NVIDIA GPU 只有在相同的PCIE根上 才能支持P2P通信

测试代码如下：

    int can_access_peer;
    cudaDeviceProp prop[64];

 
    /* check device count */
    CHECK(cudaGetDeviceCount(&dpar.ngpus));
    if (mpar.myid==0)
    {
        warn(" > CUDA-capable device count: %i", dpar.ngpus);
    }

    for (int i=0; i < dpar.ngpus; i++)
    {
         CHECK(cudaGetDeviceProperties(&prop[i], i));
    }

    // Show all the combinations of supported P2P GPUs
    if (mpar.myid==0)
    {
        for (int i = 0; i < dpar.ngpus; i++)
        {
            for (int j = 0; j < dpar.ngpus; j++)
            {
                if (i == j)
                {
                    continue;
                }
                 CHECK(cudaDeviceCanAccessPeer(&can_access_peer, i, j));
                warn(" > Peer access from %s (GPU%d) -> %s (GPU%d) : %s", prop[i].name, i,
                        prop[j].name, j, can_access_peer ? "Yes" : "No");
             }
        }
    }