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→Added OpenMP thread creation example
Our team will do a research on comparison between c++ 11 threads library and openmp. We will code problems with both libraries to demonstrate the coding differences and do the performance analysis on those two approaches. We will highlight the techniques and best practices to achieve the best performance and avoid the pitfalls.
== C++11 Threading ==
C++ threading consists of the creation of a thread that works on a function. By having multiple threads, multiple functions are able to be performed at the same time which enables parallel processing.
<syntaxhighlight lang="cpp">
template<class Function, class... args>
explicit thread( Function&& f, Args&&... args);
</syntaxhighlight>
As seen above, a single thread takes a function through overloading and performs the task by passing the arguments to the function parameter which would execute separate to other threads. Once these threads are finished
performing their function, the child thread must be joined using the .join() function in the thread library to the parent thread.
<syntaxhighlight lang="cpp">
#include <thread>
#include <iostream>
#include <chrono>
void thread1() {
std::this_thread::sleep_for(std::chrono::seconds(2));
std::cout << "Hello from thread 1" << std::endl;
}
void thread2() {
std::this_thread::sleep_for(std::chrono::seconds(3));
std::cout << "Hello from thread 2" << std::endl;
}
void thread3() {
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << "Hello from thread 3" << std::endl;
}
int main() {
std::cout << "Creating thread 1: " << std::endl;
std::thread t1(thread1); //appears 2nd
std::cout << "Creating thread 2: " << std::endl;
std::thread t2(thread2); //appears 3rd
std::cout << "Creating thread 3: " << std::endl;
std::thread t3(thread3); //appears 1st
t1.join();
t2.join();
t3.join();
}
//Creating thread 1:
//Creating thread 2:
//Creating thread 3:
//Hello from thread 3
//Hello from thread 1
//Hello from thread 2
</syntaxhighlight>
The code above gives a simple example on how threads are created and joined back to the parent thread in the end.
== OpenMP Threading ==
OpenMP threads consists of a parallel region that forks a master thread into multiple child threads that each performs a function in parallel amongst each other and joins back to its parent thread after reaching the end of the parallel region.
<syntaxhighlight lang="cpp">
#pragma omp construct [clause, ...]
structured block
</syntaxhighlight>
The construct identifies what block of code is being executed in parallel and the clause qualifies the parallel construct. In comparison to the C++ 11 thread library, the threads are performed in a region while the C++ 11 thread library have to be specifically created and joined back to the parent thread.
<syntaxhighlight lang="cpp>
#include <iostream>
#include <omp.h>
void main() {
omp_set_num_threads(3);
#pragma omp parallel
{
int tid = omp_get_thread_num();
#pragma omp critical
std::cout << "Thread " << tid + 1 << std::endl;
}
}
//Thread 1
//Thread 2
//Thread 3
</syntaxhighlight>
The code above shows the creation of 3 threads in a parallel region which are performed in sequence using critical. This highlights the difference between the c++11 thread library as no joining is required after
the threads concludes their function.
== References ==
Thread Support Library https://en.cppreference.com/w/cpp/thread
