Do threads run in parallel?

Yes, threads can run in parallel, but whether they actually do depends on the system's hardware and how threads are managed. Let's break down the concept of parallel execution and explain how it works with threads in Java:

1. Parallel Execution on Multi-Core Systems

On systems with multiple CPU cores, threads can run in parallel. This means that each thread can be assigned to a different core, allowing them to run simultaneously at the exact same time. This is true parallelism.

Example:

• If you have a quad-core CPU, you can run up to four threads in parallel, with each thread running on a separate core. This results in true simultaneous execution, improving performance for CPU-bound tasks (tasks that require heavy computation).

2. Concurrency (Single-Core Systems)

On a single-core CPU, threads cannot run in parallel, as there is only one core available. However, the operating system can still simulate concurrent execution by rapidly switching between threads. This technique is called context switching.

• Even though only one thread is executing at any given time, the operating system switches between threads so quickly that it appears as if they are running concurrently (but not simultaneously).

This is often called time-sharing, where the CPU allocates small time slices to each thread, giving the illusion that all threads are running at once.

3. How Java Handles Parallelism and Concurrency

In Java, the Thread class allows you to create and manage threads, and whether they run in parallel or concurrently depends on the system's resources:

• On multi-core systems, Java's thread scheduler (along with the OS) can allocate different threads to different CPU cores, enabling parallel execution.
• On single-core systems, Java can only simulate parallelism by using context switching (i.e., threads run concurrently).

Java also provides the ExecutorService and ForkJoinPool classes for more efficient management of parallel tasks. These are particularly useful for managing a large number of threads and achieving better parallelism and scalability.

4. Thread Pooling for Parallelism

Using a thread pool (such as ExecutorService) in Java allows multiple tasks to be executed in parallel by reusing a fixed number of threads. The thread pool can manage and distribute tasks across available cores for parallel execution.

Conclusion:

• Yes, threads can run in parallel on systems with multiple CPU cores.
• On a single-core system, threads run concurrently (not simultaneously) due to context switching.
• Java provides tools to leverage both concurrency (via Thread, ExecutorService, etc.) and parallelism (using multi-core systems and tools like ForkJoinPool).

Recommended Courses:

To understand parallelism, concurrency, and how Java handles them, you can explore the following courses from DesignGurus.io:

• Grokking Multithreading and Concurrency for Coding Interviews
• Grokking Data Structures & Algorithms for Coding Interviews
• Grokking the System Design Interview

These courses will help you understand the underlying mechanisms and best practices for handling threads in Java for both concurrency and parallelism.