Keno's answer is spot on, but maybe I can give a little more detail on what's going on and what we're planning to do about it.
Currently there is only an LLVM JIT mode:
- There's a very trivial interpreter for some simple top-level statements.
- All other code is jitted into machine code before execution. The code is aggressively specialized using the run-time types of the values that the code is being applied to, propagated through the program using dynamic type inference.
This is how Julia gets good performance even when code is written without type annotations: if you call f(1) you get code specialized for Int64 — the type of 1 on 64-bit systems; if you call f(1.0) you get a newly jitted version that is specialized for Float64 — the type of 1.0 on all systems. Since each compiled version of the function knows what types it will be getting, it can run at C-like speed. You can sabotage this by writing and using "type-unstable" functions whose return type depends on run-time data, rather than just types, but we've taken great care not to do that in designing the core language and standard library.
Most of Julia is written in itself, then parsed, type-inferred and jitted, so bootstrapping the entire system from scratch takes some 15-20 seconds. To make it faster, we have a staged system where we parse, type-infer, and then cache a serialized version of the type-inferred AST in the file sys.ji. This file is then loaded and used to run the system when you run julia. No LLVM code or machine code is cached in sys.ji, however, so all the LLVM jitting still needs to be done every time julia starts up, which therefore takes about 2 seconds.
This 2-second startup delay is quite annoying and we have a plan for fixing it. The basic plan is to be able to compile whole Julia programs to binaries: either executables that can be run or .so/.dylib shared libraries that can be called from other programs as though they were simply shared C libraries. The startup time for a binary will be like any other C program, so the 2-second startup delay will vanish.
Addendum 1: Since November 2013, the development version of Julia no longer has a 2-second startup delay since it precompiles the standard library as binary code. The startup time is still 10x slower than Python and Ruby, so there's room for improvement, but it's pretty fast. The next step will be to allow precompilation of packages and scripts so that those can startup just as fast as Julia itself already does.
Addendum 2: Since June 2015, the development version of Julia precompiles many packages automatically, allowing them to load quickly. The next step is static compilation of entire Julia programs.
