CUDA - thrust::sort on device returns only 0's

Question

I have run the following Thrust example for sorting. The problem is that after the thrust::sort, the output contains all 0's.

Please, tell me what is wrong here.

#include <thrust/host_vector.h>
#include <thrust/device_vector.h>
#include <thrust/copy.h>    
#include <thrust/sort.h>
#include <cstdlib>
#include <iostream>

using namespace std;

int main(void)
{

    thrust::host_vector<int> h_vec(32 << 20);
    thrust::generate(h_vec.begin(), h_vec.end(), rand);


    thrust::device_vector<int> d_vec=h_vec;

    for(int i = 0; i<32;i++)
        cout<<d_vec[i]<<endl;

    cout<<endl<<endl<<endl;
    thrust::sort(d_vec.begin(), d_vec.end());

    for(int i = 0; i<32;i++)
        cout<<d_vec[i]<<endl;

    cout<<endl<<endl<<endl; 

    thrust::copy(d_vec.begin(), d_vec.end(), h_vec.begin());


    for(int i = 0; i<32;i++)
        cout<<h_vec[i]<<endl;


    return 0;
}

Answer 1

The reason why you are observing all 0's is that you are generating a large number of random numbers, i.e., 32 << 20 = 33554432, between 0 and RAND_MAX, you are orderning them, but you are displaying only 32 of them.

As mentioned by Robert Crovella, on a Windows machine (the OP is working on Windows), RAND_MAX = 2^15-1 = 32767. Accordingly, you are generating 33554432 integers between 0 and 32767, which means that you will have a large number of 0's in the original array and so all 0's in the first 32 numbers of the sorted array.

I have personally verifyed that this occurs for both, Windows 32 and 64 bit machines, that is, on both Windows 32 and 64 bit systems RAND_MAX = 32767.

Again, as pointed out by Robert, this effect will show on Linux 32 bit machines, but not on Linux 64 bit machines, for which RAND_MAX = 2^31-1 since, for that case, RAND_MAX is much larger than 32 << 20.

As suggested by Robert, one may change the instruction

thrust::host_vector<int> h_vec(32 << 20);

to

thrust::host_vector<int> h_vec(min(32 << 20,RAND_MAX));

to avoid the all 0's show.