Cuda gridDim and blockDim

Question

I get what blockDim is, but I have a problem with gridDim. Blockdim gives the size of the block, but what is gridDim? On the Internet

Answer 1

In this source code, we even have 4 threds, the kernel function can access all of 10 arrays. How?

#define N 10 //(33*1024)

__global__ void add(int *c){
    int tid = threadIdx.x + blockIdx.x * gridDim.x;

    if(tid < N)
        c[tid] = 1;

    while( tid < N)
    {
        c[tid] = 1;
        tid += blockDim.x * gridDim.x;
    }
}

int main(void)
{
    int c[N];
    int *dev_c;
    cudaMalloc( (void**)&dev_c, N*sizeof(int) );

    for(int i=0; i>>(dev_c);
    cudaMemcpy(c, dev_c, N*sizeof(int), cudaMemcpyDeviceToHost );

    for(int i=0; i< N; ++i)
    {
        printf("c[%d] = %d \n" ,i, c[i] );
    }

    cudaFree( dev_c );
}

Why we do not create 10 threads ex) add<<<2,5>>> or add<5,2>>> Because we have to create reasonably small number of threads, if N is larger than 10 ex) 33*1024.

This source code is example of this case. arrays are 10, cuda threads are 4. How to access all 10 arrays only by 4 threads.

see the page about meaning of threadIdx, blockIdx, blockDim, gridDim in the cuda detail.

In this source code,

gridDim.x : 2    this means number of block of x

gridDim.y : 1    this means number of block of y

blockDim.x : 2   this means number of thread of x in a block

blockDim.y : 1   this means number of thread of y in a block

Our number of thread are 4, because 2*2(blocks * thread).

In add kernel function, we can access 0, 1, 2, 3 index of thread

->tid = threadIdx.x + blockIdx.x * blockDim.x

①0+0*2=0

②1+0*2=1

③0+1*2=2

④1+1*2=3

How to access rest of index 4, 5, 6, 7, 8, 9. There is a calculation in while loop

tid += blockDim.x + gridDim.x in while

** first call of kernel **

-1 loop: 0+2*2=4

-2 loop: 4+2*2=8

-3 loop: 8+2*2=12 ( but this value is false, while out!)

** second call of kernel **

-1 loop: 1+2*2=5

-2 loop: 5+2*2=9

-3 loop: 9+2*2=13 ( but this value is false, while out!)

** third call of kernel **

-1 loop: 2+2*2=6

-2 loop: 6+2*2=10 ( but this value is false, while out!)

** fourth call of kernel **

-1 loop: 3+2*2=7

-2 loop: 7+2*2=11 ( but this value is false, while out!)

So, all index of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 can access by tid value.

refer to this page. http://study.marearts.com/2015/03/to-process-all-arrays-by-reasonably.html I cannot upload image, because low reputation.