What Is Multithreading: A Guide to Multithreaded Applications
With the availability of GPUs and multi-core CPUs applications are becoming increasingly complex as developers leverage threads for maximum application performance and responsiveness. With this increased complexity comes a higher degree of difficulty when writing code, debugging, testing and managing the applications.
Here we explain what threads and multithreading are, what multithreaded applications are used for, TotalView vs GDB, and how to debug multithreaded applications.
What is a Thread in Programming?
A thread is an independent unit of execution created within the context of a process (or application that is being executed). When multiple threads are executing in a process at the same time, we get the term “multithreading.” Think of it as the application’s version of multitasking.
Learn more about concurrent vs parallel multithreading: How to Take Advantage of Multithreaded Programming and Parallel Programming in C/C++ (Perforce.com)
What Is Multithreading?
Multithreading is a model of program execution that allows for multiple threads to be created within a process, executing independently but concurrently sharing process resources. Depending on the hardware, threads can run fully parallel if they are distributed to their own CPU core.
What Is Multithreading Good For?
The main reason for incorporating threads into an application is to improve its performance. Performance can be expressed in multiple ways:
- A web server will utilize multiple threads to simultaneous process requests for data at the same time.
- An image analysis algorithm will spawn multiple threads at a time and segment an image into quadrants to apply filtering to the image.
- A ray-tracing application will launch multiple threads to compute the visual effects while the main GUI thread draws the final results.
Multithreading also leads to minimization and more efficient use of computing resources. Application responsiveness is improved as requests from one thread do not block requests from other threads.
Additionally, multithreading is less resource-intensive than running multiple processes at the same time. There is much more overhead, time consumption, and management involved in creating processes as compared to creating and managing threads.
Examples of Multithreaded Applications
Most of the applications that you use on a daily basis have multiple threads running behind the scenes. Consider your internet browser. At any given time, you may have numerous tabs open, each one displaying various types of content. Multiple threads of execution are used to load content, display animations, play a video, and so on.
Another example of a multithreaded program that we are all familiar with is a word processor. While you are typing, multiple threads are used to display your document, asynchronously check the spelling and grammar of your document, generate a PDF version of the document. These are all happening concurrently, with independent threads performing these tasks internally.
Common Issues in Multithreaded Applications
For all the advantages of using multiple threads, they add complexity and can create tough bugs to solve. There are some common scenarios where you may encounter challenges with debugging multithreaded applications. These include:
- Investigating data access issues where two threads are reading and modifying the same data. Without the proper use of locking mechanisms, data inconsistency and dead-lock situations can arise.
- Thread starvation and resource contention issues arise if many threads are trying to access a shared resource.
- Display issues can surface if threads are not coordinated correctly when displaying data.
See how both TotalView and GDB features handle these scenarios in a demonstration of the differences between the two debugging tools. Read the white paper.
How to Debug a Multithreaded Application
TotalView is designed for applications with hundreds of millions of lines of code and extensive parallelism and concurrency. TotalView’s comprehensive toolkit features capabilities for debugging multithreading in C, C++, and Fortran. TotalView supports debugging a large variety of threading technologies including pthreads, OpenMP, TBB, QThread, and many others, including debugging threads across a GPU.
The real thread debugging power of TotalView comes in its ability to individually control the execution of threads and proliferation of thread-level operations throughout the debugger. Users can set thread-level breakpoints to control the execution all the way down to an individual thread. They can also examine thread-specific data and easily compare the data across threads. With TotalView, developers can quickly gain an understanding of the state of their threads, examine data, and locate bugs in their code. Through debugging a multithreaded program, performance is boosted through improved parallelism, concurrency, and accuracy.
For a tutorial of debugging a multithreaded application with TotalView, the below video shows you how to easily examine and switch between threads, set thread-level breakpoints, and individually step and control threads.
Using TotalView for Debugging a Multithreaded Application
TotalView simplifies the debugging process for complex multithreaded programs. See why TotalView customers rely on our tools for all their debugging needs. Try free by clicking the button below.