Fastest way to access device vector elements directly on host

Question

I refer you to following page http://code.google.com/p/thrust/wiki/QuickStartGuide#Vectors. Please see second paragraph where it says that

Also note that individual elements of a device_vector can be accessed using the standard bracket notation. However, because each of these accesses requires a call to cudaMemcpy, they should be used sparingly. We'll look at some more efficient techniques later.

I searched all over the document but I could not find the more efficient technique. Does anyone know the fastest way to do this? i.e how to access device vector/device pointer on host fastest?

Answer 1

The "more efficient techniques" the guide alludes to are the Thrust algorithms. It's more efficient to access (or copy across the PCI-E bus) millions of elements at once than it is to access a single element because the fixed cost of CPU/GPU communication is amortized.

There's no faster way to copy data from the GPU to the CPU than by calling cudaMemcpy, because it is the most primitive way for a CUDA programmer to implement the task.