# Collapse (OpenMP)
**Collapse** is an OpenMP clause that merges multiple perfectly-nested loops into a single flat iteration space before distributing work across threads.

When a loop nest has too few outer iterations to keep all threads busy, distributing only the outer loop leaves most threads idle. `collapse(N)` multiplies the available parallelism by treating the combined iteration space as one loop. "Perfectly nested" means no statements appear between the loop headers.

```c
#pragma omp parallel for collapse(2)
for (int i = 0; i < M; i++) {
    for (int j = 0; j < N; j++) {
        a[i][j] = b[i][j] * c[i][j];
    }
}
```

Without `collapse(2)`, a 4-thread run on a loop with `M = 2` would use at most 2 threads. With it, the iteration space is `M * N`, and all 4 threads stay busy. The combined iteration space is split and scheduled according to the `schedule` clause as usual.