRIPT
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Chapter 4
Threads
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SHANDONG UNIVERSITY 2
Contents Overview Multithreading Models Threading Issues Pthreads Windows XP Threads Linux Threads Java Threads
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SHANDONG UNIVERSITY 3
Objectives To introduce the notion of a thread---a
fundamental unit of CPU utilization that forms the basis of multithread computer system.
To discuss the APIs for Pthreads, Win32, and Java thread libraries.
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SHANDONG UNIVERSITY 4
Thread A thread
A running entity of a process, and a unit that can be scheduled independently.
A basic unit of CPU utilization
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Motivation When increase the concurrence of system, the
time spent on process creation, process cancellation, process exchange will increase greatly
In addition, the communication between processes is also limited.
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Motivation --- example Suppose there is a web server
What is the result if there is only one thread? The time to create The time to exchange The space for each user
A program will accept input from user, list the menu, execute the command What is the result if there is only one thread?
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Single and Multithreaded Processes
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Benefits Responsiveness
Resource Sharing
Economy
Utilization of MP Architectures
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SHANDONG UNIVERSITY 9
Thread A thread
A running entity of a process, and a unit that can be scheduled independently.
A basic unit of CPU utilization Resources still belong to process
Code section Data section Open files signals
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SHANDONG UNIVERSITY 10
Thread & Process Process is the owner of resources
Code section Data section Open files Signals
Thread is a running unit (smallest unit) Thread has few resources (counter, register,
stack), shares all the resources that the process has.
A program has one process at least, and one process has one thread at least
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Implementation User Level Thread Kernel level thread Hybrid method
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User Threads Thread management done by user-level
threads library Kernel knows nothing about threads
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User thread
运行系统
TCB进程
线程
核心栈 进程表
用户空
间
系统空间
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User thread
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User thread Implemented by thread library
Create, cancellation Transfer data or message Save and recover the context of threads
The kernel manage the process, but know nothing about thread
When a thread have a system call, the process will be blocked. To thread library, the thread’s state is running
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User thread Three primary thread libraries:
POSIX Pthreads Win32 threads Java threads
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Advantages & Disadvantages Advantages
It does not need to call the kernel when there is thread switching.
Scheduling is determined by application, so best algorithm can be selected.
ULT can run on any platform if the thread library is install on it.
Disadvantages Most system call will result in blocking Two threads in the same process can not
simultaneously run on two processors
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Kernel Threads Supported by the Kernel All threads are managed by the kernel
Create, cancellation and schedule No thread library, but provide API Kernel maintains context of threads and processes The switch between threads needs the support of kernel
Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X
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Kernel Threads
进程
线程
核心栈 进程表
用户空
间
系统空间
TCB
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Advantages & Disadvantages Advantages
For multiprocessor system, more than one thread can run simultaneously
Just block the thread, not process Disadvantage
The switch between threads in the same process, will slow the speed.
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Hybrid model Thread is created in user space
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Multithreading Models Many-to-One
One-to-One
Many-to-Many
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Many-to-One Many user-level threads mapped to single
kernel thread Examples:
Solaris Green Threads GNU Portable Threads
Advantage Management is efficient
Disadvantages Process is blocked when one thread is blocked Can’t utilize multi-processors system
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Many-to-One Model
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One-to-One Each user-level thread maps to one kernel
thread Examples
Windows NT/XP/2000 Linux Solaris 9 and later
Advantage Can run on multiprocessor system One blocked, others can run still
Disadvantage To create one user thread, one kernel thread is
also created.
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One-to-one Model
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Many-to-Many Model Allows many user level threads to be mapped
to many kernel threads Allows the operating system to create a
sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber
package
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Many-to-Many Model
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Two-level Model Similar to M:M, except that it allows a user
thread to be bound to kernel thread Examples
IRIX HP-UX Tru64 UNIX Solaris 8 and earlier
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Two-level Model
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Threading Issues Semantics of fork() and exec() system calls Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations
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Semantics of fork() and exec() Does fork() duplicate only the calling thread or
all threads? Exec() is after fork() No exec() after fork()
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Thread Cancellation Terminating a thread before it has finished
Search database Web pages
Two general approaches: Asynchronous cancellation terminates the target
thread immediately Deferred cancellation allows the target thread to
periodically check if it should be cancelled Cancellation point
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Signal Handling Signals are used in UNIX systems to notify a process
that a particular event has occurred Synchronous Asynchronous
A signal handler is used to process signals1. Signal is generated by particular event2. Signal is delivered to a process3. Signal is handled
Options: Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific thread to receive all signals for the process
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Thread Pools Problems in multithread server:
Spend much time to create thread Resources will be exhausted if no limitation to
thread Create a number of threads in a pool where
they await work Advantages:
Usually slightly faster to service a request with an existing thread than create a new thread
Allows the number of threads in the application(s) to be bound to the size of the pool
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Thread Specific Data Allows each thread to have its own copy of
data Example—transaction processing system
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Scheduler Activations Both N:M and Two-level models require
communication to maintain the appropriate number of kernel threads allocated to the application
Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library
This communication allows an application to maintain the correct number kernel threads
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Pthreads A POSIX standard (IEEE 1003.1c) API for
thread creation and synchronization API specifies behavior of the thread library,
implementation is up to development of the library
Common in UNIX operating systems (Solaris, Linux, Mac OS X)
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Pthreads#include<pthread.h>#include<stdio.h>int sum; /*this data is shared by the thread(s) */void *runner(void *param); /*the thread*/
Main(int argc, char *argv[]){
pthread_t tid; /*the thread identifier*/pthread_attr_t attr; /* set of attributes for the thread*/pthread_attr_init(&attr);pthread_create(&tid, &attr, runner, argv[1]);pthread_join(tid, NULL);printf(“sum= %d\n”, sum);
}
void *runner(void *param){
int upper = atoi(param);int I;sum = 0;if (upper > 0) {
for (I = 1; I <= upper; I ++)sum += I;
}pthread_exit(0);
}
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Windows XP Threads Implements the one-to-one mapping Each thread contains
A thread id Register set Separate user and kernel stacks Private data storage area
The register set, stacks, and private storage area are known as the context of the threads
The primary data structures of a thread include: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block)
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Linux Threads Linux refers to them as tasks rather than threads
Thread creation is done through clone() system call
clone() allows a child task to share the address space of the parent task (process)
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Java Threads Java threads are managed by the JVM
Java threads may be created by:
Extending Thread class Implementing the Runnable interface
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Extending Thread classclass Worker1 extends Thread{
public void run() {System.out.println(“I Am a Worker Thread”);
}}
public class First{
public static void main(String args[]) {Worker1 runner = new Worker1();runner.start();
System.out.println(“I Am The Main Thread”);}
}
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Runnable interface
public interface Runnable
{public abstract void run();
}
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Implementing the Runnable interfaceclass Worker2 implements Runnable {
public void run() {System.out.println(“I Am a Worker Thread”);
}}
public class Second {public static void main(String argc[]) {
Runnable runner = new Worker2();Thread thrd = new Thread(runner);thrd.start();
System.out.println(“I Am The Main Thread”);}
}
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Java Thread States
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Joining Threadsclass JoinableWorker implements Runnable {
public void run() {System.out.println(“Worker working”);
}}
public class JoinExample {public static void main(String [] args) {
Thread task = new Thread(new JoinableWorker());task.start();
try { task.join(); }catch (InterruptedException ie) { }System.out.println(“Worker done”);
}}
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Thread cancellation
Thread thrd = new Thread(new InterruptibleThread());
thrd.start();
…
//now interrupt it
thrd.interrupt();
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Thread cancellationpublic class InterruptibleThread implements Runnable {
public void run() {while (true) {
…if
(Thread.currentThread().isInterrupted()) {System.out.println();break;
} /* 线程取消点 */}//clean up and terminate
}}
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Thread dataClass Service {
private static ThreadLocal errorCode = new ThreadLocal();
public static void transaction() {try {}catch (Exception e) {
errorCode.set(e);}
}
public static Object getErrorCode() {return errorCode.get();
}}
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class Worker implements Runnable {
private static Service provider; //线程特定数据public void run() {
provider.transaction();
System.out.println(provider.getErrorCode());
}
}
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Producer-consumer problempublic class Factory {
public Factory() {Channel mailBox = new MessageQueue();Thread producerThread = new Thread(new
Producer(mailBox));Thread consumerThread = new Thread(new
Consumer(mailBox));producerThread.start();consumerThread.start();
}
public static void main(String args[]) {Factory server = new Factory();
}}
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Producer threadclass Producer implements Runnable {
private Channel mbox;
public Producer(Channel mbox) {this.mbox = mbox;
}
public void run() {Date message;while (true) {
SleepUtilities.nap(); // 小睡片刻message = new Date();System.out.println(“Producer produced” + message);mbox.send(message);
}}
}
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Consumer threadclass Consumer implements Runnable {
private Channel mbox;
public Consumer(Channel mbox) {this.mbox = mbox;
}
public void run() {Date message;while (true) {
SleepUtilities.nap();System.out.println(“Consumer wants to consume.”);message = (Date) mbox.receive();if (message != null)
System.out.println(“Consumer consumed” + message);
}}
}
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Assignment 4.2, 4.4, 4.5
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End of Chapter 4
Any Question?