Mutual exclusion

• Use LOCK/UNLOCK around critical sections
  • Updates to shared variable diff  must be sequential
  • Heavily contended locks may degrade performance
  • Try to minimize the use of critical sections: they are sequential anyway and will limit speedup
  • This is the reason for using a local_diff  instead of accessing gm->diff  every time
  • Also, minimize the size of critical section because the longer you hold the lock, longer will be the waiting time for other processors at lock acquire

LOCK optimization

• Suppose each processor updates a shared variable holding a global cost value, only if its local cost is less than the global cost: found frequently in minimization problems

  LOCK (gm->cost_lock);                if (my_cost < gm->cost) {  gm->cost = my_cost;                 } UNLOCK (gm->cost_lock); /* May lead to heavy lock contention if everyone tries to update at the same time */ if (my_cost < gm->cost) { LOCK (gm->cost_lock); if (my_cost < gm->cost) { /* make sure*/ gm->cost = my_cost; } UNLOCK (gm->cost_lock); } /* this works because gm->cost is monotonically decreasing */

More synchronization

• Global synchronization
  • Through barriers
  • Often used to separate computation phases
• Point-to-point synchronization
  • A process directly notifies another about a certain event on which the latter was waiting
  • Producer-consumer communication pattern
  • Semaphores are used for concurrent programming on uniprocessor through P and V functions
  • Normally implemented through flags on shared memory multiprocessors (busy wait or spin)

  P₀: A = 1; flag = 1;
  P₁: while (!flag); use (A);