# Sum Reduction: Vector Dot Product¶

This section contains an exercise file RAJA/exercises/dot-product.cpp for you to work through if you wish to get some practice with RAJA. The file RAJA/exercises/dot-product_solution.cpp contains complete working code for the examples discussed in this section. You can use the solution file to check your work and for guidance if you get stuck. To build the exercises execute make dot-product and make dot-product_solution from the build directory.

Key RAJA features shown in this example are:

• RAJA::forall loop execution template and execution policies
• RAJA::TypedRangeSegment iteration space construct
• RAJA::ReduceSum sum reduction template and reduction policies

In the example, we compute a vector dot product, ‘dot = (a,b)’, where ‘a’ and ‘b’ are two vectors of length N and ‘dot’ is a scalar. Typical C-style code to compute the dot product and print its value afterward is:

  double dot = 0.0;

for (int i = 0; i < N; ++i) {
dot += a[i] * b[i];
}

std::cout << "\t (a, b) = " << dot << std::endl;


Although this kernel is serial, it is representative of a reduction operation which is a common algorithm pattern that produces a single result from a set of values. Reductions present a variety of issues that must be addressed to operate properly in parallel.

## RAJA Variants¶

Different programming models support parallel reduction operations differently. Some models, such as CUDA, do not provide direct support for reductions and so such operations must be explicitly coded by users. It can be challenging to generate a correct and high performance implementation. RAJA provides portable reduction types that make it easy to perform reduction operations in kernels. The RAJA variants of the dot product computation show how to use the RAJA::ReduceSum sum reduction template type. RAJA provides other reduction types and allows multiple reduction operations to be performed in a single kernel alongside other computations. Please see Reduction Operations for more information.

Each RAJA reduction type takes a reduce policy template argument, which must be compatible with the execution policy applied to the kernel in which the reduction is used. Here is the RAJA sequential variant of the dot product computation:

  RAJA::ReduceSum<RAJA::seq_reduce, double> seqdot(0.0);

RAJA::forall<RAJA::seq_exec>(RAJA::TypedRangeSegment<int>(0, N), [=] (int i) {
seqdot += a[i] * b[i];
});

dot = seqdot.get();


The sum reduction object is defined by specifying the reduction policy RAJA::seq_reduce matching the kernel execution policy RAJA::seq_exec, and a reduction value type (i.e., ‘double’). An initial value of zero for the sum is passed to the reduction object constructor. After the kernel executes, we use the ‘get’ method to retrieve the reduced value.

The OpenMP multithreaded variant of the kernel is implemented similarly:

  RAJA::ReduceSum<RAJA::omp_reduce, double> ompdot(0.0);

RAJA::forall<RAJA::omp_parallel_for_exec>(RAJA::RangeSegment(0, N), [=] (int i) {
ompdot += a[i] * b[i];
});

dot = ompdot.get();


Here, we use the RAJA::omp_reduce reduce policy to match the OpenMP kernel execution policy.

The RAJA CUDA variant is achieved by using appropriate kernel execution and reduction policies:

  RAJA::ReduceSum<RAJA::cuda_reduce, double> cudot(0.0);

RAJA::forall<RAJA::cuda_exec<CUDA_BLOCK_SIZE>>(RAJA::RangeSegment(0, N),
[=] RAJA_DEVICE (int i) {
cudot += a[i] * b[i];
});

dot = cudot.get();


Here, the CUDA reduce policy RAJA::cuda_reduce matches the CUDA kernel execution policy. Note that the CUDA thread block size is not specified in the reduce policy as it will use the same value as the loop execution policy.

Similarly, for the RAJA HIP variant:

  RAJA::ReduceSum<RAJA::hip_reduce, double> hpdot(0.0);

RAJA::forall<RAJA::hip_exec<HIP_BLOCK_SIZE>>(RAJA::RangeSegment(0, N),
[=] RAJA_DEVICE (int i) {
hpdot += d_a[i] * d_b[i];
});

dot = hpdot.get();


It is worth repeating how similar the code looks for each of these variants. The loop body is identical for each and only the loop execution policy and reduce policy types change.

Note

Currently available reduction capabilities in RAJA require a reduction policy type that is compatible with the execution policy for the kernel in which the reduction is used. We are developing a new reduction interface for RAJA that will provide an alternative for which the reduction policy is not required.