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Chapter 4: Threads
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Threads vs. Processes
---------------------
- **Process**
  - Heavyweight unit of execution with its own memory space.
  - Context switching between processes is expensive.
  - Processes do not share memory by default.

- **Thread**
  - Lightweight unit of execution within a process.
  - Shares code, data, and resources of the process.
  - Each thread has its own program counter, register set, and stack.
  - Cheaper and faster context switching than processes.

Benefits of Multithreading
--------------------------
- **Responsiveness**: Applications remain interactive even during long tasks.
- **Resource Sharing**: Threads share memory and resources of a process.
- **Economy**: More efficient than creating new processes.
- **Scalability**: Can take advantage of multiprocessor systems.

Multithreading Models
---------------------

Many-to-One
^^^^^^^^^^^
- Multiple user-level threads map to a single kernel thread.
- If one thread blocks, the entire process blocks.

.. code-block:: text

   User Threads:  T1   T2   T3
                    \   |   /
                     \  |  /
                   [ Kernel Thread K1 ]

One-to-One
^^^^^^^^^^
- Each user thread maps to its own kernel thread.
- More concurrency, but higher overhead.

.. code-block:: text

   User Threads:  T1   T2   T3
                   |    |    |
                   v    v    v
              Kernel:   K1   K2   K3

Many-to-Many
^^^^^^^^^^^^
- Many user-level threads multiplexed over fewer or equal kernel threads.
- Provides flexibility and concurrency.

.. code-block:: text

   User Threads:  T1   T2   T3   T4   T5
                    \   |   / \   |   /
                     \  |  /   \  |  /
                 Kernel Threads:  K1   K2

Two-Level Model
^^^^^^^^^^^^^^^
- Variation of many-to-many.
- Allows binding some user threads directly to kernel threads.

.. code-block:: text

   User Threads:  T1   T2   T3   T4
                   |   / \     |
                   v  v   v    v
                 Kernel Threads: K1   K2

Threading APIs
--------------
- **Pthreads (POSIX Threads)**
  - Standard API for thread creation and synchronization in UNIX/Linux.
  - Functions for creating/joining threads, mutexes, condition variables.

- **Windows Threads**
  - Windows API for multithreading.
  - Managed via functions like `CreateThread`, `WaitForSingleObject`.

- **Java Threads**
  - Built-in support with `Thread` class and `Runnable` interface.
  - Managed by the Java Virtual Machine (JVM).

- **Other APIs**
  - OpenMP: Directive-based parallelism for C/C++/Fortran.
  - C++11 `<thread>`: Standard threading library in modern C++.