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chore: remove demos directory from main repository
Remove demos/README.md as all demos and examples have been moved to the website repository (juliafem.github.io) where they belong as documentation rather than core library code. - Move all demos/ and examples/ files to juliafem.github.io - Update STRUCTURE.md to reflect examples/ and demos/ live in website - Remove demos/README.md from main codebase - Keep main repository focused on library code only
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Jukka Aho
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# JuliaFEM Technology Demonstrations
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This directory contains demonstrations of key technologies and architectural decisions for JuliaFEM v1.0.
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## Overview
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These demos validate that type-stable field storage enables modern high-performance computing patterns: GPU execution, MPI communication, and Krylov iterative solvers.
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## Demonstrations
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### Assembly Strategy Comparison (`cantilever_cpu_comparison.jl`)
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**Purpose:** Algorithm-level comparison of assembly strategies (RESEARCH).
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⚠️ **NOT a user-facing example!** This is low-level performance research.
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**What it demonstrates:**
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- Element-by-element assembly (traditional FEM)
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- Node-by-node assembly (GPU-friendly, contact-ready)
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- Matrix-free iterative solvers
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- Direct comparison: assembly time, iterations, accuracy
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**Why low-level:**
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- Uses `ElementAssemblyData` and `NodeToElementsMap` structures directly
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- Manually constructs element stiffness matrices
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- Not representative of user workflow
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**For users:** See `examples/linear_static.jl` or `cantilever_gmsh_gpu.jl` instead.
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**Results (135 Tet4 elements):**
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- Nodal assembly: 4.7× faster than element assembly
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- Nodal CG: 2× fewer iterations (215 vs 417)
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- All methods give identical displacements
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### 1. GPU and MPI Communication (`gpu_mpi_demo.jl`)
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**Purpose:** Prove that type-stable data structures flow efficiently to GPU and MPI.
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**What it demonstrates:**
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- Real CUDA GPU kernel execution
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- MPI data transfer between processes
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- Combined GPU+MPI workflow
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**Run:**
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```bash
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mpiexec -np 2 julia --project=. demos/gpu_mpi_demo.jl
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```
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**Documentation:** [README_GPU_MPI.md](README_GPU_MPI.md)
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### 2. Multi-GPU MPI Krylov Solver (`krylov_mpi_gpu_demo.jl`)
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**Purpose:** Complete distributed FEM solver workflow with nodal assembly.
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**What it demonstrates:**
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- Nodal assembly pattern (row-by-row matrix construction)
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- Distributed matrix-vector products
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- Conjugate Gradient solver with MPI
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- Multi-GPU execution
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- Solution verification (10×10 SPD system)
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**Run:**
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```bash
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mpiexec -np 2 julia --project=. demos/krylov_mpi_gpu_demo.jl
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```
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**Documentation:** [README_KRYLOV_DEMO.md](README_KRYLOV_DEMO.md)
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**Results:**
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- Converges in 9 iterations
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- Relative error: 7.73 × 10⁻¹⁴
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- Validates complete distributed solving workflow
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## Requirements
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### Required
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- Julia 1.9+
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- MPI installation (e.g., OpenMPI, MPICH)
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- MPI.jl package
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### Optional (for GPU demos)
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- CUDA-capable GPU
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- CUDA.jl package
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If CUDA is not available, demos will fall back to CPU execution while still demonstrating the distributed computing patterns.
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## Key Insights
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### Type Stability is Not Optional
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These demos prove that type-stable field storage is **required** (not just "nice to have") for:
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| Feature | Why Type Stability Required |
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|---------|----------------------------|
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| GPU execution | CUDA kernels cannot compile with abstract types |
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| Fast MPI | Typed buffers avoid serialization overhead |
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| Krylov solvers | Matrix-free operators need concrete types |
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| CPU performance | 9-92× speedup measured (see CPU benchmarks) |
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### Nodal Assembly Pattern
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The Krylov demo shows row-by-row matrix construction (`get_row()` abstraction), which:
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- Aligns naturally with contact mechanics (nodal constraints)
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- Enables domain decomposition (each rank owns nodes)
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- Supports matrix-free solving (never form global matrix)
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- Scales to large problems (O(N) memory vs O(N²))
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### Architectural Validation
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These demos validate the design decisions for JuliaFEM v1.0:
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**v0.5.1 (2019):**
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- Dict-based fields → Type instability
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- Element assembly → Global matrix
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- Direct solvers → O(N³) time, O(N²) memory
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- Single-threaded CPU
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**v1.0 (target, validated here):**
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- Type-stable fields → GPU/MPI capable
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- Nodal assembly → Distributed construction
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- Krylov solvers → O(N·iter) time, O(N) memory
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- Multi-GPU + MPI
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## References
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- **CPU benchmarks:** See `benchmarks/field_storage_comparison.jl` for 9-92× speedup measurements
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- **Design documentation:** See `docs/book/zero_allocation_fields_v2.md` for architectural rationale
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- **Session notes:** See `llm/sessions/2025-11-09_gpu_mpi_validation.md` for development history
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## Contributing
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These demos are educational and meant to be:
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- **Clear:** Understand what's being demonstrated
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- **Minimal:** No unnecessary complexity
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- **Runnable:** Work on typical hardware (fall back to CPU if needed)
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- **Validated:** Compare against exact solutions
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When adding new demos, follow this pattern and document thoroughly.
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