Question

I need to perform a parallel reduction to find the min or max of an array on a CUDA device. I found a good library for this, called Thrust. It seems that you can only perform a parallel reduction on arrays in host memory. My data is in device memory. Is it possible to perform a reduction on data in device memory?
I can’t figure how to do this. Here is documentation for Thrust: http://code.google.com/p/thrust/wiki/QuickStartGuide#Reductions. Thank all of you.

Answer 1

You can do reductions in thrust on arrays which are already in device memory. All that you need to do is wrap your device pointers inside thrust::device_pointer containers, and call one of the reduction procedures, just as shown in the wiki you have linked to:

// assume this is a valid device allocation holding N words of data
int * dmem;

// Wrap raw device pointer 
thrust::device_ptr<int> dptr(dmem);

// use max_element for reduction
thrust::device_ptr<int> dresptr = thrust::max_element(dptr, dptr+N);

// retrieve result from device (if required)
int max_value = dresptr[0];

Note that the return value is also a device_ptr, so you can use it directly in other kernels using thrust::raw_pointer_cast:

int * dres = thrust::raw_pointer_cast(dresptr);