From 4b07e1189eff20e9841bb0676c40c76d7347492f Mon Sep 17 00:00:00 2001 From: Jukka Aho Date: Tue, 18 Nov 2025 18:02:30 +0200 Subject: [PATCH] refactor(assemblers): Add nodal assembler placeholder - Implement NodalAssembler placeholder for future GPU implementation - Add create_cache() stub for NodalCache creation - Add assemble!() stub with planned algorithm documentation - Add compute_node_contributions!() stub for node-level assembly - Document GPU parallelization strategy (one thread per node) - 178 lines of placeholder and documentation Planned GPU algorithm: 1. Launch one thread per node 2. Each thread gets touching elements for its node 3. Compute contributions from all touching elements 4. Atomic add to global K, f (thread-safe on GPU) Expected performance: - 2-10x speedup on GPU for large problems (> 100k nodes) - Better cache locality for nodal DOFs - Natural parallelization pattern Status: - Not yet implemented - Raises error directing users to COO/CSC assemblers - Will require CUDA.jl or similar GPU framework --- src/assemblers/nodal_based.jl | 177 ++++++++++++++++++++++++++++++++++ 1 file changed, 177 insertions(+) create mode 100644 src/assemblers/nodal_based.jl diff --git a/src/assemblers/nodal_based.jl b/src/assemblers/nodal_based.jl new file mode 100644 index 0000000..87800ea --- /dev/null +++ b/src/assemblers/nodal_based.jl @@ -0,0 +1,177 @@ +# This file is a part of JuliaFEM. +# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md + +""" +Nodal-based assembly (future implementation). + +Node-by-node assembly using inverse connectivity (node-to-elements map). +Natural for GPU parallelization (one thread per node). + +**Performance**: Expected 2-10x speedup on GPU for large problems (> 100k nodes). +**Best for**: GPU acceleration, very large problems. +**Status**: Planned for future implementation. +""" + +using SparseArrays + +""" + assemble!( + cache::NodalCache, + assembler::NodalAssembler, + kernel::AbstractKernel, + mesh::AbstractMesh + ) -> Nothing + +Assemble global system using nodal traversal **in-place, zero allocations**. + +**Status**: Not yet implemented. Placeholder for future GPU-based assembly. + +# Algorithm (Planned) + +1. Reset cache +2. Loop over nodes (parallelizable on GPU): + a. Get all elements touching this node + b. For each touching element: + - Compute full element stiffness (or use cached value) + - Extract only rows/columns for this node + - Accumulate to global system (atomic add on GPU) + +# GPU Parallelization + +Each node processed by one GPU thread: +```cuda +__global__ void assemble_nodal(nodes, elements, K, f) { + int node_id = blockIdx.x * blockDim.x + threadIdx.x; + if (node_id >= nnodes) return; + + // Get touching elements + for (elem in touching_elements[node_id]) { + // Compute element contribution for this node + // Atomic add to K, f + } +} +``` + +# Arguments +- `cache`: Pre-allocated nodal cache +- `assembler`: Nodal assembler +- `kernel`: Domain kernel +- `mesh`: Finite element mesh + +# Zero-Allocation Guarantee + +All arrays pre-allocated. Atomic operations on GPU ensure thread-safety. +""" +function assemble!( + cache::NodalCache, + assembler::NodalAssembler, + kernel::AbstractKernel, + mesh::AbstractMesh +) + error("NodalAssembler not yet implemented. Use COOAssembler or CSCAssembler.") + + # Planned implementation: + # 1. Reset cache + # 2. Loop over nodes + # 3. For each node, get touching elements from cache.node_to_elements + # 4. Accumulate contributions from all touching elements + # 5. Write to global K, f (atomic on GPU) + + return nothing +end + +# ============================================================================ +# HELPER FUNCTIONS (for future implementation) +# ============================================================================ + +""" + create_cache(assembler::NodalAssembler, mesh::AbstractMesh, kernel::AbstractKernel) -> NodalCache + +Create pre-allocated cache for nodal assembly. + +Builds node-to-elements map (inverse connectivity) for efficient nodal traversal. + +# Arguments +- `assembler`: Nodal assembler +- `mesh`: Finite element mesh +- `kernel`: Domain kernel + +# Returns +- Pre-allocated nodal cache with inverse connectivity +""" +function create_cache(assembler::NodalAssembler, mesh::AbstractMesh, kernel::AbstractKernel) + return NodalCache(mesh, kernel) +end + +""" + compute_node_contributions!( + node_cache::NodeCache, + element_cache::ElementCache, + node_id::Int, + kernel::AbstractKernel, + mesh::AbstractMesh, + node_to_elements::NodeToElementsMap + ) -> Nothing + +Compute contributions to this node from all touching elements **in-place**. + +# Algorithm + +1. Get all elements touching this node +2. For each element: + a. Compute full element stiffness (using `compute_element_stiffness!`) + b. Find local node index within element + c. Extract rows/columns corresponding to this node's DOFs + d. Accumulate to node contributions + +# Arguments +- `node_cache`: Node workspace (output) +- `element_cache`: Element workspace (for kernel calls) +- `node_id`: Node index +- `kernel`: Domain kernel +- `mesh`: Finite element mesh +- `node_to_elements`: Inverse connectivity + +# Returns + +Nothing - writes to `node_cache` in-place. +""" +function compute_node_contributions!( + node_cache::NodeCache, + element_cache::ElementCache, + node_id::Int, + kernel::AbstractKernel, + mesh::AbstractMesh, + node_to_elements::NodeToElementsMap +) + error("compute_node_contributions! not yet implemented") + + # Planned implementation: + # 1. Get touching elements: get_node_spider(node_to_elements, node_id) + # 2. For each element: + # - Compute element stiffness: compute_element_stiffness!(element_cache, ...) + # - Find local node index in element + # - Extract node DOF rows/columns from Ke, fe + # - Accumulate to node_cache + # 3. Return node_cache (contains all contributions for this node) + + return nothing +end + +# ============================================================================ +# GPU KERNEL STUBS (for future CUDA implementation) +# ============================================================================ + +# """ +# assemble_nodal_gpu!(K, f, mesh, kernel, node_to_elements) +# +# GPU kernel for nodal assembly. +# +# Launches one thread per node. Each thread: +# 1. Gets touching elements for its node +# 2. Computes contributions from all elements +# 3. Atomically adds to global K, f +# +# Requires CUDA.jl or similar GPU framework. +# """ +# function assemble_nodal_gpu! end