Kokkos::deep_copy

Kokkos::deep_copy()

Header File: Kokkos_Core.hpp

Usage:

  Kokkos::deep_copy(exec_space, dest, src);
  Kokkos::deep_copy(dest, src);

Copies data from src to dest, where src and dest can be Kokkos::Views or scalars under certain circumstances.

Interface

template <class ExecSpace, class ViewDest, class ViewSrc>
void Kokkos::deep_copy(const ExecSpace& exec_space, 
                       const ViewDest& dest,
                       const ViewSrc& src);

template <class ExecSpace, class ViewDest>
void Kokkos::deep_copy(const ExecSpace& exec_space, 
                       const ViewDest& dest,
                       const typename ViewDest::value_type& src);

template <class ExecSpace, class ViewSrc>
void Kokkos::deep_copy(const ExecSpace& exec_space, 
                       ViewSrc::value_type& dest,
                       const ViewSrc& src);

template <class ViewDest, class ViewSrc>
void Kokkos::deep_copy(const ViewDest& dest,
                       const ViewSrc& src);

template <class ViewDest>
void Kokkos::deep_copy(const ViewDest& dest,
                       const typename ViewDest::value_type& src);

template <class ViewSrc>
void Kokkos::deep_copy(ViewSrc::value_type& dest,
                       const ViewSrc& src);

Parameters:

• ExecSpace: An ExecutionSpace
• ViewDest:A view-like type with a non-const value_type
• ViewSrc: A view-like type.

Requirements:

• If src and dest are Kokkos::Views, then all the following are true:
  • std::is_same<ViewDest::non_const_value_type, ViewSrc::non_const_value_type>::value == true
  • src.rank == dest.rank (or, for Kokkos::DynRankView, src.rank() == dest.rank())
  • For all k in [0, dest.rank) dest.extent(k) == src.extent(k) (or the same as dest.rank()
  • src.span_is_contiguous() && dest.span_is_contiguous() && std::is_same<ViewDest::array_layout,ViewSrc::array_layout>::value, or there exists an ExecutionSpace copy_space (either given or defaulted) such that both SpaceAccessibility<copy_space, ViewDest::memory_space>::accessible == true and SpaceAccessibility<copy_space,ViewSrc::memory_space>::accessible == true.
• If src is a Kokkos::View and dest is a scalar, then src.rank == 0 is true.

Semantics

• If no ExecutionSpace argument is provided, all outstanding operations (kernels, copy operation) in any execution spaces will be finished before the copy is executed, and the copy operation is finished before the call returns.
• If an ExecutionSpace argument exec_space is provided the call is potentially asynchronous—i.e., the call returns before the copy operation is executed. In that case the copy operation will occur only after any already submitted work to exec_space is finished, and the copy operation will be finished before any work submitted to exec_space after the deep_copy call returns is executed. Note: the copy operation is only synchronous with respect to work in the specific execution space instance, but not necessarily with work in other instances of the same type. This behaves analogous to issuing a cudaMemcpyAsync into a specific CUDA stream, without any additional synchronization.

Examples

Some Things you can and cannot do

#include <Kokkos_Core.hpp>
#include <cstdio>

int main(int argc, char* argv[]) {
  Kokkos::initialize(argc, argv);
  {
    int N = argc > 1 ? atoi(argv[1]) : 12;
    if (N < 6) N = 12;

    // Contiguous Device View
    Kokkos::View<int**, Kokkos::LayoutLeft> d_a("A", N, 10);
    // Deep Copy Scalar into every element of a view
    Kokkos::deep_copy(d_a, 3);

    // Non Contiguous Device View
    auto d_a_2 = Kokkos::subview(d_a, 2, Kokkos::ALL);
    // Deep Copy Scalar into every element of a non-contiguous view
    Kokkos::deep_copy(d_a_2, 5);
    // Non Contiguous Device View
    auto d_a_5 = Kokkos::subview(d_a, 5, Kokkos::ALL);
    // Deep Copy between two non-contiguous views with a common execution space
    Kokkos::deep_copy(d_a_2, d_a_5);

    // Contiguous Host View
    auto h_a = Kokkos::create_mirror_view(d_a);
    // Deep Copy contiguous views
    Kokkos::deep_copy(h_a, d_a);

    // Non Contiguous Host Views
    auto h_a_2 = Kokkos::subview(h_a, 2, Kokkos::ALL);
    // Deep Copy between two non-contiguous views with potentially no common
    // execution space This fails for example if you compile the code with Cuda
    // Kokkos::deep_copy(h_a_2, d_a_2);

    // A Scalar View
    auto d_a_2_5 = Kokkos::subview(d_a, 2, 5);
    int scalar;
    // Deep Copy Scalar View into a scalar
    Kokkos::deep_copy(scalar, d_a_2_5);
  }
  Kokkos::finalize();
}

How to get layout incompatible views copied

#include<Kokkos_Core.hpp>

int main(int argc, char* argv[]) {
  Kokkos::initialize(argc,argv);
  {
    int N = argc>1?atoi(argv[1]):1000000;
    int R = argc>2?atoi(argv[2]):10;


    // Create two views with different Layouts
    Kokkos::View<int**[5], Kokkos::LayoutLeft> d_view("DeviceView",N,R);
    Kokkos::View<int**[5], Kokkos::LayoutRight, Kokkos::HostSpace> h_view("HostView",N,R);

    // This would fail for example in a CUDA or HIP build:
    // Kokkos::deep_copy(d_view,h_view);

    // To copy two views with incompatible layouts between devices we need a temporary
    auto h_view_tmp = Kokkos::create_mirror_view(d_view);

    // This inherits the Layout from d_view
    static_assert(std::is_same<decltype(h_view_tmp)::array_layout,
                               Kokkos::LayoutLeft>::value);

    // This now works since h_view_tmp and h_view are both accessible 
    // from HostSpace::execution_space
    Kokkos::deep_copy(h_view_tmp,h_view);

    // Now we can copy from h_view_tmp to d_view since they are Layout compatible
    // If we just compiled for OpenMP this is a no-op since h_view_tmp and d_view
    // would reference the same data.
    Kokkos::deep_copy(d_view,h_view_tmp);
  }
  Kokkos::finalize();
}