Managed Threading Best Practices

Článek
10/17/2014

Multithreading requires careful programming. For most tasks, you can reduce complexity by queuing requests for execution by thread pool threads. This topic addresses more difficult situations, such as coordinating the work of multiple threads, or handling threads that block.

For guidelines on implementing multithreading in class libraries, see Threading Design Guidelines.

Deadlocks and Race Conditions

Multithreading solves problems with throughput and responsiveness, but in doing so it introduces new problems: deadlocks and race conditions.

Deadlocks

A deadlock occurs when each of two threads tries to lock a resource the other has already locked. Neither thread can make any further progress.

Many methods of the managed threading classes provide time-outs to help you detect deadlocks. For example, the following code attempts to acquire a lock on the current instance. If the lock is not obtained in 300 milliseconds, Monitor.TryEnter returns false.

If Monitor.TryEnter(Me, 300) Then
    Try
        ' Place code protected by the Monitor here.
    Finally
        Monitor.Exit(Me)
    End Try
Else
    ' Code to execute if the attempt times out.
End If
[C#]
if (Monitor.TryEnter(this, 300)) {
    try {
        // Place code protected by the Monitor here.
    }
    finally {
        Monitor.Exit(this);
    }
}
else {
    // Code to execute if the attempt times out.
}

Race Conditions

A race condition is a bug that occurs when the outcome of a program depends on which of two or more threads reaches a particular block of code first. Running the program many times produces different results, and the result of any given run cannot be predicted.

A simple example of a race condition is incrementing a field. Suppose a class has a private static field (Shared in Visual Basic) that is incremented every time an instance of the class is created, using code such as objCt++; (C#) or objCt += 1 (Visual Basic). This operation requires loading the value from objCt into a register, incrementing the value, and storing it in objCt.

In a multithreaded application, a thread that has loaded and incremented the value might be preempted by another thread which performs all three steps; when the first thread resumes execution and stores its value, it overwrites objCt without taking into account the fact that the value has changed in the interim.

This particular race condition is easily avoided by using methods of the Interlocked class, such as Interlocked.Increment. To read about other techniques for synchronizing data among multiple threads, see Synchronizing Data for Multithreading.

Race conditions can also occur when you synchronize the activities of multiple threads. Whenever you write a line of code, you must consider what might happen if a thread were preempted before executing the line (or before any of the individual machine instructions that make up the line), and another thread overtook it.

Number of Processors

Multithreading solves different problems for the single-processor computers that run most end-user software, and the multiprocessor computers typically used as servers.

Single-Processor Computers

Multithreading provides greater responsiveness to the computer user, and uses idle time for background tasks. If you use multithreading on a single-processor computer:

Only one thread runs at any instant.
A background thread executes only when the main user thread is idle. A foreground thread that executes constantly starves background threads of processor time.
When you call the Thread.Start method on a thread, that thread does not start executing until the current thread yields or is preempted by the operating system.
Race conditions typically occur because the programmer did not anticipate the fact that a thread can be preempted at an awkward moment, sometimes allowing another thread to reach a code block first.

Multiprocessor Computers

Multithreading provides greater throughput. Ten processors can do ten times the work of one, but only if the work is divided so that all ten can be working at once; threads provide an easy way to divide the work and exploit the extra processing power. If you use multithreading on a multiprocessor computer:

The number of threads that can execute concurrently is limited by the number of processors.
A background thread executes only when the number of foreground threads executing is smaller than the number of processors.
When you call the Thread.Start method on a thread, that thread might or might not start executing immediately, depending on the number of processors and the number of threads currently waiting to execute.
Race conditions can occur not only because threads are preempted unexpectedly, but because two threads executing on different processors might be racing to reach the same code block.

General Recommendations

Consider the following guidelines when using multiple threads:

Don't use Thread.Abort to terminate other threads. Calling Abort on another thread is akin to throwing an exception on that thread, without knowing what point that thread has reached in its processing.
Don't use Thread.Suspend and Thread.Resume to synchronize the activities of multiple threads. Do use Mutex, ManualResetEvent, AutoResetEvent, and Monitor.
Don't control the execution of worker threads from your main program (using events, for example). Instead, design your program so that worker threads are responsible for waiting until work is available, executing it, and notifying other parts of your program when finished. If your worker threads do not block, consider using thread pool threads. Monitor.PulseAll is useful in situations where worker threads block.
Do ensure that a thread that has entered a monitor always leaves that monitor, even if an exception occurs while the thread is in the monitor. The C# lock statement and the Visual Basic SyncLock statement provide this behavior automatically, employing a finally block to ensure that Monitor.Exit is called. If you cannot ensure that Exit will be called, consider changing your design to use Mutex. A mutex is automatically released when the thread that currently owns it terminates.
Do use multiple threads for tasks that require different resources, and avoid assigning multiple threads to a single resource. For example, any task involving I/O benefits from having its own thread, because that thread will block during I/O operations and thus allow other threads to execute. User input is another resource that benefits from a dedicated thread. On a single-processor computer, a task that involves intensive computation coexists with user input and with tasks that involve I/O, but multiple computation-intensive tasks contend with each other.

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