MSDN Documentation

What is Multithreading?

Multithreading is a programming model that allows a program to execute multiple threads (sequences of instructions) concurrently. Each thread runs independently within the same process, sharing the process's memory space and resources. This is in contrast to multiprocessing, where multiple independent processes run concurrently, each with its own memory space.

Think of a single thread as a single worker performing tasks sequentially. Multithreading is like having multiple workers within the same workshop, each capable of performing different tasks simultaneously, which can significantly speed up the overall work completion.

Advantages of Multithreading

• Improved Responsiveness: In applications with user interfaces (like desktop applications), one thread can handle user input while other threads perform background tasks, preventing the UI from freezing.
• Performance Gains: On multi-core processors, threads can run in parallel, leading to faster execution for CPU-bound tasks.
• Resource Sharing: Threads within the same process share memory and resources, which can be more efficient than inter-process communication.
• Simplified Program Structure: For certain problems, breaking them down into concurrent threads can lead to more modular and manageable code.

Key Concepts

Threads

A thread is the smallest unit of execution that can be scheduled by an operating system. Each thread has its own program counter, stack, and set of registers.

Concurrency vs. Parallelism

Concurrency refers to the ability to deal with multiple things at once. It doesn't necessarily mean they are happening at the exact same time. On a single-core processor, threads might interleave their execution rapidly, creating the illusion of concurrency.

Parallelism refers to the ability to do multiple things at the exact same time. This requires multiple processing units (e.g., multiple CPU cores).

Race Conditions

A race condition occurs when the outcome of a program depends on the non-deterministic interleaving of operations from multiple threads accessing shared data. For example, if two threads try to increment a counter simultaneously without proper synchronization, the final value might be incorrect.

Consider this scenario:

int sharedCounter = 0;

// Thread 1
sharedCounter = sharedCounter + 1;

// Thread 2
sharedCounter = sharedCounter + 1;
                    

If Thread 1 reads sharedCounter (which is 0), then Thread 2 reads sharedCounter (which is still 0), and then Thread 1 writes 1, followed by Thread 2 writing 1, the final value of sharedCounter will be 1, not the expected 2.

Synchronization

Synchronization mechanisms are crucial for managing access to shared resources among multiple threads, preventing race conditions and ensuring data integrity. Common synchronization primitives include:

• Mutexes (Mutual Exclusion Locks): Allow only one thread to access a critical section of code at a time.
• Semaphores: Control access to a resource by a limited number of threads.
• Monitors: Higher-level synchronization constructs that bundle data with the procedures that operate on it, ensuring exclusive access.
• Critical Sections: A specific block of code that should not be executed by more than one thread at a time.

Common Pitfalls

• Deadlocks: Occur when two or more threads are blocked forever, each waiting for the other to release a resource.
• Livelocks: Threads are not blocked but continue to change their state in response to each other without making any progress.
• Starvation: A thread is perpetually denied access to necessary resources and thus unable to make progress.
• Overhead: Creating and managing threads incurs overhead. Creating too many threads can sometimes degrade performance.

Getting Started

Modern programming languages and frameworks provide robust support for multithreading. For example, in C#, you can use the System.Threading namespace. In C++, the standard library provides <thread>. Many other languages, like Python (with its threading module) and Java, offer similar capabilities.

Here's a simple conceptual example of creating a thread in C#:

using System;
using System.Threading;

public class Example
{
    public static void ThreadWork()
    {
        Console.WriteLine("This is running on a separate thread.");
    }

    public static void Main(string[] args)
    {
        Thread newThread = new Thread(new ThreadStart(ThreadWork));
        newThread.Start(); // Start the thread execution
        Console.WriteLine("Main thread continues execution.");
        // The main thread will exit, potentially before newThread finishes.
        // For robust applications, you'd use Join() or other mechanisms.
    }
}