cuda-samples/python/2_CoreConcepts/blockwiseSum/README.md

Sample: Block-wise Array Sum (Python)

Description

Demonstrates fundamental CUDA thread cooperation: thread/block indexing, strided loops, and block-wise reduction using shared memory. This sample shows three progressively complex kernel patterns using the cuda.core API:

1. Simple indexing - One thread per element
2. Strided loop - Each thread processes multiple elements
3. Block partial sum - Shared memory reduction within each block

What You'll Learn

• How to calculate global thread ID from block and thread indices
• Strided loop pattern for processing arrays larger than grid size
• Block-level cooperation using shared memory and __syncthreads()

Key Concepts

Thread and Block Indexing

Global Thread ID = blockIdx.x * blockDim.x + threadIdx.x
Stride = blockDim.x * gridDim.x

Strided Loop Pattern

Each thread processes multiple elements, enabling fixed grid size for arbitrary array lengths:

for (size_t i = tid; i < N; i += stride) {
    output[i] = input[i] * 2.0f;
}

Key APIs

From cuda.core:

• Device - Device management and context
• Program - Compile CUDA C++ kernels
• ProgramOptions - Kernel compilation options (architecture target)
• LaunchConfig - Configure grid/block dimensions and shared memory
• launch() - Execute kernel
• EventOptions - GPU timing configuration

From CuPy:

• cp.asarray() - Transfer data to GPU
• cp.zeros_like() - Allocate GPU arrays

Requirements

Hardware:

• NVIDIA GPU with CUDA support

Software:

• CUDA Toolkit 13.0 or newer
• Python 3.10 or newer
• See requirements.txt for Python packages

Installation

pip install -r requirements.txt

How to Run

python blockwiseSum.py

Expected Output

Device: <Your GPU>
Compute Capability: sm_XX
Array size: 1,048,576 elements

Simple indexing: Test PASSED
Strided loop:    Test PASSED
Block-wise sum:  Test PASSED

Kernel time: X.XXX ms, Bandwidth: XXX.X GB/s

Done

Files

• blockwiseSum.py - Python implementation with CUDA kernels
• README.md - This file
• requirements.txt - Sample dependencies

See Also

• cuda.core Documentation
• CUDA Shared Memory
• CuPy Documentation