Plunging deeper

English: Example red-black tree (Photo credit: Wikipedia)

For the last month I have been working hard on some C/C++ code to simulate a 16 core computer.

I had already got some code that did this – written in Groovy – but the limitations of the Java VM made it just too difficult to write efficient code to do what I really wanted – which was to simulate the performance of such a 16 core device using Belady’s OPT (optimal – or so-called “clairvoyant”) page replacement policy. The groovy code was going to take two years just to generate the memory reference strings – never mind how quickly it might “execute” the code.

Using C++ (with a C interface as I am just a little bit more comfortable in that language) and some code I wrote when doing my MSc to generate a red-black tree (a semi-balanced binary tree) I was able to build some code that generates the reference strings in a few hours: it’s enough to restore your faith in traditional compiled, natively-executing, code.

But building the simulator to handle the reference strings is another matter. Last weekend, up in York, I was able to write many hundreds of lines and make quite a bit of progress, but nothing was really working.

A week later, and some crash self-tutoring in using the GDB debugger I now have got somewhere, but now have to confront a deeper problem – my red-black tree code is fundamentally broken somewhere.

I wrote this code in the summer of 2010 because I had had such a bad time of it on the C++ course at Birkbeck (my own fault, I didn’t pay enough attention). For the close-to-four-years I have looked on it as something of a coding pinnacle: and certainly it does quite efficiently generate a templatised red-black tree if you are just sticking new values in one after another. But it would appear (I am not quite sure if this is correct – but the evidence points that way) to be quite a bit broken if you subsequently start deleting some nodes in the tree and adding new nodes in: the crashes I am seeing suggest my code creates a loop – the left and right branches pointing back to the parent.

So, the work must go on and I will have to pore over some four year old code (not brilliantly documented) to fix it.

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