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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
| 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
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if ( my_cost < gm->cost) {
LOCK (gm-> cost_lock );
if ( my_cost < gm->cost) { /* make sure*/
gm->cost = my_cost ;
}
UNLOCK (gm-> cost_lock );
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} /* this works because gm->cost is
monotonically decreasing */
same time */
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