Difference between program using constant memory and global memory - cuda

Question

I have two programs. the only difference is that one uses constant memory to store input while the other uses global memory.I want to know why the global memory one is faster than the constant memory one? They both compute dot product btw 2 matrices
#include<cuda_runtime.h>
#include<cuda.h>
#include<stdio.h>
#include<stdlib.h>
#define intMin(a,b) ((a<b)?a:b)
//Threads per block
#define TPB 128
//blocks per grid
#define BPG intMin(128, ((n+TPB-1)/TPB))
const int n = 4;
__constant__ float deva[n],devb[n];
__global__ void addVal( float *c){
int tid = blockIdx.x * blockDim.x + threadIdx.x;
//Using shared memory to temporary store results
__shared__ float cache[TPB];
float temp = 0;
while(tid < n){
temp += deva[tid] * devb[tid];
tid += gridDim.x * blockDim.x;
}
cache[threadIdx.x] = temp;
__syncthreads();
int i = blockDim.x/2;
while( i !=0){
if(threadIdx.x < i){
cache[threadIdx.x] = cache[threadIdx.x] +cache[threadIdx.x + i] ;
}
__syncthreads();
i = i/2;
}
if(threadIdx.x == 1){
c[blockIdx.x ] = cache[0];
}
}
int main(){
float a[n] , b[n] , c[BPG];
//float *deva, *devb, *devc;
float *devc;
int i;
//Filling with random values to test
for( i =0; i< n; i++){
a[i] = i;
b[i] = i*2;
}
//cudaMalloc((void**)&deva, n * sizeof(float));
//cudaMalloc((void**)&devb, n * sizeof(float));
cudaMalloc((void**)&devc, BPG * sizeof(float));
//cudaMemcpy(deva, a, n *sizeof(float), cudaMemcpyHostToDevice);
//cudaMemcpy(devb, b, n *sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpyToSymbol(deva, a, n * sizeof(float));
cudaMemcpyToSymbol(devb, b, n * sizeof(float));
cudaEvent_t start, stop;
cudaEventCreate(&start);
cudaEventCreate(&stop);
cudaEventRecord(start, 0);
//Call function to do dot product
addVal<<<BPG, TPB>>>( devc);
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
float time;
cudaEventElapsedTime(&time,start, stop);
printf("The elapsed time is: %f\n", time);
//copy result back
cudaMemcpy(c, devc, BPG * sizeof(float), cudaMemcpyDeviceToHost);
float sum =0 ;
for ( i = 0 ; i< BPG; i++){
sum+=c[i];
}
//display answer
printf("%f\n",sum);
getchar();
return 0;
}
Below is the global memory version.
#include<cuda_runtime.h>
#include<cuda.h>
#include<stdio.h>
#include<stdlib.h>
#define intMin(a,b) ((a<b)?a:b)
//Threads per block
#define TPB 128
//blocks per grid
#define BPG intMin(128, ((n+TPB-1)/TPB))
const int n = 4;
__global__ void addVal(float *a, float *b, float *c){
int tid = blockIdx.x * blockDim.x + threadIdx.x;
//Using shared memory to temporary store results
__shared__ float cache[TPB];
float temp = 0;
while(tid < n){
temp += a[tid] * b[tid];
tid += gridDim.x * blockDim.x;
}
cache[threadIdx.x] = temp;
__syncthreads();
int i = blockDim.x/2;
while( i !=0){
if(threadIdx.x < i){
cache[threadIdx.x] = cache[threadIdx.x] +cache[threadIdx.x + i] ;
}
__syncthreads();
i = i/2;
}
if(threadIdx.x == 1){
c[blockIdx.x ] = cache[0];
}
}
int main(){
float a[n] , b[n] , c[BPG];
float *deva, *devb, *devc;
int i;
//Filling with random values to test
for( i =0; i< n; i++){
a[i] = i;
b[i] = i*2;
}
printf("Not using constant memory\n");
cudaMalloc((void**)&deva, n * sizeof(float));
cudaMalloc((void**)&devb, n * sizeof(float));
cudaMalloc((void**)&devc, BPG * sizeof(float));
cudaMemcpy(deva, a, n *sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(devb, b, n *sizeof(float), cudaMemcpyHostToDevice);
cudaEvent_t start, stop;
cudaEventCreate(&start);
cudaEventCreate(&stop);
cudaEventRecord(start, 0);
//Call function to do dot product
addVal<<<BPG, TPB>>>(deva, devb, devc);
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
float time;
cudaEventElapsedTime(&time,start, stop);
printf("The elapsed time is: %f\n", time);
//copy result back
cudaMemcpy(c, devc, BPG * sizeof(float), cudaMemcpyDeviceToHost);
float sum =0 ;
for ( i = 0 ; i< BPG; i++){
sum+=c[i];
}
//display answer
printf("%f\n",sum);
getchar();
return 0;
}

Answer 1

You are not getting advantage of the constant memory.
A single read from constant memory can be broadcast to a half-warp (not your case as every thread load from its own tid).
Constant memory is cached (not used in your case as you only read once from each position in the constant memory array).
As each thread in a half-warp does a single read to different data, the 16 different reads get serialized, taking 16 times the amount of time to place the request.
If they are reading from global memory, the request are done at the same time, coalesced. That's why your global memory example is better than the constant memory.
Of course, this conclusion can vary with devices of compute capability 2.x with a L1 and L2 cache.
Regards!