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From Gilles Sadowski <gil...@harfang.homelinux.org>
Subject Re: [math] Major speed improvement to 1D FFT (MATH-732)
Date Tue, 07 Feb 2012 11:34:43 GMT
Hello.

> 
> Hello S├ębastien,
> 
> > Kurt has recently proposed a patch for 1D-FFT which is much faster
> > than our current impl, while still passing the tests (of course).
> > Am I likely to hurt anyone's feelings if I replace the old impl by the new impl?
> 
> Please, go ahead, a faster implementation is always good, as long as it
> can be maintained.
> 
> > 
> > Also, I think that one of the reasons why our previous implementation
> > was so slow is because it used internally Complex objects. Since these
> > objects are immutable, we end up creating *a lot* of small objects.
> > Apparently, the GC and JIT are not so good at that ;) (not being an
> > expert, I'm not sure that the culprit is indeed the JIT or the GC...
> > but surely someone is to be blamed).
> 
> This seems strange to me. Lots of improvements have been added to Java 5
> and Java 6 about JVM optimization, and small objects are not really
> costly now. I do trust JVM implementors here and really like using small
> immutable objects.

The difference might be that there is a slight overhead in creating objects
(however small) with respect to using arrays of primitives in this scenario.
Trying to borrow from your explanation about cache-friendly implementations:
the cache will contain a larger chunk of an array of "double" than an array
of "Complex" (?).
Also, when the Complex objects array are split into their real an imaginary
constituent arrays, the computation are performed with "double", thus
bypassing method calls ("add", "subtract") and their possible precondition
checks.

> > 
> > I remember we recently had this conversation on the ML: one of Gilles
> > or Luc argued that for very low-level algorithms, it didn't really
> > matter if the parameters were passed as "very crude" objects, since it
> > was most likely that the user would have to write an adapter to its
> > own data format. So I would suggest we give up Complex[] altogether in
> > the interface of FFT, and replace it with double[] arrays, with the
> > following layout :
> > data[2 * i] = real part
> > data[2 * i + 1] = imaginary part.
> > 
> > What do you think?
> 
> I agree with this view (it may be me who expressed this). A double array
> as an interface for our library seems good to me.

I'm wary of this sort of "optimization" (as it decreases the code clarity).

-1 until it has been proven that it brings a _significant_ performance
   improvement.
At the moment, I would of course keep the internal switch to arrays of
primitives but also the API that takes and returns "Complex[]" objects.

A change[1] that might bring an added performance improvement is by
combining the "getRealArray()" and "getImaginartArray()" methods into a
single one, in order to loop only once over the array of "Complex" objects:
---CUT---
public static double[][] getRealAndImaginaryArrays(Complex[] dataC) {
    final double[][] dataRI = new double[2][dataC.length];
    final double[] dataR = dataRI[0];
    final double[] dataI = dataRI[1];
    for (int i = 0; i < dataC.length; i++) {
        final Complex c = dataC[i];
        dataR[i] = c.getReal();
        dataI[i] = c.getImaginary();
    }
    return dataRI;
}
---CUT---

Then, for "transform" we would have:
---CUT---
public Complex[] transform(Complex[] f) {
    final double[][] dataRI = getRealAndImaginaryArrays(f);
    transformInPlace(dataRI[0], dataRI[1], false);

    // etc.
}
---CUT---
and similarly for the other methods.


Two other points, unrelated to the above:
1. I think that the "scaleArray" method (the version that takes a "double"
   argument) should be moved to the "util.MathArrays" class (and renamed
   "scale").
2. Why are some methods "protected" (e.g. "fft")? IMHO, they should be
   "private".


Best,
Gilles

[1] This is untested.

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