Error while reading same mem positions on different threads

问题

I have a problem while reading a couple of positions in a double array from different threads.

I enqueue the execution with :

nelements = nx*ny;
err = clEnqueueNDRangeKernel(queue,kernelTvl2of,1,NULL,&nelements,NULL,0,NULL,NULL);

kernelTvl2of has (among other) the code

size_t k = get_global_id(0);
(...)
u1_[k] = (float)u1[k];
(...)
barrier(CLK_GLOBAL_MEM_FENCE);
forwardgradient(u1_,u1x,u1y,k,nx,ny);
barrier(CLK_GLOBAL_MEM_FENCE);

and forwardgradient has the code:

void forwardgradient(global double *f, global double *fx, global double *fy, int ker,int nx, int ny){
unsigned int rowsnotlast = ((nx)*(ny-1));
if(ker<rowsnotlast){
 fx[ker] = f[ker+1] - f[ker];
 fy[ker] = f[ker+nx] - f[ker];
}
if(ker<nx*ny){
 fx[ker] = f[ker+1] - f[ker];
 if(ker==4607){
  fx[0] = f[4607];
  fx[1] = f[4608];
  fx[2] = f[4608] - f[4607];
  fx[3] = f[ker];
  fx[4] = f[ker+1];
  fx[5] = f[ker+1] - f[ker];
 }
}
if(ker==(nx*ny)-1){
 fx[ker] = 0;
 fy[ker] = 0;
}
if(ker%nx == nx-1){
 fx[ker]=0;
}
fx[6] = f[4608];
}

When I get the contents of the first positions of fx, they are:

-6 0 6 -6 0 6 -6

And here's my problem: when I query fx[ker+1] or fx[4608] on thread with id 4607 I get a '0' (positions second and fifth of the output array), but from other threads I get a '-6' last position of the output array)

Anyone has a clue on what I'm doing wrong, or where I could look to?

Thanks a lot,

Anton

回答1:

Within a kernel, global memory consistency is only achievable within a single work-group. This means that if a work-item writes a value to global memory, a barrier(CLK_GLOBAL_MEM_FENCE) only guarantees that other work-items within the same work-group will be able to read the updated value.

If you need global memory consistency across multiple work-groups, you need to split your kernel into multiple kernels.

来源：https://stackoverflow.com/questions/33277945/error-while-reading-same-mem-positions-on-different-threads

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multithreading

opencl