parallel algorithm course 01

gcc -fopenmp

omp_num_thread(int): request

multi-data

omp_get_thread_num, get id

---

SMP: equal-time access cost, in theory

NUMA: different .., practically

---

False Sharing

cache line

two processors may have access to the same region, repeat many useless write back

---

Synchronization, to avoid data racing, false sharing (avoid global array) d barrier

#pragma omp barrier

critical

only one thread can enter (often cost cheap), mutual exclusion, avoid data racing

(software support)

#pragma omp critical

atomic

only support (hardware support)

• x binopr= expr

• x++, ++x, x–, –x

#pragma omp atomic