Multithreading

🧠 What is Multithreading?

Multithreading is the ability of a single process to have multiple threads of execution, which run concurrently (or in parallel on multi-core CPUs).

A thread is the smallest unit of CPU execution inside a process.
All threads in a process share the same memory space but have independent execution paths.

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💡 Analogy:

Think of a web browser (process):

• One thread renders the UI,

• Another fetches network data,

• Another plays audio/video.

All are part of one app, running in parallel or concurrently.

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🔍 Thread vs Process

Feature	Process	Thread
🧠 Memory	Has its own memory space	Shares memory with other threads in the same process
📦 Overhead	Heavy (OS allocates new memory, PCB)	Lightweight (uses shared data structures)
🚀 Switching	Slower (more context to save/restore)	Faster (less state to manage)
🔌 Isolation	Processes are isolated	Threads are tightly coupled
🔧 Use Case	Run separate apps	Run parallel tasks within the same app

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⚙️ How Multithreading Works

Shared between all threads:

• Code

• Data segment (global variables)

• Open files

• Heap (dynamic memory)

Each thread has its own:

• Stack

• Registers

• Program Counter

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🧪 Real-World Use Cases

System	Use of Threads
Web server	One thread per request (Apache, Nginx)
Game engines	Separate threads for rendering, physics, AI
Android apps	Main thread (UI), background threads (network, DB)
IDEs	UI thread + background compiler thread

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⚠️ Multithreading Challenges

Challenge	Why it’s Hard
🔄 Race Conditions	Multiple threads access same data → inconsistency
🔒 Deadlocks	Two threads wait for each other’s resources
🛠️ Synchronization Overhead	Locks/mutexes slow things down
🧪 Debugging	Timing issues make bugs hard to reproduce

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🔧 Threading Models

Model	Description
User-Level Threads	Managed by user-level libraries (e.g., green threads)
Kernel-Level Threads	Managed directly by OS (e.g., POSIX threads)
Hybrid	Combines both (most modern OSs)

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📜 Code Example (Java)

public class HelloThread extends Thread {
    public void run() {
        System.out.println("Thread running: " + Thread.currentThread().getName());
    }
 
    public static void main(String[] args) {
        HelloThread t1 = new HelloThread();
        HelloThread t2 = new HelloThread();
        t1.start();
        t2.start();
    }
}

Both t1 and t2 will run in parallel (if multicore) or concurrently (if single-core).

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🧠 Interview-Ready Definition:

Multithreading is the execution of multiple threads within a single process, allowing tasks to run concurrently. Threads share the same memory space but operate independently, enabling more efficient CPU use and responsive applications — especially on multi-core systems.