CSC/ECE 517 Summer 2008/wiki1 4 wm
Introduction
A thread is a basic unit of CPU utilization. A traditional process has a single thread of control. Many modern operating systems provide features enabling a process to contain multiple threads of control. If the process has multiple threads of control, it can do more than one task at a time, such as, displaying graphics and reading keystrokes. Threads are used mainly to run asynchronous tasks and pass the application’s tasks to an executor. They are useful because they reduce the overall time of execution of programs. All threads belonging to the same process share its code section, data section and other operating system resources, such as open files and signals. Thus multithreading is more efficient than having parallel processes running for the same program, which require a huge overhead.
Multi-threaded programming
Multithreading support for single processors works by giving each of the threads a “time slice” of the CPU, similar to the parallel execution of processes. On multi-core machines, different threads can run on the different processors and the threads truly run simultaneously. Support for threads may be provided at either the user level, for user or green threads, or by the kernel, for kernel or native threads. User threads are supported above the kernel and are managed without kernel support, whereas kernel threads are supported and managed directly by the operating system.
Multi-threading models
There are three common multithreading models
One-to-one
This model maps each user-thread to a kernel thread. It provides more concurrency than the many-to-one model by allowing another thread to run when a thread makes a blocking system call. It also allows multiple threads to run in parallel on multiprocessors. The main disadvantage of this model is the overhead of creating each kernel thread for each user thread. This burdens the performance of an application and limits the number of threads supported by the system.
Many-to-one
This model maps many user-level threads to one kernel thread. It is the model used by green threads. Green threads are scheduled by a Virtual Machine instead of natively by the OS. They emulate multithreaded environments without relying on any native OS capabilities. These user-level threads are lightweight and very efficient. They are an easy way to achieve parallelism in a program. An application can have as many user-level threads as it needs, but true concurrency is not achieved because the kernel can schedule only one thread at a time. The developers can create as many user threads as necessary. However, there are two main disadvantages: 1) If a thread makes a blocking system call, the entire process will block. 2) If you are running on a multi-core machine, the multiple threads are unable to run in parallel.
Many-to-Many
This model multiplexes many user-level threads to a smaller or equal number of kernel threads. This model has the best of both worlds. Developers can create as many user threads as they need. When a user thread performs a blocking system call, the kernel can schedule another thread for execution and the kernel threads can run in parallel on multi-core systems.
Benefits of multi-threading
Resource sharing
Memory, code and process resource sharing - allocating memory and resources for process creation is costly and time consuming. It is more economical to create threads than new processes. Applications can benefit by having several different threads of activity within the same address space.
Responsiveness
Multithreading an interactive application may allow a program to continue running, even if part of it is blocked doing IO or performing a lengthy operation, thereby increasing responsiveness to the user. A web browser might have one thread display images or text while another thread retrieves data from the network. A busy web server may have many clients concurrently accessing it. If the web server ran as a traditional single-threaded process, it would be able to service only one client at a time.
Potential problems of multi-threading
Many threaded programs have shared resources that must be accessed by more than one thread. These programs must implement a form of mutual exclusion to ensure that only one of the threads can access the resource at a time. These shared resources must execute in code called critical sections. These critical sections must be protected by either hardware or software means.
Java threads
The Java programming language has a thread library for creating and managing threads at the user-level. The Java virtual machine (JVM) manages the mapping of the user threads to the operating system kernel threads. The particular mapping model that is used depends on the operating system on which the Java program is running. Java’s Thread Scheduler monitors all the threads running in Java programs. It decides which threads to run at any given time and when to switch between threads to run based on the thread’s priority with higher running threads running before lower priority threads. The Java scheduler uses either preemptive or non-preemptive scheduling based on the operating system on which it is running. With preemptive scheduling, each thread is given a constant period of time to run, after which time the thread will be suspended() to allow the next thread to resume() running. With non-preemptive scheduling, the running thread is allowed to run until the thread completes or until it issues a yield() to allow other threads to run while it waits for some other processing to occur. A Java thread can be created either by having your Java class extend the Java Thread class or by having your Java class implement the Runnable interface and coding a run() method. The two options are shown in Figures 1 and 2. The preferred method for creating a thread is by implementing the Runnable interface. It is more flexible and useful in complex applications. Java does not support multiple inheritance, but a class may implement multiple interfaces. By using the interface method, the Client class is open to inherit another class, if the need arose in the future.
Figure 1: Client Extends Thread Class
public class Client extends Thread {
public void run() {
System.out.println(“Client thread”);
}
}
public class Server {
public static void main(String args[]){
Thread runner = new Client();
runner.start();
System.out.println(“Server thread”);
}
}
Figure 2: Client Implements Runnable Interface
public class Client implements Runnable {
public void run() {
System.out.println(“Client thread”);
}
}
public class Server {
public static void main(String args[]) {
Thread runner = new Thread(new Client());
runner.start();
System.out.println(“Server thread”);
}
}
If the main Java thread wants to wait for any threads it creates to finish their run() method, the thread method join() can be executed. The join() method is useful in situations where the creating thread can continue only after a worker thread has completed. Java threads can be asynchronously terminated using the stop() method. However, this method has been deprecated and its used is discouraged. The preferred cancellation technique is to have the target thread periodically check whether it should terminate by using the interrupt() method.
States of Java threads
New
A thread is in this state when the thread object is first created with the new() method.
Runnable
Calling the start() method allocates memory for the new thread in the JVM and calls the run() method for the thread object.
Blocked
A thread becomes blocked if it performs a blocking statement, such as doing I/O or if it invokes a sleep() method.
Dead
A thread moves to the dead state when its run() method terminates.
Mutual exclusion
The Java language implements mutual exclusion in a couple of ways. There exists a MutualExclusion interface. The Client class can implement this interface and code the methods, enteringCriticalSection() and leavingCriticalSection(), as shown in Figure 3. The yield() method tells the Thread Scheduler to allow another thread to run.
Figure 3: Client Implements MutualExclusion Interface
public class Client implements MutualExclusion{
public Client(){
flag0 = false; flag1 = false; turn = TURN_0;
}
public void enteringCriticalSection(int t) {
int other = 1 - t;
if (t == 0) {
flag0 = true; turn = other;
while ((flag1 == true) && (turn == other))
Thread.yield();
} else {
flag1 = true; turn = other;
while ((flag0 == true) && (turn == other))
Thread.yield();
}
}
public void leavingCriticalSection(int t) {
if(t == 0) flag0 = false;
else flag1 = false;
}
private volatile int turn, boolean flag0, flag1;
}
An alternative method of handling mutual exclusion is to code a synchronization tool called a semaphore. A semaphore is variable that is accessed only through two standard operations: acquire() and release(). The semaphore is acquired, then the critical section of an object is entered and after the critical code completes, the semaphore is released. An example of this code is in Figure 4.
Figure 4: Client Has Semaphore
public class Client implements Runnable {
public Client (Semaphore sem) {
this.sem = sem;
}
public void run() {
sem.acquire();
System.out.println("Entering critical section");
SleepUtilities.nap(3);
System.out.println("Leaving critical section");
sem.release();
}
private Semaphore sem;
}