Try timing Julia's IO, or it's string processing, or it's hash tables, or its array allocation or it's GC - you know, nearly everything other that mathematical operations on floats or ints. Nearly everything leaves performance on the table.

Sure Julia is fast, but not on a C level in these areas. There is still lots of room for optimisation, though.

Most the speed advantage of the C here is due to reusing the same memory over and over, which you can do pretty easily in Julia as well. Here's the Julia code using in-place operations and it's still quite a bit more readable than the C version: https://gist.github.com/StefanKarpinski/e57f5a36b7890b261a0d.... I timed this and it's the same speed as the C version. When developing this, it follows the same general outline as the C version, but you have several benefits: (1) You can use asserts to compare to the easy version; (2) There are niceties like bounds checks and array indexing. In C you can't do (1) because there is on easy version to compare with. And doing the array index computations in C is kind of a nightmare—it's so easy to accidentally screw them up.

However, for many, many small tasks, today's compilers are smart enough that you can express your idea in a high-level language and the generated code will be maximally efficient. The real killer feature of Julia is that, where ever you can gain maximal performance with high-level syntax, you can just choose to do that. A more correct but less sexy slogan for Julia is that it has the best performance/expressiveness tradeoff you have ever seen.

Here is the code, perhaps someone else would know how to optimize it.

CODE:

    function main(max_a0)
       longest = max_len = len = a = 0
       for a0 in 1:max_a0
          a = a0      
          len = 0
          while a != 1
             len += 1
             a = ((a%2==0) ? a : 3*a+1)/2
          end
          if len > max_len
             max_len = len
             longest = a0
          end
       end
       println("($max_len, $longest)")
    end

    @time main(1000000)

    $ julia collatz.jl 
    (329, 837799)
    elapsed time: 16.436623922 seconds (4206873264 bytes allocated)

    $ julia --version
    julia version 0.3.0-prerelease+143

> It's amazing to realize that writing down the first thing that comes to your head is usually like 80% as fast as a good, performant implementation. (As someone who has done a decent amount of work in performance engineering for embedded platforms, I really enjoy squeezing out the last drop of performance from most programs, but doing this for every single first-pass at a program, like in Python, is rather annoying if this is what is needed to get a usable implementation.)

Yes, Julia is not necessarily faster than a good C implementation (that doesn't leak, etc), but, like Python, what would be a 300-line C implementation, where one has to somewhat carefully manage typing and the abstraction is really rather complicated for something that is mathematically simple, we can usually write 20 lines of very performant Julia that is 95% as fast.

Attempting to do something relatively similar in Python is often slow enough that giving a practical implementation essentially needs to be coded in C and interfaced with Python, where we return (again!) back to the same problem we had before: writing a 300+ line C file for something that should be rather simple, mathematically speaking.

Actually want to qualify this a bit. Nothing gets automatically fast by just picking a fast language. That Julia is close to C in performance mainly means it is possible to achieve this through optimization without too much effort. However you still need to have some idea of what makes something slow or fast in Julia.

Personally I find it very easy to optimize Julia code because you can quickly see how the JIT transforms an individual function and what you may need to change.

The C function is of course naive. I'm pretty sure that a hand-unrolled loop would be faster.

This is not necessarily a bad thing. Just an observation from me translating my Python code to Julia.

Sometimes, a function really is performance critical so you spend a ton of time fiddling with it to make it blazing fast, other times you write something non-optimal but straightforward. It's all the same language though, and unlike tools like Cython, you don't lose all your nice high level language features when you drop down to 'high performance julia code'.