Articolo
11/17/2011

Best Practices for Managed Threading

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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.

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

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 that 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.

Static Members and Static Constructors

A class is not initialized until its class constructor (static constructor in C#, Shared Sub New in Visual Basic) has finished running. To prevent the execution of code on a type that is not initialized, the common language runtime blocks all calls from other threads to static members of the class (Shared members in Visual Basic) until the class constructor has finished running.

For example, if a class constructor starts a new thread and the thread procedure calls a static member of the class, the new thread blocks until the class constructor completes.

This applies to any type that can have a static constructor.

Blocking Calls on the User Interface Thread

Avoid making blocking calls such as Join or WaitOne on the main application thread of your Silverlight-based application. The user interface is unresponsive until these calls return.

General Recommendations

Consider the following guidelines when using multiple threads:

Do not 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. The Monitor.PulseAll method is useful in situations where worker threads block.
Do not use types as lock objects. That is, avoid code such as lock(typeof(X)) in C# or SyncLock(GetType(X)) in Visual Basic, or the use of the Monitor.Enter method with Type objects. For a given type, there is only one instance of System.Type per application domain. If the type you take a lock on is public, code other than your own can take locks on it, leading to deadlocks.
Use caution when locking on instances, for example lock(this) in C# or SyncLock(Me) in Visual Basic. If other code in your application, external to the type, takes a lock on the object, deadlocks could occur.
Make sure 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.
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 will 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.
Consider using methods of the Interlocked class for simple state changes, instead of using the lock statement (SyncLock in Visual Basic). The lock statement is a good general-purpose tool, but the Interlocked class provides better performance for updates that must be atomic. Internally, it executes a single lock prefix if there is no contention. In code reviews, watch for code like that shown in the following examples. In the first example, a state variable is incremented:
```
SyncLock lockObject
    myField += 1
End SyncLock
```
```
lock(lockObject) 
{
    myField++;
}
```
You can improve performance by using the Increment method instead of the lock statement, as follows:
```
System.Threading.Interlocked.Increment(myField)
```
```
System.Threading.Interlocked.Increment(myField);
```
In the second example, a reference type variable is updated only if it is a null reference (Nothing in Visual Basic).
```
If x Is Nothing Then
    SyncLock lockObject
        If x Is Nothing Then
            x = y
        End If
    End SyncLock
End If
```
```
if (x == null)
{
    lock (lockObject)
    {
        if (x == null)
        {
            x = y;
        }
    }
}
```
Performance can be improved by using the CompareExchange method instead, as follows:
```
System.Threading.Interlocked.CompareExchange(x, y, Nothing)
```
```
System.Threading.Interlocked.CompareExchange(ref x, y, null);
```

Recommendations for Class Libraries

Consider the following guidelines when designing class libraries for multithreading:

Avoid the need for synchronization, if possible. This is especially true for heavily used code. For example, an algorithm might be adjusted to tolerate a race condition rather than eliminate it. Unnecessary synchronization decreases performance and creates the possibility of deadlocks and race conditions.
Make static data (Shared in Visual Basic) thread safe by default.
Do not make instance data thread safe by default. Adding locks to create thread-safe code decreases performance, increases lock contention, and creates the possibility for deadlocks to occur. In common application models, only one thread at a time executes user code, which minimizes the need for thread safety. For this reason, the .NET Framework class libraries are not thread safe by default.
Avoid providing static methods that alter static state. In common server scenarios, static state is shared across requests, which means multiple threads can execute that code at the same time. This opens up the possibility of threading bugs. Consider using a design pattern that encapsulates data into instances that are not shared across requests. Furthermore, if static data is synchronized, calls between static methods that alter state can result in deadlocks or redundant synchronization, adversely affecting performance.

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