Defining Multithreading
Multithreading, as the term suggests, is the ability to run threads concurrently through a single executable code. Within a process there may exist multiple threads and each thread is an independent sequence of execution in a program. A program is an instance of a process that runs on a computer’s operating system. Each program’s instance can be made to have multiple threads with different tasks. The principal advantage of multithreading is that it enables a program to execute multiple operations in parallel, instead of completing them sequentially.Most computer processors available today possess more than one core and that by itself hints at the ability to perform multithreading. Needless to say, multithreading makes the best use of the cores where the program being executed is able to perform several functions at a time. A good example would be, while one thread is busy taking user input, another can conduct some necessary background processing allowing the application to function smoothly.
Why Use Multithreading?
With most recent advances in technology, it would be wrong to say that multithreading has no advantages. It is especially useful in situations where several threads can do several pieces of work at the same time. For example, in a video game, one thread can be devoted to rendering the visual graphics, another thread to receiving data entered by the players while the third one can be allocated the network tasks. These can all take place simultaneously so that the smooth gaming experience is maintained.Another such application can be in programs meant for analyzing and processing data. A program can partition the job into logical sections and each section assigned to an independent thread where multiple threads can be working on it simultaneously. This helps in drastically improving the time it would take to finish the task.
Requests from multiple clients can be handled independently using multithreading on server applications. This provision makes it possible for a server to continue responding to requests even when multiple requests are being simultaneously processed.
The C++ Multithreading Library
The thread library allows threads to be created and managed in the C++ programming language for server applications. This C++ feature was supported from the version of the language designated C++11. Components of this library facilitate the creation, management as well as synchronization between threads.The first step in the creation of a thread is to specify the function to be called by the thread before the thread is launched. C++ allows for easy creation of thread management interfaces that enable developers to create threads, wait for them to complete, and even throw and catch exceptions.
Threads are created separately from the main thread and thus run separately. With regards to execution context, each thread possesses its own set of local variables and even a different program counter. However, all threads within a process share the same memory space which allows for communication and sharing of data. This shared memory is a double-edged sword, as it can lead to issues if multiple threads try to modify the same data simultaneously.
Synchronization in Multithreading
If a single data structure or location is used by multiple threads, proper precautions must be taken to ensure that concurrent threads can read and write to that location without interfering with one another. Without orderly control, multiple threads can potentially modify the same data simultaneously, resulting in unpredictable performance and errors.
To address these concurrency concerns, the C++ standards define several mechanisms, including mutex and lock . A mutex (which stands for mutual exclusion) is a construct to eliminate more than one thread accessing the same critical resource at the same time. First, if a thread wants to use the shared resource, it must unlock the mutex. The problem is when another thread has already taken the lock. Then, the thread must wait for the mutex to become free.
Other than mutex, C++ also offers other types of synchronisation like condition variables. There are a variety of advanced locks available including split locks and even lock-free synchronization methods. These tools help to reduce race conditions, where several threads are racing against one another and producing results which are not the expected behavior.
Barriers to Multithreading
Even though multithreading has the potential to enhance performance, it also presents a number of issues. The most troublesome of those is Race Conditions. A race condition occurs when two or more threads attempt to access the same shared resource simultaneously, yielding unpredictable results. Race conditions can only be avoided with effective synchronization, but the effective management of such synchronizations becomes more difficult the greater the number of threads there are.Another problem presents itself in Deadlocks. A deadlock results when two (or more) threads need a resource already held by another thread only for them to get blocked forever. Once again, deadlocks tend to be challenging – though not always impossible – to troubleshoot and overcome and greatly compromise a system’s performance. Multithreaded applications should be carefully constructed to avoid deadlocks by ensuring consistent resource acquisition ordering per thread.
Finally, as is the case for most systems, Thread Management Overhead becomes the bane of multithreading as the overhead of managing multiple threads greatly outweighs its benefits. Making threads, managing these threads as well as synchronizing them consumes both time and resources and so creating too many threads is counterproductive and results in a performance dip instead of a boost.
Thread Safety
In computing, one of the common terms that is used is the term thread safety. This term refers to code that is written in such a way that it will function as intended when multiple threads access it simultaneously. In order for a piece of code to be thread-safe, developers have to design users into their systems such that shared resources are not used at the same time by more than one thread or that a thread does not make modifications to a resource that will yield indecisive or invalid results.There are a number of techniques that can be invoked to enhance or build thread safety. Some of these methods or techniques are:
- Mutexes and locks: These tools help ensure that only one thread at a time can access a shared resource.
- Atomic operations: Setting locks is not necessary as atomic operations permit threads to change a resource in a way that maintains its coherence.
- Immutability: There are times when it is appropriate to render data immutable or read-only which can resolve problems involving more than one thread attempting to change the same resource.
By using such techniques, developers are able to create codes that are secure to be executed in a multi-threaded environment.
