Add reduction example using global memory

Author: SimeonEhrig

examples/basic/CMakeLists.txt

@@ -32,3 +32,14 @@ target_sources(${_TARGET_RANDOM_2D_MATRIX}
 set_target_properties(${_TARGET_RANDOM_2D_MATRIX} PROPERTIES
   CUDA_CXX_STANDARD 17
 )
+
+set(_TARGET_REDUCE reduce)
+
+add_executable(${_TARGET_REDUCE})
+target_sources(${_TARGET_REDUCE}
+   PRIVATE
+   reduce.cu)
+set_target_properties(${_TARGET_REDUCE} PROPERTIES
+  CXX_STANDARD 17
+  CUDA_CXX_STANDARD 17
+)

examples/basic/reduce.cu

@@ -0,0 +1,126 @@
+#include <iostream>
+#include <random>
+#include <vector>
+#include <algorithm>
+
+// The wrapper macro is required, that __LINE__ is correct pointing to the line, where the check fails
+#define checkCudaError(ans)                            \
+    {                                                  \
+        checkCudaErrorFunc((ans), __FILE__, __LINE__); \
+    }
+
+inline void checkCudaErrorFunc(cudaError_t err, const char *file, int line)
+{
+    if (err != cudaSuccess)
+    {
+        std::cout << "\r" << file << ":" << line << " -> Cuda Error " << err << ": " << cudaGetErrorString(err) << std::endl;
+        std::cout << "Aborting..." << std::endl;
+        exit(0);
+    }
+}
+
+// The reduction algorithms divide all elements in logical blocks with the size of threads.
+// Each local block is reduced to a single element.
+// A grid stride loop maps the logical blocks to cuda blocks (both has the same size).
+// The output array has the size of the number of logical blocks.
+__global__ void reduce_gm(unsigned int const size, unsigned int *const input, unsigned int *const output)
+{
+    int const id = threadIdx.x + blockIdx.x * blockDim.x;
+    int const stride = blockDim.x * gridDim.x;
+    // use grid stride loop to distribute the logical blocks to cuda blocks.
+    for (int block_offset_id = id, virtual_block_id = blockIdx.x; block_offset_id < size; block_offset_id += stride, virtual_block_id += gridDim.x)
+    {
+        // reduce all elements of logical block to a single element.
+        for (int max_threads_blocks = blockDim.x / 2; max_threads_blocks > 0; max_threads_blocks /= 2)
+        {
+            if (threadIdx.x < max_threads_blocks)
+            {
+                input[block_offset_id] += input[block_offset_id + max_threads_blocks];
+            }
+            __syncthreads();
+        }
+        if (threadIdx.x == 0)
+        {
+            // write single element to output
+            output[virtual_block_id] = input[block_offset_id];
+        }
+        __syncthreads();
+    }
+}
+
+// Helper function -> should be replaced by html visualization ;-) 
+template <typename T>
+void print_vec(std::vector<T> vec)
+{
+    for (auto const v : vec)
+    {
+        std::cout << v << " ";
+    }
+    std::cout << std::endl;
+}
+
+int main(int argc, char **argv)
+{
+    int const blocks = 10;
+    int const threads = 32;
+
+    // number of input elements
+    unsigned int const size = 1000;
+    size_t const data_size_byte = sizeof(unsigned int) * size;
+
+    // number of logical blocks
+    size_t output_elements = size / threads;
+    // add an extra element, if logical blocks does not fit in cuda blocks 
+    output_elements += (size % threads == 0) ? 0 : 1;
+    size_t const output_size_byte = sizeof(unsigned int) * output_elements;
+
+    std::vector<unsigned int> h_data(size);
+    std::vector<unsigned int> h_output(output_elements, 0);
+
+    // initialize data matrix with random numbers betweem 0 and 10
+    std::uniform_int_distribution<unsigned int> distribution(
+        0,
+        10);
+    std::default_random_engine generator;
+    std::generate(
+        h_data.begin(),
+        h_data.end(),
+        [&distribution, &generator]()
+        { return distribution(generator); });
+
+    // calculate result for verification
+    unsigned int const expected_result = std::reduce(h_data.begin(), h_data.end());
+
+    unsigned int *d_data = nullptr;
+    unsigned int *d_output = nullptr;
+
+    checkCudaError(cudaMalloc((void **)&d_data, data_size_byte));
+    checkCudaError(cudaMalloc((void **)&d_output, output_size_byte));
+    checkCudaError(cudaMemcpy(d_data, h_data.data(), data_size_byte, cudaMemcpyHostToDevice));
+
+    reduce_gm<<<blocks, threads>>>(size, d_data, d_output);
+    checkCudaError(cudaGetLastError());
+
+    checkCudaError(cudaMemcpy(h_output.data(), d_output, output_size_byte, cudaMemcpyDeviceToHost));
+
+    unsigned int sum = 0;
+
+    // Reduce all sums of the logical blocks on CPU.
+    // Otherwise a second kernel or cuda cooperative groups are required to performe block synchronization.   
+    for (unsigned int const v : h_output)
+    {
+        sum += v;
+    }
+
+    if (sum == expected_result)
+    {
+        std::cout << "reduction kernel works correctly" << std::endl;
+    }
+    else
+    {
+        std::cout << "sum: " << sum << std::endl;
+        std::cout << "expected result: " << expected_result << std::endl;
+    }
+
+    return 0;
+}