Communications - March 2009

of its time executing within the highly tuned runtime system written in C. For example, when copying bulk data between network sockets, interpreted Erlang performs on par with a custom C program to do the same task. ⁴

The significant test of the implementation’s efficiency is the practicality of the worker process idiom, as demonstrated by the multicall2 code shown earlier. Spawning worker processes would seem to be much less efficient than sending messages directly. Not only does the parent have to spawn and destroy a process, but also the worker needs extra message hops to return the results. In most programming environments, these overheads would be prohibitive, but in Erlang, the concurrency primitives ( including process spawning) are lightweight enough that the overhead is usually negligible.

Not only do worker processes have negligible overhead, but they also increase efficiency in many cases. When a process exits, all of its memory can be immediately reclaimed. A short-lived process might not even need a collection cycle. Per-process heaps also eliminate global collection pauses, achieving soft real-time levels of latency. For this reason, Erlang programmers avoid reusable pools of processes and instead create new processes when needed and discard them afterward.

Since values in Erlang are immutable, it’s up to the implementation whether the message is copied when sent or whether it is sent by reference. Copying would seem to be the slower option in all situations, but sending messages by reference requires coordination of garbage collection between processes: either a shared heap space or maintenance of interregion links. For many applications, the overhead of copying is small compared with the benefit from short collection times and fast reclamation of space from ephemeral processes. The low penalty for copying is driven by an important exception in send-by-copy: raw binary data is always sent by reference, which doesn’t complicate garbage collection since the raw binary data cannot contain pointers to other structures.

The Erlang emulator can create a new Erlang process in less than a microsecond and run millions of process-

es simultaneously. Each process takes less than a kilobyte of space. Message passing and context switching take hundreds of nanoseconds.

Because of its performance characteristics and language and library support, Erlang is particularly good for:

˲ Irregular concurrency— applications that need to derive parallelism from disparate concurrent tasks

˲ Network servers

˲ Distributed systems

˲ Parallel databases

˲ GUIs and other interactive programs

˲ Monitoring, control, and testing tools

So when is Erlang not an appropriate programming language, for efficiency or other reasons? Erlang tends not to be good for:

˲ Concurrency more appropriate to synchronized parallel execution

˲ Floating-point-intensive code

˲Code requiring nonportable instructions

˲Code requiring an aggressive compiler (Erlang entries in language benchmark shoot-outs are unimpressive—except for process spawning and message passing)

˲Projects to implement libraries that must run under other execution environments, such as JVM (Java Virtual Machine) or CLR (Common Language Runtime)

˲ Projects that require the use of extensive libraries written in other languages

Erlang can still form part of a larger solution in combination with other languages, however. At a minimum, Erlang programs can speak text or binary protocols over standard interprocess communication mechanisms. In addition, Erlang provides a C library that other applications can link with that will allow them to send and receive Erlang messages and be monitored by an Erlang controlling program, appearing to it as just another (Erlang) process.

conclusion

With the increasing importance of concurrent programming, Erlang is seeing growing interest and adoption. Indeed, Erlang is branded as a “concurrency-oriented” language. The standard Erlang distribution is under

active development. Many high-quality libraries and applications are freely available for:

˲ Network services

˲ GUIs for 3D modeling

˲ Batch typesetting

˲ Telecom protocol stacks

˲ Electronic payment systems

˲ HTML and XML generation and parsing

˲Database implementations and ODBC (Open Database Connectivity) bindings

Several companies offer commercial products and services implemented in Erlang for telecom, electronic payment systems, and social networking chat. Erlang-based Web servers are notable for their high performance and scalability. ³

Concurrent programming will never be easy, but with Erlang, developers have a chance to use a language built from the ground up for the task and with incredible resilience engineered in its runtime system and standard libraries.

The standard Erlang implementation and its documentation, ported to Unix and Microsoft Windows platforms, is open source and available for free download from http://erlang.org. You can find a community forum at http://trapexit.org, which also mirrors several mailing lists.

The author would like to thank Romain Lenglet and JJ Furman for their feedback.

References

1. armstrong, J. making reliable distributed systems in the presence of software errors. Ph.d. thesis (2003). swedish institute of computer science; www.erlang. org/download/armstrong_thesis_2003.pdf.

2. armstrong, J. Programming Erlang: Software for a Concurrent World. the Pragmatic Bookshelf, raleigh, nc, 2007.

3. Brown, B. application server performance testing, includes django, erly Web, rails, and others; http:// berlinbrowndev.blogspot.com/2008/08/application- server-benchmarks-including.html.

4. lystig fritchie, s., larson, J., christenson, n., Jones, d., and ohman, l. sendmail meets erlang: experiences using erlang for email applications. in Proceedings of the Erlang User’s Conference, 2000; www.erlang.se/ euc/00/ euc00-sendmail.ps.

5. steele, g.l. and raymond, e.s. The New Hacker’s Dictionary, 3rd edition. mit Press, cambridge, ma, 1996.

Jim Larson ( jimlarson@google.com) is a software engineer at google, implementing large-scale distributed storage systems. he has worked with erlang for commercial products since 1999 and is the architect of the replication engine of the amazon simpledB Web service at amazon.com.