Beautiful Code

Chapter 24. Beautiful Concurrency

Simon Peyton Jones

The free lunch is over.^[50] We have grown used to the idea that our programs will go faster when we buy a next-generation processor, but that time has passed. While that next-generation chip will have more CPUs, each individual CPU will be no faster than the previous year’s model. If we want our programs to run faster, we must learn to write parallel programs.^[51]

Parallel programs execute in a nondeterministic way, so they are hard to test, and bugs can be almost impossible to reproduce. For me, a beautiful program is one that is so simple and elegant that it obviously has no mistakes, rather than merely having no obvious mistakes.^[52] If we want to write parallel programs that work reliably, we must pay particular attention to beauty. Sadly, parallel programs are often less beautiful than their sequential cousins; in particular they are, as we shall see, less modular.

In this chapter, I’ll describe Software Transactional Memory (STM), a promising new approach to programming shared-memory parallel processors that seems to support modular programs in a way that current technology does not. By the time we are done, I hope you will be as enthusiastic as I am about STM. It is not a solution to every problem, but it is a beautiful and inspiring attack on the daunting ramparts of concurrency.

A Simple Example: Bank Accounts

Here is a simple programming task.

Write a procedure to transfer money from one bank account to another. To keep ...