Solving the Concurrency Issue in C Pthreads Quick Sort Implementation

Discover how to refine your C pthreads implementation for concurrent execution of a quick sort algorithm, and learn the key strategies to improve thread performance.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Threads not working concurrently in C pthreads

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Concurrency Issues in C Pthreads Quick Sort

When working with multithreading in C using pthreads, one common challenge that developers face is ensuring that threads work concurrently rather than sequentially. This guide will dive into a specific issue encountered with a quick sort algorithm implementation and how to resolve it for better performance.

The Problem

Imagine you have an efficient quick sort algorithm that you're attempting to run with the efficiency of multithreading. The idea is to divide the array into two halves so that each half can be processed concurrently by two threads. However, upon running your program, you notice that the threads are not executing in parallel, and rather, one thread is waiting for the other to finish.

The Symptoms

When running the quick sort algorithm with random input, the output appears with a noticeable delay:

[[See Video to Reveal this Text or Code Snippet]]

This indicates that while the threads are created, they do not execute simultaneously as expected, which results in inefficiency during the sorting process.

Solution Overview

To improve concurrency between the threads, you need to adjust how the workloads are divided among them. Here’s how you can do that:

1. Analyze Work Distribution

The initial implementation divides work based on the midpoint of the array, but this can lead to an uneven distribution of work. Often, one thread might be assigned a significantly larger workload than the other.

2. Monitor Workloads

You can modify your thread creation code to output the size of the workload assigned to each thread. Update your pthread_create section as follows:

[[See Video to Reveal this Text or Code Snippet]]

By observing the output from the above changes, you may see discrepancies in the amounts of work each thread is assigned:

Example Outputs:

[[See Video to Reveal this Text or Code Snippet]]

or

[[See Video to Reveal this Text or Code Snippet]]

3. Workload Balancing

Instead of dividing the array into just two large chunks, consider implementing a system where you break your task into smaller units or chunks. This allows each thread to pull tasks from a queue and ensures that workload distribution is more dynamic and balanced.

Implementing a Task Queue

To enhance concurrency:

Create a Task Queue: Implement a queue from which each thread can dynamically pull tasks as they finish their current task.

Load Balancing: This design allows threads to quickly take on more work, improving overall execution time and resource utilization.

Conclusion

By re-evaluating how workload is distributed among threads and modifying your approach to concurrency, you can unlock the full performance potential of your multi-threaded quick sort implementation in C. Remember, the more balanced your workload, the more effective your parallel execution will be.

If you come across concurrency issues in your future programming endeavors, try implementing a task queue and monitor how your threads perform with more evenly distributed work. Happy coding!