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Multithreading is a core concept in computer science that allows multiple threads to execute concurrently within a single process, improving the performance and responsiveness of applications. Understanding its fundamental principles is crucial for designing efficient and reliable software. Here are the first principles of multithreading that every developer should know:

1. Concurrency vs. Parallelism: Concurrency involves multiple threads making progress within a single application, often interleaved, while parallelism involves executing multiple threads or processes simultaneously, usually on different processors or cores. Understanding the distinction helps in designing appropriate multithreading strategies.

2. Thread Safety: This principle is about ensuring that a function or data structure operates correctly when accessed by multiple threads simultaneously. This often involves mechanisms to prevent data races and other forms of corruption.

3. Synchronization: To maintain thread safety, synchronization mechanisms like mutexes, semaphores, and locks are used to control access to shared resources or critical sections, ensuring that only one thread accesses a shared resource at a time.

4. Deadlock Prevention: A deadlock occurs when two or more threads are waiting on each other to release resources, causing them to hang indefinitely. Understanding how to prevent or resolve deadlocks, through careful design and deadlock detection algorithms, is crucial.

5. Thread Local Storage (TLS): TLS allows threads to have their own instance of a variable, avoiding interference between threads and reducing the need for synchronization.

6. Task Parallelism and Data Parallelism: These are strategies to divide work among threads. Task parallelism involves distributing different tasks across threads, while data parallelism involves dividing data across threads, allowing similar operations to be performed concurrently.

7. Scalability: It's essential to design multithreaded applications that can scale with the number of available processors or cores. This means ensuring that adding more resources (like CPU cores) will proportionately increase the application's performance.

8. Efficiency and Overhead: Multithreading introduces overhead, including context switching and synchronization. Effective multithreaded design minimizes these costs to ensure that the benefits of parallel execution outweigh the overhead.

9. Immutable Objects: Using immutable objects can simplify multithreading by eliminating the need for synchronization, as data cannot change once it has been created.

10. Memory Visibility and Ordering: Ensuring that changes made by one thread to shared data are visible to other threads is a key consideration. This involves understanding memory models and using memory barriers or volatile variables as needed to prevent caching issues or reordering of instructions by compilers and processors.

These principles form the foundation of effective and efficient multithreaded programming, guiding developers in creating applications that are safe, responsive, and scalable.