Ensure that thrust doesnt memcpy from host to device

Question

I have used the following method, expecting to avoid memcpy from host to device. Does thrust library ensure that there wont be a memcpy from host to device in the process?

void EScanThrust(float * d_in, float * d_out)
{
     thrust::device_ptr<float> dev_ptr(d_in);
     thrust::device_ptr<float> dev_out_ptr(d_out);

     thrust::exclusive_scan(dev_ptr, dev_ptr + size, dev_out_ptr);
}

Here d_in and d_out are prepared using cudaMalloc and d_in is filled with data using cudaMemcpy before calling this function

There shouldn't be any memcopying from host to device. According to your description, there are no host pointers or data anywhere, so how would thrust copy anything from the host? Anyway, it's trivial to verify this using a profiler such as nvprof --print-gpu-trace ... A cudaMemcpy H->D will show up quite clearly in the trace if there is one. — Robert Crovella

Robert Crovella Robert Crovella · Accepted Answer · 2016-05-04T00:57:30

Does thrust library ensure that there wont be a memcpy from host to device in the process?

The code you've shown shouldn't involve any host->device copying. (How could it? There are no references anywhere to any host data in the code you have shown.)

For actual codes, it's easy enough to verify the underlying CUDA activity using a profiler, for example:

nvprof --print-gpu-trace ./my_exe

If you keep your profiled code sequences short, it's pretty easy to line up the underlying CUDA activity with the thrust code that generated that activity. If you want to profile just a short segment of a longer sequence, then you can turn profiling on and off or else use NVTX markers to identify the desired range in the profiler output.

Ensure that thrust doesnt memcpy from host to device

1 Answers