Threads Flashcards
Parallel
A system can support multiple tasks executing simultaneously
Concurrent
The system can support multiple tasks in progress at the same time
Join
use Join to wait for another thread to end.
Thread.CurrentThread
property which gives you the currently executing thread
Processes and Threads
In concurrent programming, there are two basic units of execution: processes and threads. In the Java programming language, concurrent programming is mostly concerned with threads. However, processes are also important.
A computer system normally has many active processes and threads. This is true even in systems that only have a single execution core, and thus only have one thread actually executing at any given moment. Processing time for a single core is shared among processes and threads through an OS feature called time slicing.
It’s becoming more and more common for computer systems to have multiple processors or processors with multiple execution cores. This greatly enhances a system’s capacity for concurrent execution of processes and threads — but concurrency is possible even on simple systems, without multiple processors or execution cores.
Processes
A process has a self-contained execution environment. A process generally has a complete, private set of basic run-time resources; in particular, each process has its own memory space.
Processes are often seen as synonymous with programs or applications. However, what the user sees as a single application may in fact be a set of cooperating processes. To facilitate communication between processes, most operating systems support Inter Process Communication (IPC) resources, such as pipes and sockets. IPC is used not just for communication between processes on the same system, but processes on different systems.
Most implementations of the Java virtual machine run as a single process. A Java application can create additional processes using a ProcessBuilder object. Multiprocess applications are beyond the scope of this lesson.
Threads
Threads are sometimes called lightweight processes. Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process.
Threads exist within a process — every process has at least one. Threads share the process’s resources, including memory and open files. This makes for efficient, but potentially problematic, communication.
Multithreaded execution is an essential feature of the Java platform. Every application has at least one thread — or several, if you count “system” threads that do things like memory management and signal handling. But from the application programmer’s point of view, you start with just one thread, called the main thread. This thread has the ability to create additional threads, as we’ll demonstrate in the next section.
asynchronous
When you execute something synchronously, you wait for it to finish before moving on to another task. When you execute something asynchronously, you can move on to another task before it finishes.
That being, said, in the context of computers this translates into executing a process or task on another “thread.” A thread is a series of commands–a block of code–that exists as a unit of work. The operating system can manage multiple threads and assign a thread a piece (“slice”) of processor time before switching to another thread to give it a turn to do some work. At its core (pardon the pun), a processor can simply execute a command–it has no concept of doing two things at one time. The operating system simulates this by allocating slices of time to different threads.
Now, if you introduce multiple cores/processors into the mix, then things CAN actually happen at the same time. The operating system can allocate time to one thread on the first processor, then allocate the same block of time to another thread on a different processor.
All of this is about allowing the operating system to manage the completion of your task while you can go on in your code and do other things. Asynchronous programming is a complicated topic because of the semantics of how things tie together when you can do them at the same time. There are numerous articles and books on the subject; have a look!
synchronous vs asynchronous
Synchronous:
|—-A—–| |—–B———–| |——-C——|
Asynchronous:
|—-A—–|
|—–B———–|
|——-C——|
Thread Objects
Thread Objects
Each thread is associated with an instance of the class Thread. There are two basic strategies for using Thread objects to create a concurrent application.
To directly control thread creation and management, simply instantiate Thread each time the application needs to initiate an asynchronous task.
To abstract thread management from the rest of your application, pass the application’s tasks to an executor.
This section documents the use of Thread objects. Executors are discussed with other high-level concurrency objects.
Defining and Starting a Thread
An application that creates an instance of Thread must provide the code that will run in that thread. There are two ways to do this:
Provide a Runnable object. The Runnable interface defines a single method, run, meant to contain the code executed in the thread. The Runnable object is passed to the Thread constructor, as in the HelloRunnable example:
public class HelloRunnable implements Runnable {
public void run() {
System.out.println("Hello from a thread!");
}
public static void main(String args[]) {
(new Thread(new HelloRunnable())).start();
}
} Subclass Thread. The Thread class itself implements Runnable, though its run method does nothing. An application can subclass Thread, providing its own implementation of run, as in the HelloThread example:
public class HelloThread extends Thread {
public void run() {
System.out.println("Hello from a thread!");
}
public static void main(String args[]) {
(new HelloThread()).start();
}
}
Notice that both examples invoke Thread.start in order to start the new thread.
Which of these idioms should you use? The first idiom, which employs a Runnable object, is more general, because the Runnable object can subclass a class other than Thread. The second idiom is easier to use in simple applications, but is limited by the fact that your task class must be a descendant of Thread. This lesson focuses on the first approach, which separates the Runnable task from the Thread object that executes the task. Not only is this approach more flexible, but it is applicable to the high-level thread management APIs covered later.
The Thread class defines a number of methods useful for thread management. These include static methods, which provide information about, or affect the status of, the thread invoking the method. The other methods are invoked from other threads involved in managing the thread and Thread object. We’ll examine some of these methods in the following sections.
join and sleep
You can wait for another thread to end by calling its Join method. For example: static void Main()
Shared data is the primary cause of complexity and obscure errors in multithreading. Although often essential, it pays to keep it as simple as possible.
{
}
Thread t = new Thread (Go); t.Start(); t.Join(); Console.WriteLine (“Thread t has ended!”);
static void Go() {
for (int i = 0; i < 1000; i++) Console.Write (“y”);
}
This prints “y” 1,000 times, followed by “Thread t has ended!” immediately afterward. You can include a timeout when calling Join, either in milliseconds or as a TimeSpan. It then returns true if the thread ended or false if it timed out.
© 2006-2010 Joseph Albahari, O’Reilly Media, Inc. All rights reserved. www.albahari.com/threading/ 6
Thread.Sleep pauses the current thread for a specified period: Thread.Sleep (TimeSpan.FromHours (1)); // sleep for 1 hour
Thread.Sleep (500); // sleep for 500 milliseconds
While waiting on a Sleep or Join, a thread is blocked and so does not consume CPU resources.
How Threading Works
Multithreading is managed internally by a thread scheduler, a function the CLR typically delegates to the operating system. A thread scheduler ensures all active threads are allocated appropriate execution time, and that threads that are waiting or blocked (for instance, on an exclusive lock or on user input) do not consume CPU time.
On a single-processor computer, a thread scheduler performs time- slicing—rapidly switching execution between each of the active threads. Under Windows, a time-slice is typically in the tens-of- milliseconds region—much larger than the CPU overhead in actually switching context between one thread and another (which is typically in the few-microseconds region).
On a multi-processor computer, multithreading is implemented with a mixture of time-slicing and genuine concurrency, where different threads run code simultaneously on different CPUs. It’s almost certain there will still be some time-slicing, because of the operating system’s need to service its own threads—as well as those of other applications.
A thread is said to be preempted when its execution is interrupted due to an external factor such as time-slicing. In most situations, a thread has no control over when and where it’s preempted.
Threads vs Processes
A thread is analogous to the operating system process in which your application runs. Just as processes run in parallel on a computer, threads run in parallel within a single process. Processes are fully isolated from each other; threads have just a limited degree of isolation. In particular, threads share (heap) memory with other threads running in the same application. This, in part, is why threading is useful: one thread can fetch data in the background, for instance, while another thread can display the data as it arrives.
Thread.Sleep(0) relinquishes the thread’s current time slice immediately, voluntarily handing over the CPU to other threads. Framework 4.0’s new Thread.Yield() method does the same thing—except that it relinquishes only to threads running on the same processor.
Sleep(0) or Yield is occasionally useful in production code for advanced performance tweaks. It’s also an excellent diagnostic tool for helping to uncover thread safety issues: if inserting Thread.Yield() anywhere in your code makes or breaks the program, you almost certainly have a bug.
indeterminacy
As related to threading - what will happen
How threading works
How Threading Works
Multithreading is managed internally by a thread scheduler, a function the CLR typically delegates to the operating system. A thread scheduler ensures all active threads are allocated appropriate execution time, and that threads that are waiting or blocked (for instance, on an exclusive lock or on user input) do not consume CPU time.
On a single-processor computer, a thread scheduler performs time- slicing—rapidly switching execution between each of the active threads. Under Windows, a time-slice is typically in the tens-of- milliseconds region—much larger than the CPU overhead in actually switching context between one thread and another (which is typically in the few-microseconds region).
On a multi-processor computer, multithreading is implemented with a mixture of time-slicing and genuine concurrency, where different threads run code simultaneously on different CPUs. It’s almost certain there will still be some time-slicing, because of the operating system’s need to service its own threads—as well as those of other applications.
A thread is said to be preempted when its execution is interrupted due to an external factor such as time-slicing. In most situations, a thread has no control over when and where it’s preempted.
Sleep and yield
Thread.Sleep(0) relinquishes the thread’s current time slice immediately, voluntarily handing over the CPU to other threads. Framework 4.0’s new Thread.Yield() method does the same thing—except that it relinquishes only to threads running on the same processor.
Sleep(0) or Yield is occasionally useful in production code for advanced performance tweaks. It’s also an excellent diagnostic tool for helping to uncover thread safety issues: if inserting Thread.Yield() anywhere in your code makes or breaks the program, you almost certainly have a bug.
