Computation values turn out to be incorrect

• 03-24-2010, 07:05 PM
ssarangi
Computation values turn out to be incorrect
Hi All,
I am implementing a simple bounding box algorithm on opencl and I have problem with the computations.
I have a set of vertices which I pass into the kernel and I compute the bounding box accordingly. I also pass in another array just to read back the vertices back
from the kernel to find that they are all different. I am running on WinXP with Quadro FX 570M and 197.15 (Nvidia Driver).
The code for the C part is this.
Code :

``` float bbox[8], results[8]; memset(results, 0, 8 * sizeof(float));   bbox[0] = (float)mesh->vertex[0].pt[0]; bbox[4] = (float)mesh->vertex[0].pt[0]; bbox[1] = (float)mesh->vertex[0].pt[1]; bbox[5] = (float)mesh->vertex[0].pt[1]; bbox[2] = (float)mesh->vertex[0].pt[2]; bbox[6] = (float)mesh->vertex[0].pt[2];   // GPU Calculation float* vertices = static_cast<float *>(malloc(4 * mesh->vertex_count * sizeof(float))); float* fnew_verts = static_cast<float *>(malloc(4 * mesh->vertex_count * sizeof(float)));   memset(vertices, 0, 4 * mesh->vertex_count * sizeof(float)); memset(fnew_verts, 0, 4 * mesh->vertex_count * sizeof(float));   float* vertex = vertices; for (int i = 0; i < mesh->vertex_count; i++) { *vertex++ = (float)mesh->vertex[i].pt[0]; *vertex++ = (float)mesh->vertex[i].pt[1]; *vertex++ = (float)mesh->vertex[i].pt[2]; *vertex++; }   ShaderManager* shm = ShaderManager::Instance();   cl_int err; cl_mem vertices_mem, bbox_mem, new_verts; vertices_mem = clCreateBuffer(shm->opencl_context(), CL_MEM_READ_ONLY, 4 * mesh->vertex_count * sizeof(float), NULL, NULL); err = clEnqueueWriteBuffer(shm->opencl_command_queue(), vertices_mem, CL_TRUE, 0, 4 * mesh->vertex_count * sizeof(float), (void *)vertices, 0, NULL, NULL);   bbox_mem = clCreateBuffer(shm->opencl_context(), CL_MEM_READ_WRITE, 8 * sizeof(float), NULL, NULL); err = clEnqueueWriteBuffer(shm->opencl_command_queue(), bbox_mem, CL_TRUE, 0, 8 * sizeof(float), (void *)bbox, 0, NULL, NULL);   new_verts = clCreateBuffer(shm->opencl_context(), CL_MEM_WRITE_ONLY, 4 * mesh->vertex_count * sizeof(float), NULL, NULL);   clFinish(shm->opencl_command_queue());   err = clSetKernelArg(shm->opencl_kernel(), 0, sizeof(cl_mem), &vertices_mem); err |= clSetKernelArg(shm->opencl_kernel(), 1, sizeof(cl_mem), &bbox_mem); err |= clSetKernelArg(shm->opencl_kernel(), 2, sizeof(cl_mem), &new_verts);   size_t global_work_size = mesh->vertex_count; err = clEnqueueNDRangeKernel(shm->opencl_command_queue(), shm->opencl_kernel(), 1, NULL, &global_work_size, NULL, 0, NULL, NULL);   #pragma warning(disable:4189) const char* str = errorString(err); clFinish(shm->opencl_command_queue());     err = clEnqueueReadBuffer(shm->opencl_command_queue(), bbox_mem, CL_TRUE, 0, 8 * sizeof(float), results, 0, NULL, NULL); err = clEnqueueReadBuffer(shm->opencl_command_queue(), new_verts, CL_TRUE, 0, 4 * mesh->vertex_count * sizeof(float), fnew_verts, 0, NULL, NULL);   clFinish(shm->opencl_command_queue());   // Verify with input data for (int i = 0; i < 4 * mesh->vertex_count; i++) assert(fnew_verts[i] == vertices[i]);   for (int i = 0; i < mesh->vertex_count; i++) { mesh->vertex[i].pt[0] = (double)(fnew_verts[0]); mesh->vertex[i].pt[1] = (double)(fnew_verts[1]); mesh->vertex[i].pt[2] = (double)(fnew_verts[2]); fnew_verts += 4; }   free(vertices);```

The kernel is defined like this as follows:
Code :

``` __kernel void bound_mesh(__global float4* vertices, __global float4* bbox, __global float4 new_verts) { int gid = get_global_id(0);   if (vertices[gid].x < bbox[0].x) bbox[0].x = vertices[gid].x;   if (vertices[gid].y < bbox[0].y) bbox[0].y = vertices[gid].y;   if (vertices[gid].z < bbox[0].z) bbox[0].z = vertices[gid].z;   if (vertices[gid].x > bbox[1].x) bbox[1].x = vertices[gid].x;   if (vertices[gid].y > bbox[1].y) bbox[1].y = vertices[gid].y;   if (vertices[gid].z > bbox[2].z) bbox[1].z = vertices[gid].z;   new_verts[gid = vertices[gid];   bbox[0].w = -1.0; bbox[1].w = -1.0; }```

So new_verts should atleast give me the correct vertices considering I am assigning them the same value for the vertices.
So neither the bounding box nor the new_verts has the correct values in them. Any suggestions as to what I might be doing wrong.
Thanks for your help.
• 04-06-2010, 07:50 AM
Psch
Re: Computation values turn out to be incorrect
Hi,

I didn't tried your kernel, but it might be caused by fact the vertices parameter is defined as:
Quote:

__global float4* vertices
while the new_verts:
Quote:

__global float4 new_verts
(first as pointer, second not)

Maybe changing it to:
Quote:

__global float4* new_verts
could fix the problem.

You might also try using:
Quote:

new_verts[gid].xyzw = vertices[gid].xyzw;
If I remember correctly, it helped me to workaround similar problem long time ago with older drivers.

Petr
• 04-06-2010, 10:30 PM
dbs2
Re: Computation values turn out to be incorrect
It looks like you are setting bbox[0] in all your work-items at the same time. This means they may overwrite each other in a non-determinisitic fashion. (E.g., work-item 302 might replace the value of bbox[0].y at the same time that work-item 303 is trying to check it, resulting in the check getting the old number and the replacement not being noticed.) Effectively this looks like a data-race between all your threads. You need to make sure that they all write to different locations or that you explicitly synchronize your writes.