Stencil Computations (View Offsets)¶
Key RAJA features shown in the following example:
RAJA::Kernel
loop execution template- RAJA kernel execution policies
RAJA::View
multi-dimensional data accessRAJA:make_offset_layout
method to apply index offsets
This example applies a five-cell stencil sum to the interior cells of a
two-dimensional square lattice and stores the resulting sums in a second
lattice of equal size. The five-cell stencil accumulates values from each
interior cell and its four neighbors. We use RAJA::View
and
RAJA::Layout
constructs to simplify the multi-dimensional indexing so
that we can write the stencil operation as follows:
output(row, col) = input(row, col) +
input(row - 1, col) + input(row + 1, col) +
input(row, col - 1) + input(row, col + 1)
A lattice is assumed to have \(N_r \times N_c\) interior cells with unit values surrounded by a halo of cells containing zero values for a total dimension of \((N_r + 2) \times (N_c + 2)\). For example, when \(N_r = N_c = 3\), the input lattice and values are:
0 0 0 0 0 0 1 1 1 0 0 1 1 1 0 0 1 1 1 0 0 0 0 0 0
After applying the stencil, the output lattice and values are:
0 0 0 0 0 0 3 4 3 0 0 4 5 4 0 0 3 4 3 0 0 0 0 0 0
For this \((N_r + 2) \times (N_c + 2)\) lattice case, here is our (row, col) indexing scheme.
(-1, 3) (0, 3) (1, 3) (2, 3) (3, 3) (-1, 2) (0, 2) (1, 2) (2, 2) (3, 2) (-1, 1) (0, 1) (1, 1) (2, 1) (3, 1) (-1, 0) (0, 0) (1, 0) (2, 0) (3, 0) (-1, -1) (0, -1) (1, -1) (2, -1) (3, -1)
Notably \([0, N_r) \times [0, N_c)\) corresponds to the interior index range over which we apply the stencil, and \([-1,N_r] \times [-1, N_c]\) is the full lattice index range.
RAJA Offset Layouts¶
We use the RAJA::make_offset_layout
method to construct a
RAJA::OffsetLayout
object that defines our two-dimensional indexing scheme.
Then, we create two RAJA::View
objects for each of the input and output
lattice arrays.
const int DIM = 2;
RAJA::OffsetLayout<DIM> layout =
RAJA::make_offset_layout<DIM>({{-1, -1}}, {{N_r+1, N_c+1}});
RAJA::View<int, RAJA::OffsetLayout<DIM>> inputView(input, layout);
RAJA::View<int, RAJA::OffsetLayout<DIM>> outputView(output, layout);
Here, the row index range is \([-1, N_r]\), and the column index
range is \([-1, N_c]\). The first argument to each call to the
RAJA::View
constructor is a pointer to an array that holds the data for
the view; we assume the arrays are properly allocated before these calls.
The offset layout mechanics of RAJA allow us to write loops over
data arrays using non-zero based indexing and without having to manually
compute the proper offsets into the arrays. For more details on the
RAJA::View
and RAJA::Layout
concepts we use in this example, please
refer to View and Layout.
RAJA Kernel Implementation¶
For the RAJA implementations of the example computation, we use two
RAJA::RangeSegment
objects to define the row and column iteration
spaces for the interior cells:
RAJA::RangeSegment col_range(0, N_r);
RAJA::RangeSegment row_range(0, N_c);
Here, is an implementation using RAJA::kernel
multi-dimensional loop
execution with a sequential execution policy.
using NESTED_EXEC_POL1 =
RAJA::KernelPolicy<
RAJA::statement::For<1, RAJA::seq_exec, // row
RAJA::statement::For<0, RAJA::seq_exec, // col
RAJA::statement::Lambda<0>
>
>
>;
RAJA::kernel<NESTED_EXEC_POL1>(RAJA::make_tuple(col_range, row_range),
[=](int col, int row) {
outputView(row, col) =
inputView(row, col)
+ inputView(row - 1, col)
+ inputView(row + 1, col)
+ inputView(row, col - 1)
+ inputView(row, col + 1);
});
Since the stencil operation is data parallel, any parallel execution policy
may be used. The file RAJA/examples/tut_offset-layout.cpp
contains a
complete working example code with various parallel implementations. For more
information about using the RAJA::kernel
interface, please see
Complex Loops (RAJA::kernel).