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Writing Parallel Programs with Erlang

Erlang, a language for concurrency, is a good choice for writing parallel programs to fully exploit current and future multicore CPUs.

or years, heating problems have prevented CPU manufacturers from building viable CPUs with clock rates higher than 4.0GHz. This single-core processor speed limitation means that the only viable way to make programs run faster is to run them on more than one processor. Consequently, manufacturers started making processors with multiple cores (i.e., multiprocessor CPUs). In fact, you can expect to see processors with more than 100 cores in the near future. In order to fully exploit the potential of these new processors, software developers will be forced to write parallel programs. As Herb Sutter wrote regarding Moore's law: "The Free Lunch Is Over." The transition won't be easy, though, because writing concurrent programs is hard work.

The main framework for parallel computing in the past 30 years has been single processes, threads, or programs that communicate with one another by reading and writing in a shared memory area and using locks and semaphores. In multiprocessor CPUs, the individual cores talk to each other mainly by sending asynchronous messages. This framework requires you to pay a lot of attention to mutually exclusive algorithms to avoid the simultaneous use of common resources or side effects such as deadlocks from using semaphores.

This article introduces the functional language Erlang, a good choice for writing parallel programs, and explains how you can use it to fully exploit current and future multicore CPUs.

Meet Erlang

Designed at Ericsson laboratories in 1986, Erlang (named for Danish mathematician Agner Krarup Erlang) is used largely in the telecommunications industry. Nevertheless, it is a general-purpose, functional programming language that belongs to a more comprehensive rather than imperative class of declarative languages. One of its most important features is its implementation of concurrent computation according to the Actor model, a mathematical schema that implies that everything is an actor (much like the object-oriented philosophy of everything is an object). Because an actor sends and receives messages and accordingly acts and creates other actors, the Actor model is inherently concurrent, whereas the object-oriented paradigm is practically sequential.

The Erlang software concurrency paradigm leaves no space for state. You can assign variables only once and must perform iteration through recursion. No state means you do not need memory protection, which minimizes side effects as well as the amount of data shared between processes.

Erlang functions are similar to mathematical functions: with the same input they return the same value, regardless of the context of the call. Erlang functions pass each parameter by value, so anything that you can reference is a value. This feature is called reference transparency.

Although variable assignments are subject to strict rules, Erlang partially makes up for that restriction using a powerful mechanism called pattern matching. This feature is similar to the way human beings interpret reality—by determining similarities between real objects and the conceptual schemas in their minds.

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