Comment by semi-extrinsic | Hacker News Reader

Comment by semi-extrinsic | Hacker News Reader

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He's absolutely right, this is a crappy benchmark. Increase the array sizes by at least a factor of 100 to get anything meaningful.

The fact that someone who is "the review manager for std.ndslice's inclusion into the standard library" does not understand how to profile numerical algorithms makes me very skeptical of using D for any numerical project.

Plus, the syntax looks god-awfully unintuitive. A main advantage of Python is that you often get "code that looks like what it does". The D "basic example with a benchmark" OTOH looks almost obfuscated. To wit; a Fortran version is more readable, is fewer lines of code(!) and of course kicks D's butt when it comes to speed:

  program p
  real, dimension(100,1000) :: data
  real, dimension(1000) :: means
  
  n=1
  forall(i=1:100,j=1:1000)
    data(i,j)=n
    n=n+1
  end forall

  forall(i=1:1000000,j=1:1000)
    means(j) = sum(data(:,j))/size(data(:,j))
  end forall

Disclaimer: I wrote this on my phone, only 95% sure that it will compile and run correctly. Save it in means.f90 and compile with `gfortran -Ofast means.f90`. This calculates the means 1 mill. times (the loop over i in the second forall); I bet you it will be an order of magnitude faster than D per means calculation if you time it with plain time (the *nix command).

"Increase the array sizes by at least a factor of 100 to get anything meaningful."

Ok, let's do that and see what happens:

    python -m timeit -s 'import numpy; data = numpy.arange(10000000).reshape((1000, 10000))' 'means = numpy.mean(data, axis=0)'

D code

    import std.range : iota;
    import std.array : array;
    import std.algorithm;
    import std.experimental.ndslice;
    import std.datetime;
    import std.conv : to;
    import std.stdio;

    enum testCount = 10_000;

    void f0() {
        auto means = 10_000_000.iota
            .sliced(1000, 10000)
            .transposed
            .map!(r => sum(r) / r.length)
            .array;
    }

    void main() {
       auto r = benchmark!(f0)(testCount);
        auto f0Result = to!Duration(r[0] / testCount);
        f0Result.writeln;
    }

Results

    Python: 14.1 msec
    D:      39   μs
    D is 361.5x faster

Ok. So three points remain:

* how do I know that the calculations aren't actually optimized away in all that code I can't understand? E.g. what happens if you make f0() return the means array instead of being a void function?

* Given the same number of lines (or characters), are you sure you can't write equally fast Python code?

* I tested the Fortran version on a slightly slower computer (got 14.7 msec on the Python version). Fortran runs at 0.7 μs, i.e. > 55x faster than D, with less code that's more readable to boot.

"how do I know that the calculations aren't actually optimized away in all that code I can't understand? E.g. what happens if you make f0() return the means array instead of being a void function?"

If you can't understand the code, how did you know it was a void function. Please stop with the hyperbole, it's not adding anything to the discussion.

Updated code:

    import std.range : iota;
    import std.array : array;
    import std.algorithm;
    import std.experimental.ndslice;
    import std.datetime;
    import std.conv : to;
    import std.stdio;

    enum testCount = 10_000;

    auto f0() {
        auto means = 10_000_000.iota
            .sliced(1000, 10000)
            .transposed
            .map!(r => sum(r) / r.length)
            .array;
        return means;
    }

    void main() {
        auto r = benchmark!(f0)(testCount);
        auto f0Result = to!Duration(r[0] / testCount);
        f0Result.writeln;
    }

Results:

    Python: 14.1 msec
    D:      41   μs
    D is 343.9x faster

"Given the same number of lines (or characters), are you sure you can't write equally fast Python code?"

IMO program size is an almost meaningless statistic outside of code golf challenges. LOC is not an indicative measure of code readability, usefulness, or organization.

For example, your Fortran code was 11 lines while the D function (with the return) is eight lines.

"I tested the Fortran version on a slightly slower computer (got 14.7 msec on the Python version). Fortran runs at 0.7 μs, i.e. > 55x faster than D, with less code that's more readable to boot."

Just goes to show why Fortran is still used in a lot of scientific areas.

But two things:

1. Fortran is a much simpler language than D or Python and is much harder to do multipurpose work in it (so I'm told from Fortran programmers, I don't know Fortran myself). So when your program needs to do anything other than number crunching, it's normally done in a separate language. Using D you can have everything in one code base.

2. This article was about Numpy and std.ndslice because those are two areas that I know about and Numpy is a very popular library. Bringing up Fortran's speed here is like commenting on how much faster C++ is in a thread about Ruby.

Also, readability is a subjective idea; I believe the D code is more readable than the Fortran you wrote. Different strokes.

> If you can't understand the code, how did you know it was a void function. Please stop with the hyperbole, it's not adding anything to the discussion.

I understand "void" perfectly fine, but understanding how D optimizes your code is a completely different matter. Playing devil's advocate further, you

> IMO program size is an almost meaningless statistic outside of code golf challenges. LOC is not an indicative measure of code readability, usefulness, or organization.

No, but the numpy code is undoubtedly much simpler, and very general. How does the D example look for a 3D array where you want to average over the second dimension?

> For example, your Fortran code was 11 lines while the D function (with the return) is eight lines.

You're neglecting the library imports.

>So when your program needs to do anything other than number crunching, it's normally done in a separate language. Using D you can have everything in one code base.

I do agree other languages are much better for e.g. string processing. But for the applications where you can afford to trade simpler code for 50x slower performance, I'd say you're not really caring about performance at all, so why not just use Python? If performance is mission critical, a two-language code base (Python + C/C++/Fortran/CUDA) is not that hard to do, fairly common, and will give you the required performance.

> Bringing up Fortran's speed here is like commenting on how much faster C++ is in a thread about Ruby.

No; once you start saying "I want more performance than Python", it's obviously interesting to see what level of performance a "low-level" language gets, to put the result in perspective.

Now, I'm not trying to beat down on D, so please try to interpret this as constructive criticism. Finding "where does it fit in the scientific toolbox" is what makes or breaks a language's adoption in the scientific community. Just look at R, it's at the same level of performance/abstraction but in a separate niche from Python, and doing very well. The same goes for Matlab (which has e.g. Simulink).

Is D or the compiler doing something sneaky and inlining something, because I'd expect doing 100x more things to take 100x more time for a linear algorithm,but it takes you on the order of 10x more time. Does the original function run in <1ns for you?

Edit:

Because as I showed above, Pypy managed to JIT inline the entire benchmark, so I wouldn't be surprised at all if a clever compiler managed to do something sneaky.

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