Bandwidth test between cudaMallocManaged buffer and malloc buffer in Xavier

We allocate two kinds of buffer to test the bandwidth:
1)malloc : not visible for GPU, only for CPU
2)cudaMallocManaged : both for CPU and GPU

we use different API to test the bandwidth:
a)memcpy
b)cudaMemcpy

data size: 4MiB, loop count: 100, copy direction: malloc buffer -> cudaMallocManaged buffer

We found a interesting result:
1.memcpy only use 39.18ms
2.cudaMemcpy use 70.47ms

#include <stdio.h>
#include <sys/time.h>


int main() {
        int *m, *u, *d;
        int n = 1024*1024;
        m = new int[n];
        cudaMallocManaged(&u, n*sizeof(int));

        for (int i = 0; i < n; ++i)
                m[i] = i;

        // iter
        int niter = 100;
        struct timeval start, stop;
        double t1, t2;

        // malloc to managed use cudamemcpy
        gettimeofday(&start, NULL);
        for (int i = 0; i < niter; ++i)
                cudaMemcpy(u, m, n*sizeof(int), cudaMemcpyHostToDevice);
        cudaStreamSynchronize(NULL);
        gettimeofday(&stop, NULL);
        t1 = (stop.tv_sec-start.tv_sec)*1e3+(stop.tv_usec-start.tv_usec)/1e3;


        // malloc to managed
        gettimeofday(&start, NULL);
        for (int i = 0; i < niter; ++i)
                memcpy(u, m, n*sizeof(int));
        cudaStreamSynchronize(NULL);
        gettimeofday(&stop, NULL);
        t2 = (stop.tv_sec-start.tv_sec)*1e3+(stop.tv_usec-start.tv_usec)/1e3;
        
        printf("t1: %f, t2: %f\n", t1, t2);

        delete []m;
        cudaFree(u);
        return 0;
}

Hi,

You doesn’t need to apply memory copy on an unified memory since the data is synchronized.
If you want to test cudaMemcpy, it’s recommended to allocate standalone CPU and GPU buffer.

Here is some introduction of memory type of Tegra for your reference:
https://docs.nvidia.com/cuda/cuda-for-tegra-appnote/index.html#memory-management

Thanks.