Is Parallel Programming Hard, And, If So, What Can You Do About It?

A.1. WHAT DOES “AFTER” MEAN? 155TimeProducerss.t = dgettimeofday();ss.a = ss.c + 1;ss.b = ss.a + 1;ss.c = ss.b + 1;ProducerConsumercurssc.tc = gettimeofday();curssc.t = ss.t;curssc.a = ss.a;curssc.b = ss.b;curssc.c = ss.c;ss.t = dgettimeofday();ss.a = ss.c + 1;ss.b = ss.a + 1;ss.c = ss.b + 1;FigureA.3: EffectofLockingonSnapshotCollectionprior holder of that lock. No need to worry aboutwhich CPU did or did not execute a memory barrier,no need to worry about the CPU or compilerreordering operations – life is simple. Of course, thefact that this locking prevents these two pieces ofcode from running concurrently might limit the program’sability to gain increased performance on multiprocessors,possibly resulting in a “safe but slow”situation. Chapter 5 describes ways of gaining performanceand scalability in many situations.However, in most cases, if you find yourself worryingabout what happens before or after a given pieceofcode, youshouldtakethisasahinttomakebetteruse of the standard primitives. Let these primitivesdo the worrying for you.