# Mesh Vertex Sum Example: Iteration Space Coloring¶

Key RAJA features shown in this example:

• RAJA::forall loop execution template method
• RAJA::ListSegment iteration space construct
• RAJA::IndexSet iteration space segment container and associated execution policies

The example computes a sum at each vertex on a logically-Cartesian 2D mesh as shown in the figure. A portion of the area of each mesh element is summed to the vertices surrounding the element.

Each sum is an average of the area of the mesh elements that share the vertex. In many “staggered mesh” applications, such an operation is common and is often written in a way that presents the algorithm clearly but prevents parallelization due to potential data races. That is, multiple loop iterates over mesh elements may attempt to write to the same shared vertex memory location at the same time. The example shows how RAJA constructs can be used to enable one to express such an algorithm in parallel and have it run correctly without fundamentally changing how it looks in source code.

After defining the number of elements in the mesh, necessary array offsets and an array that indicates the mapping between an element and its four surrounding vertices, a C-style version of the vertex sum calculation is:

  for (int j = 0 ; j < N_elem ; ++j) {
for (int i = 0 ; i < N_elem ; ++i) {
int ie = i + j*jeoff ;
int* iv = &(elem2vert_map[4*ie]);
vertexvol_ref[ iv ] += elemvol[ie] / 4.0 ;
vertexvol_ref[ iv ] += elemvol[ie] / 4.0 ;
vertexvol_ref[ iv ] += elemvol[ie] / 4.0 ;
vertexvol_ref[ iv ] += elemvol[ie] / 4.0 ;
}
}


## RAJA Sequential Variant¶

A nested loop RAJA variant of this kernel is:

  using EXEC_POL1 =
RAJA::KernelPolicy<
RAJA::statement::For<1, RAJA::seq_exec,    // j
RAJA::statement::For<0, RAJA::seq_exec,  // i
RAJA::statement::Lambda<0>
>
>
>;

RAJA::kernel<EXEC_POL1>( RAJA::make_tuple(RAJA::RangeSegment(0, N_elem),
RAJA::RangeSegment(0, N_elem)),
[=](int i, int j) {
int ie = i + j*jeoff ;
int* iv = &(elem2vert_map[4*ie]);
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
});


Note that this version cannot be guaranteed to run correctly in parallel by simply changing the loop execution policies as we have done in other examples. We would like to use RAJA to enable parallel execution and without changing the way the kernel looks in source code. By applying a RAJA index set and suitably-defined list segments, we can accomplish this.

## RAJA Parallel Variants¶

To enable the kernel to run safely in parallel, by eliminating the race conditions, we partition the element iteration space into four subsets (or colors) indicated by the numbers in the figure below, which represents a portion of our logically-Cartesian 2D mesh.

 2 3 2 3 0 1 0 1 2 3 2 3 0 1 0 1

Note that none of the elements with the same number share a common vertex. Thus, we can iterate over all elements with the same number (i.e., color) in parallel.

First, we define four vectors to gather the mesh element indices for each color:

  std::vector<int> idx0;
std::vector<int> idx1;
std::vector<int> idx2;
std::vector<int> idx3;

for (int j = 0 ; j < N_elem ; ++j) {
for (int i = 0 ; i < N_elem ; ++i) {
int ie = i + j*jeoff ;
if ( i % 2 == 0 ) {
if ( j % 2 == 0 ) {
idx0.push_back(ie);
} else {
idx2.push_back(ie);
}
} else {
if ( j % 2 == 0 ) {
idx1.push_back(ie);
} else {
idx3.push_back(ie);
}
}
}
}


Then, we create a RAJA index set with four list segments, one for each color, using the vectors:

  using SegmentType = RAJA::TypedListSegment<int>;

RAJA::TypedIndexSet<SegmentType> colorset;

colorset.push_back( SegmentType(&idx0, idx0.size()) );
colorset.push_back( SegmentType(&idx1, idx1.size()) );
colorset.push_back( SegmentType(&idx2, idx2.size()) );
colorset.push_back( SegmentType(&idx3, idx3.size()) );


Now, we can use an index set execution policy that iterates over the segments sequentially and executes each segment in parallel using OpenMP multi-threading (and RAJA::forall):

  using EXEC_POL3 = RAJA::ExecPolicy<RAJA::seq_segit,
RAJA::omp_parallel_for_exec>;

RAJA::forall<EXEC_POL3>(colorset, [=](int ie) {
int* iv = &(elem2vert_map[4*ie]);
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
});


We no longer need to use the offset variable to compute the element index in terms of ‘i’ and ‘j’ since the loop is no longer nested and the element indices are directly encoded in the list segments.

For completeness, here is the RAJA variant where we iterate over the segments sequentially, and execute each segment in parallel via a CUDA kernel launch on a GPU:

  using EXEC_POL4 = RAJA::ExecPolicy<RAJA::seq_segit,
RAJA::cuda_exec<CUDA_BLOCK_SIZE>>;

RAJA::forall<EXEC_POL4>(colorset, [=] RAJA_DEVICE (int ie) {
int* iv = &(elem2vert_map[4*ie]);
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
vertexvol[ iv ] += elemvol[ie] / 4.0 ;
});


Here, we have marked the lambda loop body with the ‘RAJA_DEVICE’ macro and specified the number of threads in a CUDA thread block in the segment execution policy.

The file RAJA/examples/tut_vertexsum-coloring.cpp contains the complete working example code.