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## CUDA

CUDA (Compute Unified Device Architecture) is a platform for parallel computing on NVIDIA GPUs. The application programming interface of CUDA gives access to GPU's instruction set and computation resources (Execution of compute kernels).

#### Waiting for CUDA streams

CUDA operation such as compute kernels or memory transfer (between host and device) can be grouped/queued by CUDA streams. are executed on the GPUs. Boost.Fiber enables a fiber to sleep (suspend) till a CUDA stream has completed its operations. This enables applications to run other fibers on the CPU without the need to spawn an additional OS-threads. And resume the fiber when the CUDA streams has finished.

    __global__
    void kernel( int size, int * a, int * b, int * c) {
        int idx = threadIdx.x + blockIdx.x * blockDim.x;
        if ( idx < size) {
            int idx1 = (idx + 1) % 256;
            int idx2 = (idx + 2) % 256;
            float as = (a[idx] + a[idx1] + a[idx2]) / 3.0f;
            float bs = (b[idx] + b[idx1] + b[idx2]) / 3.0f;
            c[idx] = (as + bs) / 2;
        }
    }

    boost::fibers::fiber f([&done]{
        cudaStream_t stream;
        cudaStreamCreate( & stream);
        int size = 1024 * 1024;
        int full_size = 20 * size;
        int * host_a, * host_b, * host_c;
        cudaHostAlloc( & host_a, full_size * sizeof( int), cudaHostAllocDefault);
        cudaHostAlloc( & host_b, full_size * sizeof( int), cudaHostAllocDefault);
        cudaHostAlloc( & host_c, full_size * sizeof( int), cudaHostAllocDefault);
        int * dev_a, * dev_b, * dev_c;
        cudaMalloc( & dev_a, size * sizeof( int) );
        cudaMalloc( & dev_b, size * sizeof( int) );
        cudaMalloc( & dev_c, size * sizeof( int) );
        std::minstd_rand generator;
        std::uniform_int_distribution<> distribution(1, 6);
        for ( int i = 0; i < full_size; ++i) {
            host_a[i] = distribution( generator);
            host_b[i] = distribution( generator);
        }
        for ( int i = 0; i < full_size; i += size) {
            cudaMemcpyAsync( dev_a, host_a + i, size * sizeof( int), cudaMemcpyHostToDevice, stream);
            cudaMemcpyAsync( dev_b, host_b + i, size * sizeof( int), cudaMemcpyHostToDevice, stream);
            kernel<<< size / 256, 256, 0, stream >>>( size, dev_a, dev_b, dev_c);
            cudaMemcpyAsync( host_c + i, dev_c, size * sizeof( int), cudaMemcpyDeviceToHost, stream);
        }
        auto result = boost::fibers::cuda::waitfor_all( stream); // suspend fiber till CUDA stream has finished
        BOOST_ASSERT( stream == std::get< 0 >( result) );
        BOOST_ASSERT( cudaSuccess == std::get< 1 >( result) );
        cudaFreeHost( host_a);
        cudaFreeHost( host_b);
        cudaFreeHost( host_c);
        cudaFree( dev_a);
        cudaFree( dev_b);
        cudaFree( dev_c);
        cudaStreamDestroy( stream);
    });
    f.join();

#### Synopsis

    #include <boost/fiber/cuda/waitfor.hpp>

    namespace boost {
    namespace fibers {
    namespace cuda {

    std::tuple< cudaStream_t, cudaError_t > waitfor_all( cudaStream_t st);
    std::vector< std::tuple< cudaStream_t, cudaError_t > > waitfor_all( cudaStream_t ... st);

    }}}

##### Non-member function `cuda::waitfor()`

    #include <boost/fiber/cuda/waitfor.hpp>

    namespace boost {
    namespace fibers {
    namespace cuda {

    std::tuple< cudaStream_t, cudaError_t > waitfor_all( cudaStream_t st);
    std::vector< std::tuple< cudaStream_t, cudaError_t > > waitfor_all( cudaStream_t ... st);

    }}}

Effects:

  Suspends active fiber till CUDA stream has finished its operations.

Returns:

  tuple of stream reference and the CUDA stream status