Thursday, 26 December 2013

parallel prefix sum 2

This morning I had a quick look at the next stage of the OpenCLification of the GA code. This requires trying to create a fitness measure from the raw score values.

I'm using a sort of approximation of a sum of the area under the ROC curve:

  for each sorted result
    sum += true positive rate so far - false positive rate so far
  sum /= result.length
This gives a value between -0.5 and +0.5, equating to perfectly incorrect ROC and perfectly correct ROC curve respectively. Like the ROC curve itself this calculation is independent of the actual score values and only their order and class is important.

At first I didn't think I would be able to parallise it but on further inspection I realised I could implement it using a parallel prefix sum followed by a parallel sum reduction. The first prefix sum calculates the running totals of the occurance of each class (positive training examples, negative training examples). These values are then used to calculate the two rates above and from those partial summands which are then parallel summed. Because I don't need to keep the intermediate results I process this in batches of work-group sized blocks sequentially across each result array and it is a simple matter to accumulate results across blocks internally.

Then I realised I'd forgotten how to implement a parallel prefix sum properly ... it's been nearly 2 years ... as I found out when using a search engine and finding my own rather useless post on the matter comes up on the first page of results. I had a look at the code generator in socles and distilled it's essence out.

Mostly for my own future reference, the simplest version boils down to this:

  lx = get_local_id(0);
  buffer[lx] = sum;
  for (int i=1; i < N; i <&kt; 1) {
     if (lx - i >= 0)
       sum += buffer[lx - i];
     buffer[lx] = sum;

This of course assumes that the problem is only as wide as the work-group or can be processed in work-group sized chunks; which happens often enough to be useful. There is a very simple trick to remove the internal branch but it's probably not very important on current GPU designs. There are also some fairly simple modifications to widen this to some integer multiple of the work-group size and reduce the processing by the same factor but I don't think I need that optimisation here.

I can also use basic parallel reduction techniques to calculate the lowest-positive-rank and highest-positive-rank values, should I want to experiment with them as fitness values.

This is now most of the work required in evaluating each individual in the population which is the main processor intensitve part of the problem. If I had an APU i'd just drop back to the CPU to do the breeding but since I don't i'll look at doing the breeding on the GPU too, so long as it doesn't get too involved.

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