sorting - How to partly sort arrays on CUDA? -


problem

provided have 2 arrays:

   const int n = 1000000;    float a[n];    mystruct *b[n];

the numbers in can positive or negative (e.g. a[n]={3,2,-1,0,5,-2}), how can make array partly sorted (all positive values first, not need sorted, negative values)(e.g. a[n]={3,2,5,0,-1,-2} or a[n]={5,2,3,0,-2,-1}) on gpu? array b should changed according (a keys, b values).

since scale of a,b can large, think sort algorithm should implemented on gpu (especially on cuda, because use platform). surely know thrust::sort_by_key can work, muck work since not need array a&b sorted entirely.

has come across kind of problem?

thrust example

thrust::sort_by_key(thrust::device_ptr<float> (a),              thrust::device_ptr<float> ( + n ),               thrust::device_ptr<mystruct> ( b ),               thrust::greater<float>() );

thrust's documentation on github not up-to-date. @jaredhoberock said, thrust::partition way go since supports stencils. may need copy github repository:

git clone git://github.com/thrust/thrust.git

then run scons doc in thrust folder updated documentation, , use these updated thrust sources when compiling code (nvcc -i/path/to/thrust ...). new stencil partition, can do:

#include <thrust/partition.h> #include <thrust/execution_policy.h> #include <thrust/iterator/zip_iterator.h> #include <thrust/tuple.h>  struct is_positive { __host__ __device__ bool operator()(const int &x) {   return x >= 0; } };   thrust::partition(thrust::host, // if want test on host                   thrust::make_zip_iterator(thrust::make_tuple(keyvec.begin(), valvec.begin())),                   thrust::make_zip_iterator(thrust::make_tuple(keyvec.end(), valvec.end())),                   keyvec.begin(),                   is_positive());

this returns:

before:   keyvec =   0  -1   2  -3   4  -5   6  -7   8  -9   valvec =   0   1   2   3   4   5   6   7   8   9 after:   keyvec =   0   2   4   6   8  -5  -3  -7  -1  -9   valvec =   0   2   4   6   8   5   3   7   1   9

note 2 partitions not sorted. also, order may differ between original vectors , partitions. if important you, can use thrust::stable_partition:

stable_partition differs partition in stable_partition guaranteed preserve relative order. is, if x , y elements in [first, last), such pred(x) == pred(y), , if x precedes y, still true after stable_partition x precedes y.

if want complete example, here is:

#include <thrust/host_vector.h> #include <thrust/device_vector.h> #include <thrust/partition.h> #include <thrust/iterator/zip_iterator.h> #include <thrust/tuple.h>  struct is_positive { __host__ __device__ bool operator()(const int &x) {   return x >= 0; } };  void print_vec(const thrust::host_vector<int>& v) {   for(size_t = 0; < v.size(); i++)     std::cout << "  " << v[i];   std::cout << "\n"; }  int main () {   const int n = 10;    thrust::host_vector<int> keyvec(n);   thrust::host_vector<int> valvec(n);    int sign = 1;   for(int = 0; < n; ++i)   {     keyvec[i] = sign * i;     valvec[i] = i;     sign *= -1;   }    // copy host device   thrust::device_vector<int> d_keyvec = keyvec;   thrust::device_vector<int> d_valvec = valvec;    std::cout << "before:\n  keyvec = ";   print_vec(keyvec);   std::cout << "  valvec = ";   print_vec(valvec);    // partition key-val on device   thrust::partition(thrust::make_zip_iterator(thrust::make_tuple(d_keyvec.begin(), d_valvec.begin())),                     thrust::make_zip_iterator(thrust::make_tuple(d_keyvec.end(), d_valvec.end())),                     d_keyvec.begin(),                     is_positive());    // copy result host   keyvec = d_keyvec;   valvec = d_valvec;    std::cout << "after:\n  keyvec = ";   print_vec(keyvec);   std::cout << "  valvec = ";   print_vec(valvec); }

update

i made quick comparison thrust::sort_by_key version, , thrust::partition implementation seem faster (which naturally expect). here obtain on nvidia visual profiler, n = 1024 * 1024, sort version on left, , partition version on right. may want same kind of tests on own.

sort vs partition


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