Threads and Concurrency

 

Threads and Concurrency


1. Introduction: From Processes to Threads

Traditionally, the operating system used the process as the basic unit of execution. Each process has:

  • Its own address space

  • Its own resources

  • Its own execution context

However, modern applications (web browsers, servers, IDEs) need multiple activities to occur concurrently within the same program. Creating multiple processes for this purpose is expensive.

This leads to the concept of a thread.

2. Concept of a Thread

What Is a Thread?

A thread is the basic unit of CPU utilization within a process.

A thread consists of:

  • Program Counter (PC)

  • Register set

  • Stack

Threads share the following with other threads in the same process:

  • Code section

  • Data section

  • Heap

  • Open files and resources

👉 A process may contain one or more threads.

Single-Threaded vs Multithreaded Process

Single-threaded process

  • One execution path

  • If the thread blocks (e.g., I/O), the entire process blocks

Multithreaded process

  • Multiple execution paths within the same process

  • One thread can continue even if another is blocked

📌 Example (Web Browser):

  • One thread for UI

  • One thread for network operations

  • One thread for rendering


3. Benefits of Multithreading

There are  four major benefits of multithreading:

1️⃣ Responsiveness

  • Multithreading allows an application to remain responsive even if part of it is blocked.

  • Especially important for interactive applications.

Example:

  • A word processor continues responding to user input while saving a file in the background.

2️⃣ Resource Sharing

  • Threads share memory and resources of the process.

  • Communication between threads is faster and easier than inter-process communication (IPC).

Example:

  • Multiple threads accessing the same in-memory data structure.

3️⃣ Economy

  • Creating and managing threads is cheaper than processes.

  • Lower overhead in:

    • Creation

    • Context switching

    • Termination

4️⃣ Scalability

  • Multithreading allows programs to take advantage of multiple CPU cores.

  • Different threads can run on different processors simultaneously.

Example:

  • A database server using separate threads for client requests on different cores.

4. Threads and Concurrency

What Is Concurrency?

Concurrency refers to:

The ability of a system to execute multiple tasks overlapping in time.

  • On a single-core system → achieved via time-sharing

  • On a multi-core system → achieved via true parallelism

Threads are the primary mechanism used by the OS to support concurrency within a process.

5. Multithreading Models

Lets categorizes threading models based on the relationship between user threads and kernel threads.

User Threads vs Kernel Threads

User Threads

  • Managed by user-level thread library

  • Kernel is unaware of them

  • Faster to create and manage

Kernel Threads

  • Managed directly by the OS

  • Kernel schedules them

  • Slower but more powerful

6. Multithreading Models

1️⃣ Many-to-One Model

Description

  • Many user threads mapped to one kernel thread

  • Thread management done in user space

Advantages

  • Fast thread creation

  • Efficient context switching

Disadvantages

  • If one thread blocks, all threads block

  • No parallelism on multi-core systems

Example

  • Early Java thread libraries



2️⃣ One-to-One Model

Description

  • Each user thread maps to a kernel thread

Advantages

  • True parallelism

  • One thread blocking does not affect others

Disadvantages

  • Overhead of kernel thread creation

  • OS may limit number of threads

Examples

  • Linux

  • Windows

  • macOS

📌 This is the most commonly used model today.

3️⃣ Many-to-Many Model

Description

  • Many user threads mapped to many kernel threads

  • OS decides how many kernel threads to create

Advantages

  • Combines flexibility of user threads

  • Allows parallelism

  • Avoids blocking problem

Disadvantages

  • Complex to implement

Example

  • Solaris (older versions)



4️⃣ Two-Level Model (Variation)

  • Similar to many-to-many

  • Allows binding specific user threads to kernel threads

7. Multithreading Issues (Brief Overview)

Major challenges:

  • Thread synchronization

  • Race conditions

  • Deadlocks

  • Data consistency

These issues arise because threads share resources.

8. Summary Table

Aspect            Threads
Basic unit                CPU execution
Resource sharing                Yes (within process)
Creation cost                Low
Communication                Fast
Enables                Concurrency & parallelism

9. Key Takeaways 

  • A thread is a lightweight process

  • Multithreading improves performance, responsiveness, and scalability

  • Modern OSes use the one-to-one model

  • Threads are essential for concurrent and parallel programming

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