Aparapi add sample

Question

I'm studing Aparapi (https://code.google.com/p/aparapi/) and have a strange behaviour of one of the sample included. The sample is the first, "add". Building and executing it, is ok. I also put the following code for testing if the GPU is really used

if(!kernel.getExecutionMode().equals(Kernel.EXECUTION_MODE.GPU)){
    System.out.println("Kernel did not execute on the GPU!");
}

and it works fine. But, if I try to change the size of the array from 512 to a number greater than 999 (for example 1000), I have the following output:

!!!!!!! clEnqueueNDRangeKernel() failed invalid work group size
after clEnqueueNDRangeKernel, globalSize[0] = 1000, localSize[0] = 128
Apr 18, 2013 1:31:01 PM com.amd.aparapi.KernelRunner executeOpenCL
WARNING: ### CL exec seems to have failed. Trying to revert to Java ###
JTP

Kernel did not execute on the GPU!

Here's my code:

  final int size = 1000;

  final float[] a = new float[size];
  final float[] b = new float[size];

  for (int i = 0; i < size; i++) {
     a[i] = (float)(Math.random()*100);
     b[i] = (float)(Math.random()*100);
  }

  final float[] sum = new float[size];

  Kernel kernel = new Kernel(){
     @Override public void run() {
        int gid = getGlobalId();
        sum[gid] = a[gid] + b[gid];
     }
  };

  Range range = Range.create(size);
  kernel.execute(range);

  System.out.println(kernel.getExecutionMode());
  if (!kernel.getExecutionMode().equals(Kernel.EXECUTION_MODE.GPU)){
     System.out.println("Kernel did not execute on the GPU!");
  }

  kernel.dispose();

}

I tried specifying the size using

Range range = Range.create(size, 128);

as suggested in a Google group, but nothing changed.

I'm currently running on Mac OS X 10.8 with Java 1.6.0_43. Aparapi version is the latest (2012-01-23).

Am I missing something? Any ideas?

Thanks in advance

Answer 1

Aparapi inherits a 'Grid Style' of implementation from OpenCL. When you specify a range of execution (say 1024), OpenCL will break this 'range' into groups of equal size. Possibly 4 groups of 256, or 8 groups of 128.

The group size must be a factor of range (so assert(range%groupSize==0)).

By default Aparapi internally selects the group size.

But you are choosing to fully specify the range and group size to using

Range r= Range.range(n,128)

You are responsible for ensuring that n%128==0.

From the error, it looks like you chose Range.range(1000,128).

Sadly 1000 % 128 != 0 so this range will fail.

If you specifiy

Range r = Range.range(n)

Aparapi will choose a valid group size, by finding the highest common factor of n.

Try dropping the 128 as the the second arg.

Gary