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+---
+category: tool
+tool: OpenMP
+filename: learnopenMP.cpp
+contributors:
+ - ["Cillian Smith", "https://github.com/smithc36-tcd"]
+---
+
+**OpenMP** is a library used for parallel programming on shared-memory machines.
+OpenMP allows you to use simple high-level constructs for parallelism,
+while hiding the details, keeping it easy to use and quick to write.
+OpenMP is supported by C, C++, and Fortran.
+
+## Structure
+
+Generally an OpenMP program will use the following structure.
+
+- **Master**: Start a Master thread, which will be used to set up the environment and
+initialize variables
+
+- **Slave**: Slave threads are created for sections of code which are marked by a special
+directive, these are the threads which will run the parallel sections.
+
+Each thread will have its own ID which can be obtained using the function
+`omp_get_thread_num()`, but more on that later.
+
+```
+ __________ Slave
+ /__________ Slave
+ /
+Master ------------- Master
+ \___________ Slave
+ \__________ Slave
+
+```
+
+## Compiling and running OpenMP
+
+A simple "hello world" program can be parallelized using the `#pragma omp parallel` directive
+
+```cpp
+#include
+
+int main() {
+ #pragma omp parallel
+ {
+ printf("Hello, World!\n");
+ }
+ return 0;
+}
+```
+
+Compile it like this
+
+```bash
+# The OpenMP flat depends on the compiler
+# intel : -openmp
+# gcc : -fopenmp
+# pgcc : -mp
+gcc -fopenmp hello.c -o Hello
+```
+
+Running it should output
+
+```
+Hello, World!
+...
+Hello, World!
+```
+
+The exact number of "`Hello, Worlds`" depends on the number of cores of your machine,
+for example I got 12 my laptop.
+
+## Threads and processes
+
+You can change the default number of threads using `export OMP_NUM_THREADS=8`
+
+Here are some useful library functions in the `omp.h` library
+
+```cpp
+// Check the number of threads
+printf("Max Threads: %d\n", omp_get_max_threads());
+printf("Current number of threads: %d\n", omp_get_num_threads());
+printf("Current Thread ID: %d\n", omp_get_thread_num());
+
+// Modify the number of threads
+omp_set_num_threads(int);
+
+// Check if we are in a parallel region
+omp_in_parallel();
+
+// Dynamically vary the number of threads
+omp_set_dynamic(int);
+omp_get_dynamic();
+
+// Check the number of processors
+printf("Number of processors: %d\n", omp_num_procs());
+```
+
+## Private and shared variables
+
+```cpp
+// Variables in parallel sections can be either private or shared.
+
+/* Private variables are private to each thread, as each thread has its own
+* private copy. These variables are not initialized or maintained outside
+* the thread.
+*/
+#pragma omp parallel private(x, y)
+
+/* Shared variables are visible and accessible by all threads. By default,
+* all variables in the work sharing region are shared except the loop
+* iteration counter.
+*
+* Shared variables should be used with care as they can cause race conditions.
+*/
+#pragma omp parallel shared(a, b, c)
+
+// They can be declared together as follows
+#pragma omp parallel private(x, y) shared(a,b,c)
+```
+
+## Synchronization
+
+OpenMP provides a number of directives to control the synchronization of threads
+
+```cpp
+#pragma omp parallel {
+
+ /* `critical`: the enclosed code block will be executed by only one thread
+ * at a time, and not simultaneously executed by multiple threads. It is
+ * often used to protect shared data from race conditions.
+ */
+ #pragma omp critical
+ data += data + computed;
+
+
+ /* `single`: used when a block of code needs to be run by only a single
+ * thread in a parallel section. Good for managing control variables.
+ */
+ #pragma omp single
+ printf("Current number of threads: %d\n", omp_get_num_threads());
+
+ /* `atomic`: Ensures that a specific memory location is updated atomically
+ * to avoid race conditions. */
+ #pragma omp atomic
+ counter += 1;
+
+
+ /* `ordered`: the structured block is executed in the order in which
+ * iterations would be executed in a sequential loop */
+ #pragma omp for ordered
+ for (int i = 0; i < N; ++i) {
+ #pragma omp ordered
+ process(data[i]);
+ }
+
+
+ /* `barrier`: Forces all threads to wait until all threads reach this point
+ * before proceeding. */
+ #pragma omp barrier
+
+ /* `nowait`: Allows threads to proceed with their next task without waiting
+ * for other threads to complete the current one. */
+ #pragma omp for nowait
+ for (int i = 0; i < N; ++i) {
+ process(data[i]);
+ }
+
+ /* `reduction` : Combines the results of each thread's computation into a
+ * single result. */
+ #pragma omp parallel for reduction(+:sum)
+ for (int i = 0; i < N; ++i) {
+ sum += a[i] * b[i];
+ }
+
+}
+```
+
+Example of the use of `barrier`
+
+```c
+#include
+#include
+
+int main() {
+
+ // Current number of active threads
+ printf("Num of threads is %d\n", omp_get_num_threads());
+
+#pragma omp parallel
+ {
+ // Current thread ID
+ printf("Thread ID: %d\n", omp_get_thread_num());
+
+#pragma omp barrier <--- Wait here until other threads have returned
+ if(omp_get_thread_num() == 0)
+ {
+ printf("\nNumber of active threads: %d\n", omp_get_num_threads());
+ }
+ }
+ return 0;
+}
+```
+
+## Parallelizing Loops
+
+It is simple to parallelise loops using OpenMP. Using a work-sharing directive we can do the following
+
+```c
+#pragma omp parallel
+{
+ #pragma omp for
+ // for loop to be parallelized
+ for() ...
+}
+```
+
+A loop must be easily parallelisable for OpenMP to unroll and facilitate the assignment amoungst threads.
+If there are any data dependancies between one iteration to the next, OpenMP can't parallelise it.
+
+## Speed Comparison
+
+The following is a C++ program which compares parallelised code with serial code
+
+```cpp
+
+#include
+#include
+#include
+#include
+#include
+
+int main() {
+ const int num_elements = 100000000;
+
+ std::vector a(num_elements, 1.0);
+ std::vector b(num_elements, 2.0);
+ std::vector c(num_elements, 0.0);
+
+ // Serial version
+ auto start_time = std::chrono::high_resolution_clock::now();
+ for (int i = 0; i < num_elements; i++) {
+ c[i] = a[i] * b[i];
+ }
+ auto end_time = std::chrono::high_resolution_clock::now();
+ auto duration_serial = std::chrono::duration_cast(end_time - start_time).count();
+
+ // Parallel version with OpenMP
+ start_time = std::chrono::high_resolution_clock::now();
+ #pragma omp parallel for
+ for (int i = 0; i < num_elements; i++) {
+ c[i] = a[i] * b[i];
+ }
+ end_time = std::chrono::high_resolution_clock::now();
+ auto duration_parallel = std::chrono::duration_cast(end_time - start_time).count();
+
+ std::cout << "Serial execution time: " << duration_serial << " ms" << std::endl;
+ std::cout << "Parallel execution time: " << duration_parallel << " ms" << std::endl;
+ std::cout << "Speedup: " << static_cast(duration_serial) / duration_parallel << std::endl;
+
+ return 0;
+}
+```
+
+This results in
+
+```
+Serial execution time: 488 ms
+Parallel execution time: 148 ms
+Speedup: 3.2973
+```
+
+It should be noted that this example is fairly contrived and the actual speedup
+depends on implementation and it should also be noted that serial code may run
+faster than parallel code due to cache preformace.
+
+## Example
+
+The following example uses OpenMP to calculate the Mandlebrot set
+
+```cpp
+#include
+#include
+#include
+#include
+#include
+
+const int width = 2000;
+const int height = 2000;
+const int max_iterations = 1000;
+
+int mandelbrot(const std::complex &c) {
+ std::complex z = c;
+ int n = 0;
+ while (abs(z) <= 2 && n < max_iterations) {
+ z = z * z + c;
+ n++;
+ }
+ return n;
+}
+
+int main() {
+ std::vector> values(height, std::vector(width));
+
+ // Calculate the Mandelbrot set using OpenMP
+ #pragma omp parallel for schedule(dynamic)
+ for (int y = 0; y < height; y++) {
+ for (int x = 0; x < width; x++) {
+ double real = (x - width / 2.0) * 4.0 / width;
+ double imag = (y - height / 2.0) * 4.0 / height;
+ std::complex c(real, imag);
+
+ values[y][x] = mandelbrot(c);
+ }
+ }
+
+ // Prepare the output image
+ std::ofstream image("mandelbrot_set.ppm");
+ image << "P3\n" << width << " " << height << " 255\n";
+
+ // Write the output image in serial
+ for (int y = 0; y < height; y++) {
+ for (int x = 0; x < width; x++) {
+ int value = values[y][x];
+ int r = (value % 8) * 32;
+ int g = (value % 16) * 16;
+ int b = (value % 32) * 8;
+
+ image << r << " " << g << " " << b << " ";
+ }
+ image << "\n";
+ }
+
+ image.close();
+ std::cout << "Mandelbrot set image generated as mandelbrot_set.ppm." << std::endl;
+
+ return 0;
+}
+```
+
+## Resources
+
+- [Intro to parallel programming](https://tildesites.bowdoin.edu/~ltoma/teaching/cs3225-GIS/fall17/Lectures/openmp.html)
+- [Tutorials currated by OpenMP](https://www.openmp.org/resources/tutorials-articles/)
+- [OpenMP cheatsheet](https://www.openmp.org/wp-content/uploads/OpenMPRefCard-5-2-web.pdf)