What makes Erlang unsuitable for computationally expensive work?

Question

At the beginning of Programming Erlang, there is the following:

What makes Erlang the best choice for your project? It depends on what you are looking to build. If you are looking into writing a number-crunching application, a graphics- intensive system, or client software running on a mobile handset, then sorry, you bought the wrong book.

The implied message is that Erlang isn't suitable for computationally expensive work. What makes Erlang so unsuitable, or have I misinterpreted?

Answer 1

Erlang shines for I/O-bound applications, that is, problems whose limiting factor is the latency and throughput of I/O operations rather than the rate at which instructions can be pushed through a CPU pipeline. Web servers and databases are good examples of I/O-bound applications: the liming factors are likely to be the disk and network rather than the CPU. Traditionally "compute-heavy" applications include cryptographic tools and scientific simulations.

As to why Erlang fails to match languages like C and Fortran when it comes to computationally intensive problems, we must consider things like code generation and cache-friendliness... I'll give it a try:

• Code generation: Normally when you start an Erlang program, it will be run in BEAM, a virtual machine based on threaded code. While BEAM performs well enough for most purposes, it has much greater overhead per logical "instruction" than does the kind of code generated by a modern optimizing C compiler. The HiPE project provides a native code compiler for Erlang that was integrated into main OTP source tree a couple of years ago*. While it certainly improves Erlang's number crunching capacity, it will still have a hard time matching a well-written C or Fortran program.
• Cache-friendliness: The memory system is a major bottleneck in modern computers: a read from main memory can take hundreds of processor cycles! To solve this problem, CPU designers introduce several levels of cache to hide the memory latency. Caches exploit two key properties of computer programs: temporal and spatial locality -- that is, regions of memory that were recently referenced (and nearby regions) are likely to be referenced again. Languages like C and Fortran offers a great deal of control over where and how memory is allocated, enabling the programmer to tune algorithms to play nicely with the caches. The same doesn't generally hold for dynamic languages like Erlang, where memory allocation is hidden from the programmer and handled automatically by the virtual machine.
• Code size: The argument about spatial locality holds for code as well; Erlang code, whether in native or bytecode form, will generally be larger than the corresponding compiled C code. This leads to more frequent misses in the instruction cache.

Bear in mind that this is just the tip of the iceberg, and that I am by no means an expert in Erlang or language implementation. Don't let the fact that Erlang will probably never run scientific simulations scare you, though; for many applications, it's an absolutely fantastic language.

*HiPE is available through the erlang-base-hipe package in Debian, or ./configure --enable-hipe from a source tarball.

Answer 2

It's just that C code might be considerable faster most of the time. Erlang is great at fault tolerance, distributed computing, and concurrency. Programmers tend to be equally proficient in writing erlang or other languages, but if you want speed, use C or C++, maybe from an erlang port, so this code is usable from your own erlang application.