Question

I want to move the content of a volume texture along the vector vecShift. I think of a kernel like this:

__global__ void
moveVolume(int* vecShift)
{
  // Determine position of current voxel as ptDest

  // Determine position of voxel we copy the content from as ptSrc

  // Read value at ptSrc and store it to voxelColor

  // __threadfence()

  // Write voxelColor to voxel at position ptDest
}

The threadfence will ensure that ALL voxels have read the contents of their “partner” and there will be no write to ptDest before every voxel has done the read-operation, does it?

If this is true, why I (sometimes) get artifacts of a blurry kind? Or do I have a wrong opinion on the functionality of threadfence?

Answer 1

As talonmies explains in the comments, using __threadfence() here is neither necessary nor sufficient. __threadfence() does not provide global barrier synchronization, it simply ensures that before the thread that calls __threadfence() proceeds, all writes by that thread before the fence are visible to all other active threads in the kernel launch.

What you really want here is to double buffer your volume data (i.e. write to a different array than you read). You cannot overwrite other parts of the array unless you can guarantee that they are only read by other threads in the same thread block. Otherwise you have a race condition and your program is incorrect.

Note: even in a sequential (CPU) implementation, you would need to double buffer your data for this type of computation!

What you are implementing is very similar to an advection kernel, as would be used in fluid dynamics simulations, and I’m sure there are multiple examples of what you want on the web (parallel or sequential).

Mark