Differences

This shows you the differences between two versions of the page.

--- openmp [June 10, 2026 at 21:21] – created Ivan Janevski
+++ openmp [August 22, 2026 at 15:22] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 # OpenMP
-**OpenMP** is a standardized API for
+**OpenMP** is a shared-memory parallelism API for C, C++, and Fortran. Add a `#pragma omp` directive before a loop or block to parallelize it across available cores. The compiler handles thread creation and synchronization; if it doesn't support OpenMP, directives are silently ignored and the program runs serially, making debugging easy.
+The execution model is **fork-join**: a single thread forks into a worker team at `#pragma omp parallel`, they execute concurrently, then join back. Thread count defaults to the number of logical cores, configurable via `OMP_NUM_THREADS=N` at runtime or `omp_set_num_threads(n)` in code. Compile with `-fopenmp` and include `<omp.h>` for the `omp_*` runtime functions.
 ```c
-// Compile: gcc -fopenmp main.c -o main
+// compile: gcc -O2 -fopenmp -o sum sum.c
-// Run:     ./main
+// run: OMP_NUM_THREADS=4 ./sum
+// description: parallel reduction using a single pragma
+#include <omp.h>
 #include <stdio.h>
-int main() {
+int main(void) {
+    long n = 1000000000L, sum = 0;
+    double t = omp_get_wtime();
+    #pragma omp parallel for reduction(+:sum)
+    for (long i = 0; i < n; i++)
+        sum += i;
+    printf("sum=%ld  time=%.2fs\n", sum, omp_get_wtime() - t);
+    return 0;
 }
 ```
+On a 4-core machine this runs roughly 4× faster with `-fopenmp` than without. [[amdahls-law|Amdahl's law]] limits speedup when part of the program is inherently serial; reducing overhead like false sharing and load imbalance matters more than just adding threads.
+## Concepts
+. [[openmp-data-sharing|Data sharing]]
+. [[openmp-parallel-loops|Parallel loops]]
+. [[openmp-collapse|Collapse]]
+. [[openmp-reduction|Reduction]]
+. [[openmp-scheduling|Scheduling]]
+. [[openmp-simd|SIMD]]
+. [[openmp-tasks|Tasks]]
+. [[openmp-single|Single]]
+. [[openmp-master|Master]]
+. [[openmp-sections|Sections]]
+. [[openmp-barrier|Barrier]]
+. [[openmp-nowait|Nowait]]
+. [[openmp-critical-sections|Critical sections]]
+. [[openmp-atomic|Atomic]]
+. [[openmp-flush|Flush]]
+. [[openmp-false-sharing|False sharing]]
+. [[openmp-thread-affinity|Thread affinity]]
+. [[openmp-time-measurement|Time measurement]]
+. [[openmp-overview|Overview]]