OpenCL crash on big 2d range

Question

In my program, i need to run the kernel once on every item of the large 2d-array. The program works correctly for small ranges - up to around 50x50, sometimes up to 100x100.

For bigger datasets however, calling the kernel causes the video card driver to crash.

I have tested this program on two computers with different AMD cards, and they exhibit the exact same behaviour. Other, one-dimensional kernels work properly, even for huge datasets of ~10 000 x 10 000 items.

Also, removing the i variable from the matrix[i + (N + 1) * j] expression causes the kernel to work without errors.

Am i setting the range incorrectly, making a mistake in the kernel, or maybe the problem lies elsewhere?

enqueued range:

cl::EnqueueArgs args(queue,cl::NDRange(offset, offset+1),cl::NDRange(N+1, N),cl::NullRange);

kernel:

void kernel sub(global float* matrix, global const float* vec, int N, int offset) {
  int i = get_global_id(0);
  int j = get_global_id(1);         
  matrix[i + (N + 1) * j] -= matrix[i + (N + 1) * offset] * vec[j]; 
}

Answer 1

One of possible reasons - if your kernel is running for too long, driver may drop it. Dice up problem area into smaller blocks.

Answer 2

Consider this, for a 100x100 input array you will use N=100, hence the maximum value of i in your kernel will be 100 because of the N+1 used in the enqueue args, while the maximum for j will be 99. I have assumed that offset = 0. Therefore i + (N + 1) * j = 100 + 101*99 = 10099 which is outside of your 2D array.

When offset = 1, the minimums for i and j will be 1 and 2 respectively, while the maximums will be 101 and 100. Therefore i + (N + 1) * j = 101 + 101*100 = 10201.

In my experience, GPUs are not very good at catching segmentation faults when accessing global memory. Your attempt at purposefully creating one may work on some cards sometimes but no guarantees.

Answer 3

The problem could be caused by local-work-size and global-work-size. It is important while using two dimensional arrays to properly calculate them. It could be that for big values your global_id(0) is bigger than you specified in clEnqueueNDRangeKernel().