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Thread pool in C++

A thread pool is a collection of threads, each with a particular task. As a result, various threads do distinct types of jobs. As a result, each thread specializes in different tasks. A thread is responsible for executing a specific set of similar functions, while another thread performs a distinct set of identical functions, and this pattern continues for additional threads. Each threads should focus on comparable functions because separate threads have distinct parameters.

This thread pool must be maintained in C++. C++ needs a library for generating and managing thread pools. It is likely because there are several methods for building a thread pool. As a result, a C++ programmer must design a thread pool depending on the requirements.

What is a thread?

A thread is a component created by creating the thread class. In common instantiation, the first parameter to the thread constructor is usually the name of a top-level method. The remainder of the thread constructor inputs are function arguments. The function begins to execute as soon as the thread is created. The C++ main() function is an upper-level function. Additional responsibilities in that global scope are top-level operations. The main() function is a thread that does not require explicit declaration, unlike other threads.

What properties should a thread pool have?

A thread pool is just a collection of threads that may be used. It can be expressed in C++ as an array of std::thread or as a vector. <std::vectorstd::thread> is appropriate for any expansions.

At some time, each thread within the thread pool may be assigned a job. When the thread of work is created, the particular job is unknown. In C++, this indicates that a thread in a thread pool:

  • It should be able to perform arbitrary functions that allows any set of arguments and any return value type.
  • It should be allowed to communicate task execution results back to the task's publisher.
  • It should be able to be woken up to do a job when needed while not using excessive CPU resources when not required.
  • When necessary, the controller thread must be able to regulate it to halt tasks, stop accepting tasks, reject unfinished jobs, etc.

The current C++ method for the first would be to leverage the framework given by the functionality of the header file (std::bind, std::function, etc.) when combined with the template parameters package. The traditional technique for the second is to implement the callback function simultaneously as the task is published; the newer C++ approach would involve using std::packaged_task in conjunction with std::future. Consider std::condition_variable for less frequent jobs or std::this_thread::yield for more frequent tasks for the third. For the fourth, it may be performed by making an internal variable a representation of the token and making each worker thread verify that token regularly.

The Thread Pool Requirements and Architecture in C++

The design for the thread pool explained in the previous paragraph is simple. It implies that the thread pool does not have any unusual needs. Thus, we may assume:

  • We can pass any function that accepts any form of input parameter. For ease of use, we may argue for our tasks std:: variants in this scenario.
  • The queue must be thread-safe because the thread pool requires a queue to contain jobs and their parameters.
  • The thread pool will only stop running after all the jobs in the queue have been completed.

The key ideas required for our execution are summarized in the list below.

  • Tasks - A structure that represents a task that our thread pool may execute. In its most basic form, this object contains a lambda (or function pointer) of the operation that must be performed and its arguments.
  • A task queue- It is a container for jobs that will be picked out from the thread pool.
  • The thread pool - It is a struct or class that contains the thread pool functionality. It contains the task queue, a std::vector of std::jthreads, and the logic for pushing and popping from the task queues.

Implementing a Thread Pool in C++

Looking at the implementation and some key points on the code after that.

  • The thread pool is a management layer over the task queue.
  • Pushing to the thread pool is the same as adding a job to the queue.
  • There is no need for a popping mechanism because our basic architecture requires the thread pool to terminate only after completing all tasks.
  • The destructor has to ensure that all threads within the pool are stopped. As a result, it should send a stop task to every thread in the pool.

How do the Threads execute Tasks?

We can see n_threads std::jthread items in the _threads vector in the thread pool code above. However, what code is executed by each thread? Simply the logic to pop from the queue and stop when necessary.

Using the Thread Pool

Simple application that displays an incremented integer by using the thread pool. It also provides the thread id to confirm that each count is indeed being processed on a distinct thread.

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