CUDA矩阵乘法，执行时间长

我是CUDA的新手，我一直在努力找出我做错了什么。CUDA所花费的时间比仅仅使用CPU来乘以矩阵要长。如果我做错了什么，请告诉我。这是我的代码：

#include "cuda_runtime.h"
#include "device_launch_parameters.h"
#include <stdio.h>
#include <stdlib.h>
#include <cstdlib>
#include <assert.h>
#include <time.h>
#define size 100   // Matrix size
#define cols size   // Matrix width
#define rows size   // Matrix height
void checkCUDAError(const char *msg)
{
    cudaError_t err = cudaGetLastError();
    if( cudaSuccess != err) 
    {
        fprintf(stderr, "Cuda error: %s: %s.n", msg, cudaGetErrorString( err) );
        exit(EXIT_FAILURE);
    }                         
}
__global__ void matrixMul( int *A, int *B, int *C)
{   
    int bx = blockIdx.x; // Block index
    int tx = threadIdx.x; // Thread index
    int ts = blockDim.x; // number of threads   
    // Declaration of the shared memory C element
    extern __shared__ int c_element_sum[];
    c_element_sum[tx] = A[tx+((bx/ts)*ts)] * B[(bx%ts)+(tx*ts)];
    //Block until all threads in the block have written their data to shared mem
    __syncthreads();
    int sum;
    for(int i=0; i<ts; i++){
        if(i==0){
            sum=c_element_sum[i];
        }
        else{
            sum+=c_element_sum[i];
        }
    }
    C[bx] = sum;
}

/////////////////////////////////////////////////////////
// Program main
/////////////////////////////////////////////////////////
int main(int argc, char** argv)
{
   //create timer.
   clock_t t1, t2;
   //start timer
   t1=clock();
   //allocate host memory for matrices
   unsigned int size_A = cols * rows;
   unsigned int mem_size_A = sizeof(int) * size_A;
   int* mA = (int*) malloc(mem_size_A);
   unsigned int size_B = cols * rows;
   unsigned int mem_size_B = sizeof(int) * size_B;
   int* mB = (int*) malloc(mem_size_B);
   unsigned int size_C = cols * rows;
   unsigned int mem_size_C = sizeof(int) * size_C;
   int* mC = (int*) malloc(mem_size_C);
   //initialize host memory
   for (int i = 0; i < size_A; ++i){
       mA[i] = 1;
       mB[i] = 1;
       mC[i] = 0;
   }
   // allocate device memory
   int* d_mA;
   int* d_mB;
   int* d_mC;
   cudaMalloc((void**) &d_mA, mem_size_A);
   cudaMalloc((void**) &d_mB, mem_size_B);
   cudaMalloc((void**) &d_mC, mem_size_C);
   //copy host memory to device (A and B)
   cudaMemcpy(d_mA, mA, mem_size_A, cudaMemcpyHostToDevice);
   cudaMemcpy(d_mB, mB, mem_size_B, cudaMemcpyHostToDevice);
   cudaMemcpy(d_mC, mC, mem_size_C, cudaMemcpyHostToDevice);
   // setup execution parameters
   int numThreadsPerBlock = cols;
   int numBlocks = (cols * rows);
   int sharedMemSize = numThreadsPerBlock * sizeof(int);
   dim3 dimGrid(numBlocks);
   dim3 dimBlock(numThreadsPerBlock);
   // execute the kernel
   matrixMul <<< dimGrid, dimBlock, sharedMemSize >>>(d_mA, d_mB, d_mC);
   //Block until device has completed
   cudaThreadSynchronize();
   // check if kernel execution generated an error
   // Check for any CUDA errors
   checkCUDAError("kernel invocation");
   //copy result from device to host
   cudaMemcpy(mC, d_mC, mem_size_C, cudaMemcpyDeviceToHost);
   // Check for any CUDA errors
   checkCUDAError("memcpy");
   //stop timer
   t2 = clock();
   //check results
   for (int i = 0; i < size_C; ++i){
       assert(mC[i] == cols);
   }
   //clean up memory
   free(mA);
   free(mB);
   free(mC);
   cudaFree(d_mA);
   cudaFree(d_mB);
   cudaFree(d_mC);
   printf("WITH CUDA - clocks: %d nn", t2-t1);
   //////////////////////////////
   ///////// CPU ONLY //////////
   /////////////////////////////
   //create timer.
   clock_t cpu_t1, cpu_t2;
   //start timer
   cpu_t1=clock();
   //allocate host memory for matrices
   unsigned int cpu_size_A = cols * rows;
   unsigned int cpu_mem_size_A = sizeof(int) * cpu_size_A;
   int* cpu_mA = (int*) malloc(cpu_mem_size_A);
   unsigned int cpu_size_B = cols * rows;
   unsigned int cpu_mem_size_B = sizeof(int) * cpu_size_B;
   int* cpu_mB = (int*) malloc(cpu_mem_size_B);
   unsigned int cpu_size_C = cols * rows;
   unsigned int cpu_mem_size_C = sizeof(int) * cpu_size_C;
   int* cpu_mC = (int*) malloc(cpu_mem_size_C);
   //initialize host memory
   for (int i = 0; i < cpu_size_A; ++i){
       cpu_mA[i] = 1;
       cpu_mB[i] = 1;
       cpu_mC[i] = 0;
   }
   int ts = cols;
   for(int bx=0; bx<(cols*rows);bx++){
       int sum = 0;
       for(int tx=0; tx<cols; tx++){
          sum += cpu_mA[tx+((bx/ts)*ts)] * cpu_mB[(bx%ts)+(tx*ts)];
       }
       cpu_mC[bx]=sum;
   }
   //stop timer
   cpu_t2 = clock();
   //check results
   for (int i = 0; i < cpu_size_C; ++i){
       assert(cpu_mC[i] == cols);
   }
   //clean up memory
   free(cpu_mA);
   free(cpu_mB);
   free(cpu_mC);
   printf("CPU ONLY - clocks: %d nn", cpu_t2-cpu_t1);
   return 0;
}