Differences

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

--- openmp [June 10, 2026 at 21:49] – Ivan Janevski
+++ openmp [June 13, 2026 at 03:13] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
 # OpenMP
-**OpenMP** is a parallel computing API mostly used for CPU parallelism. It's created for C, C++ and Fortran programming languages.
+**OpenMP** is a shared-memory parallelism API for C, C++, and Fortran. If you have a loop that takes too long and you want it to use all the cores on your machine instead of just one, OpenMP is usually the shortest path there. It works via compiler directives (`#pragma omp` in C/C++), a small runtime library (`libomp`), and a set of environment variables. In contrast to distributed-memory models like [[mpi|MPI]], where each process has its own memory, OpenMP threads all share the same address space — you get parallelism without touching your data layout.
-It's a bit weird when you first see it, but essentially OpenMP relies on a lesser used compiler directive known as `#pragma`. This directive tells the compiler to "do something special". In OpenMP's case, it injects code that creates threads and locks in order to establish a parallel region. It spawns multiple worker threads.
+The execution model is **fork-join**: the program starts as a single thread. When it hits a `#pragma omp parallel` block, it forks into a team of worker threads that all execute the block concurrently, then join back into one thread at the closing brace. You do not write any thread creation, management, or teardown code — the compiler inserts all of that. If the compiler does not support OpenMP, it silently ignores all `#pragma omp` directives and the program runs serially, which is useful for debugging: drop `-fopenmp` from the compile command and you get a clean serial build without changing a line of source.
-OpenMP is
-## Example
 ```c
-// #include <omp.h>
 #pragma omp parallel
 {
     int tid = omp_get_thread_num();
-    printf("Hello, I'm thread %d\n", tid);
+    int nthreads = omp_get_num_threads();
+    printf("thread %d of %d\n", tid, nthreads);
 }
 ```
-For example, if your environment variable is set to `export OMP_NUM_THREADS=5` you would see the following. Each worker thread
+The thread count defaults to the number of logical cores. You can override it at runtime with `OMP_NUM_THREADS=N ./prog` without recompiling, or with `omp_set_num_threads(n)` inside the program, or with a `num_threads(N)` clause directly on the pragma. Output order from a parallel region is non-deterministic — threads are scheduled by the OS, not by rank. Compile with `-fopenmp` and include `<omp.h>` for the `omp_*` runtime functions.
+Adding more threads does not always mean proportionally faster code. [[amdahls-law|Amdahl's law]] says that if a fraction $s$ of the program is inherently serial, the maximum speedup is $1/s$ no matter how many threads you add. A loop that accounts for 80% of runtime can at best give 5× speedup. This is why reducing the serial fraction matters more than just adding more cores, and why eliminating overhead like false sharing, load imbalance, and unnecessary barriers compounds those gains.
+## Practice
+Compile the hello-world above and run it:
+```bash
+$ gcc -fopenmp -o hello hello.c
+$ OMP_NUM_THREADS=4 ./hello
+thread 2 of 4
+thread 0 of 4
+thread 3 of 4
+thread 1 of 4
 ```
-Hello, I'm thread 4
-Hello, I'm thread 1
+The output order is non-deterministic — run it a few times and you will get different permutations. Now try dropping `-fopenmp`:
-Hello, I'm thread 3
-Hello, I'm thread 2
+```bash
-Hello, I'm thread 0
+$ gcc -o hello hello.c
+$ ./hello
+thread 0 of 1
 ```
+Without the flag, every `#pragma omp` directive is silently ignored and the program runs as a single thread. This is the most useful OpenMP debugging technique: if your parallel output looks wrong, remove `-fopenmp` and check whether the serial output is correct first.
+Here is a more realistic example: parallelising a sum of a billion integers with a single pragma.
+```c
+// compile: gcc -O2 -fopenmp -o sum sum.c
+// run: OMP_NUM_THREADS=4 ./sum
+// description: parallel reduction; try with -fopenmp omitted for serial baseline
+#include <omp.h>
+#include <stdio.h>
+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. The only additions to the loop are the pragma line and the `reduction(+:sum)` clause. Without the clause you would get a data race and a wrong answer — the clause is what makes it correct. When it works this cleanly, it genuinely feels like cheating.
+Try varying `OMP_NUM_THREADS` from 1 up to your core count and beyond. Speedup will plateau or even decline past the hardware thread count — that is thread management overhead and Amdahl's law at work.
+## Concepts
+. [[data-sharing-openmp|Data sharing]]
+. [[parallel-loops-openmp|Parallel loops]]
+. [[collapse-openmp|Collapse]]
+. [[reduction-openmp|Reduction]]
+. [[scheduling-openmp|Scheduling]]
+. [[simd-openmp|SIMD]]
+. [[tasks-openmp|Tasks]]
+. [[single-openmp|Single]]
+. [[master-openmp|Master]]
+. [[sections-openmp|Sections]]
+. [[barrier-openmp|Barrier]]
+. [[nowait-openmp|Nowait]]
+. [[critical-sections-openmp|Critical sections]]
+. [[atomic-openmp|Atomic]]
+. [[flush-openmp|Flush]]
+. [[false-sharing-openmp|False sharing]]
+. [[thread-affinity-openmp|Thread affinity]]
+. [[time-measurement-openmp|Time measurement]]
+## Overview
+### Directives
+```c
+#pragma omp parallel                         // fork a team of threads; join at closing brace
+#pragma omp parallel for                     // distribute loop iterations across the team
+#pragma omp parallel for reduction(+:s)     // loop with a parallel reduction
+#pragma omp parallel sections                // distribute independent blocks across the team
+#pragma omp section                          // one block inside a sections region
+#pragma omp single                           // one thread runs the block; others wait at end
+#pragma omp master                           // thread 0 only; no implicit barrier
+#pragma omp task                             // package work for any idle thread to execute
+#pragma omp taskwait                         // wait for all child tasks to finish
+#pragma omp barrier                          // all threads wait until every thread arrives
+#pragma omp critical                         // mutual exclusion — one thread at a time
+#pragma omp atomic                           // single hardware-atomic read-modify-write
+#pragma omp simd                             // assert the loop is safe to vectorise
+#pragma omp flush                            // enforce memory visibility across threads
+```
+### Functions
+```c
+omp_get_thread_num()      // ID of the calling thread (0 … N-1)
+omp_get_num_threads()     // number of threads in the current team
+omp_get_max_threads()     // threads that would be used if a parallel region started now
+omp_set_num_threads(n)    // set the default thread count at runtime
+omp_get_num_procs()       // number of logical processors available to the program
+omp_get_wtime()           // wall-clock time in seconds; use for timing parallel regions
+omp_in_parallel()         // 1 if called from inside a parallel region, 0 otherwise
+```
+### Environment variables
+^ Variable ^ Default ^ Description ^
+| `OMP_NUM_THREADS` | core count | Number of threads to use in each parallel region |
+| `OMP_SCHEDULE` | `static` | Default schedule kind and optional chunk size, e.g. `dynamic,4` |
+| `OMP_PROC_BIND` | `false` | Thread-to-core affinity policy: `close`, `spread`, or `master` |
+| `OMP_PLACES` | (unset) | Placement units for affinity: `cores`, `threads`, or `sockets` |
+| `OMP_MAX_ACTIVE_LEVELS` | `1` | Maximum nesting depth of simultaneously active parallel regions |
+| `OMP_DISPLAY_ENV` | `false` | Print OpenMP version and active settings at startup: `TRUE` or `VERBOSE` |