97 Things Every Programmer Should Know

Chapter 57. Message Passing Leads to Better Scalability in Parallel Systems

Russel Winder

PROGRAMMERS ARE TAUGHT from the very outset of their study of computing that concurrency—and especially parallelism, a special subset of concurrency—is hard, that only the very best can ever hope to get it right, and even they get it wrong. Invariably, there is great focus on threads, semaphores, monitors, and how hard it is to get concurrent access to variables to be thread-safe.

True, there are many difficult problems, and they can be very hard to solve. But what is the root of the problem? Shared memory. Almost all the problems of concurrency that people go on and on about relate to the use of shared mutable memory: race conditions, deadlock, livelock, etc. The answer seems obvious: either forgo concurrency or eschew shared memory!

Forgoing concurrency is almost certainly not an option. Computers have more and more cores on an almost quarterly basis, so harnessing true parallelism becomes more and more important. We can no longer rely on ever-increasing processor clock speeds to improve application performance. Only by exploiting parallelism will the performance of applications improve. Obviously, not improving performance is an option, but it is unlikely to be acceptable to users.

So can we eschew shared memory? Definitely.

Instead of using threads and shared memory as our programming model, we ...