plugins.rst

Plugins

RAJA provides a plugin mechanism to support optional components that provide additional functionality to make writing applications easier. Currently, there is only one RAJA plugin that we support, CHAI.

CHAI

RAJA provides abstractions for parallel execution, but does not support a memory model for managing data in heterogeneous memory spaces. CHAI is an array abstraction that can be used to copy data transparently from one memory space to another as needed to run a RAJA-based kernel. The data can be accessed inside any RAJA kernel, and regardless of where that kernel executes, CHAI will make the data available.

To build RAJA with CHAI integration, you need to download and install CHAI. Please see the CHAI project for details.

After CHAI is installed, RAJA can be configured to use it by passing two additional arguments to CMake:

$ cmake -DRAJA_ENABLE_CHAI=On -Dchai_DIR=/path/to/chai

After RAJA has been built with CHAI support enabled, applications can use chai::ManangedArray objects to access data inside RAJA kernels; for example:

chai::ManagedArray<float> array(1000);

RAJA::forall<RAJA::cuda_exec<16> >(0, 1000, [=] __device__ (int i) {
    array[i] = i * 2.0f;
});

RAJA::forall<RAJA::seq_exec>(0, 1000, [=] (int i) {
  std::cout << "array[" << i << "]  is " << array[i] << std::endl;
});

Here, the data held by array is allocated on the host CPU. Then, it is initialized on a CUDA GPU device. CHAI sees that the data lives on the CPU and is needed in a GPU device data environment. So it copies the data from CPU to GPU, making it available for access in the first RAJA kernel. Next, it is printed in the second kernel which runs on the CPU. So CHAI copies the data back to the host CPU. All necessary data copies are done transparently on demand as needed for each kernel.