Another Lesson on Performance

Just another story you can tell someone who fears that “XYZ might be too slow”:

I’m toying with the idea to write a new text editor. I mean, I’ve written my own OS, my own XML parser and I once maintained XDME, an editor written originally by Matthew Dillon. XDME has a couple of bugs and major design flaws that I always wanted to fix but never really got to it. Anyway.

There are various data structures which are suitable for a text editor and some of those depend on copying data around (see gap buffers). The question is: How effective is that? The first instinct of a developer is to avoid copying large amounts data and to optimize the data structure instead.

After years of training, I’ve yet to overcome this instinct and start to measure:

    public static void main (String[] args)
    {
        long start = System.currentTimeMillis ();
        
        int N = 10000;
        for (int i=0; i<N; i++)
        {
            int[] buffer = new int[1024*1024];
            System.arraycopy (buffer, 0, buffer, 1,
                buffer.length-1);
        }
        
        long duration = System.currentTimeMillis () - start;
        System.out.println (duration);
        System.out.println (duration / N);
    }

On my computer at work (which is pretty fast but not cutting edge), prints: “135223” and “13”. That’s thirteen milliseconds to copy 4MB of RAM. Okay. It’s obviously not worth to spend a second to think about the cost of moving data around in a big block of bytes.

Lesson: If you’re talking about performance and you didn’t measure, you have no idea what you’re talking about.

Still not convinced? Read this.

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