Problem about get_global_id()

Accelerated Computing CUDA CUDA Programming and Performance
1

Hi all,

I have a problem about get_global_id() …

I try to record the current running thread ID in the Kernel function and can not get the global_id.

It seems like the kernel does not run in parallel…

see the following code:

__kernel void BuildResponseLayer(__global float* responses,

								 __global float* laplacian,

								 __global float* img,

								 uint h,

								 uint w,

								 uint s,

								 uint stride,

					 __global int*   deviceGid)

{

 [b] int gid = get_global_id(0);

  deviceGid[1] = gid; [/b]

  int ss = step[gid];								 // step size for this filter

  int b = (filter[gid] - 1) / 2 + 1;				  // border for this filter

  int l = filter[gid] / 3;							// lobe for this filter (filter size / 3)

  int ww = filter[gid];							   // filter size

  float inverse_area = 1.f/(ww*ww);				   // normalisation factor

  float Dxx, Dyy, Dxy;

for(int r, c, ar = 0, index = 0; ar < h * co[gid]; ++ar)

  {

	for(int ac = 0; ac < w * co[gid]; ++ac, index++)

	{

	  r = ar * ss;

	  c = ac * ss;

	  // Compute response components

	  Dxx = BoxIntegral(img, stride, h ,ww, r - l + 1, c - b, 2*l - 1, ww)

		  - BoxIntegral(img, stride, h, ww, r - l + 1, c - l / 2, 2*l - 1, l)*3;

	  Dyy = BoxIntegral(img, stride, h, ww, r - b, c - l + 1, ww, 2*l - 1)

		  - BoxIntegral(img, stride, h, ww, r - l / 2, c - l + 1, l, 2*l - 1)*3;

	  Dxy = + BoxIntegral(img, stride, h, ww, r - l, c + 1, l, l)

			+ BoxIntegral(img, stride, h, ww, r + 1, c - l, l, l)

			- BoxIntegral(img, stride, h, ww, r - l, c - l, l, l)

			- BoxIntegral(img, stride, h, ww, r + 1, c + 1, l, l);

	  // Normalise the filter responses with respect to their size

	  Dxx *= inverse_area;

	  Dyy *= inverse_area;

	  Dxy *= inverse_area;

	  // Get the determinant of hessian response & laplacian sign

	  responses[gid * h * w + index] = (Dxx * Dyy - 0.81f * Dxy * Dxy);

	  laplacian[gid * h * w + index] = (Dxx + Dyy >= 0 ? 1 : 0);

	}

  }

}

After we transfer the deviceGid back to the host and print it. These elements seem strange and similar to some uninitialized data.

Thanks for any help! :rolleyes:

2

I think it is because you fill only element 1 of deviceGid, while other remain uninitialized. Probably you should have written something like “deviceGid[gid] = gid;”.

3

I think it should be deviceGid[0] = gid. Because now you write to the second index (1), but maybe that’s what you wanted (and there are 2 ints allocated).

4

Actually I did initialize the deviceGid to 0 and also tried your advice but it didn’t work.

After we run the kernel and transfer the deviceGid back to the host.All the elements are 0!

It seems like the kernel does not even touch the deviceGid …

Thank you for your suggestions :rolleyes:

5

Are you sure it is allowed to declare variable in the for loop?

The langage for writting kernel is based on C99. Unlike in C++, I think it is not allowed to do so in standard C. Maybe you can try to change this.

How are you allocating your memory objects and giving them in parameters to the function?

6

Since deviceGid is __global, I think any thread can write to deviceGid[1], so it could be any number within the range of 0~number_of_threads.

Am I right?

7

I guess it isn’t a good idea to let several threads write to global mem at once. Reading is ok, since there should be some broadcast mechanism. I might be wrong too :-D

8

I guess it isn’t a good idea to let several threads write to global mem at once. Reading is ok, since there should be some broadcast mechanism. I might be wrong too :-D