fast CUDA thrust custom comparison operator

Question

I'm evaluating CUDA and currently using Thrust library to sort numbers.

I'd like to create my own comparer for thrust::sort, but it slows down drammatically! I created my own less implemetation by just copying code from functional.h. However it seems to be compiled in some other way and works very slowly.

1. default comparer: thrust::less() - 94ms
2. my own comparer: less() - 906ms

I'm using Visual Studio 2010. What should I do to get the same performance as at option 1?

Complete code:

#include <stdio.h>

#include <cuda.h>

#include <thrust/host_vector.h>
#include <thrust/device_vector.h>
#include <thrust/generate.h>
#include <thrust/sort.h>

int myRand()
{
        static int counter = 0;
        if ( counter++ % 10000 == 0 )
                srand(time(NULL)+counter);
        return (rand()<<16) | rand();
}

template<typename T>
struct less : public thrust::binary_function<T,T,bool>
{
  __host__ __device__ bool operator()(const T &lhs, const T &rhs) const {
     return lhs < rhs;
  }
}; 

int main()
{
    thrust::host_vector<int> h_vec(10 * 1000 * 1000);
    thrust::generate(h_vec.begin(), h_vec.end(), myRand);

    thrust::device_vector<int> d_vec = h_vec;

    int clc = clock();
    thrust::sort(d_vec.begin(), d_vec.end(), less<int>());
    printf("%dms\n", (clock()-clc) * 1000 / CLOCKS_PER_SEC);

    return 0;
}

Answer 1

The reason you are observing a difference in performance is because Thrust is implementing the sort with different algorithms depending on the arguments provided to thrust::sort.

In case 1., Thrust can prove that the sort can be implemented in linear time with a radix sort. This is because the type of the data to sort is a built-in numeric type (int), and the comparison function is the built-in less than operation -- Thrust recognizes that thrust::less<int> will produce the equivalent result as x < y.

In case 2., Thrust knows nothing about your user-provided less<int>, and has to use a more conservative algorithm based on a comparison sort which has different asymptotic complexity, even though in truth your less<int> is equivalent to thrust::less<int>.

In general, user-defined comparison operators can't be used with more restrictive, faster sorts which manipulate the binary representation of data such as radix sort. In these cases, Thrust falls back on a more general, but slower sort.