sparseforge-hessian-mask

name	sparseforge-hessian-mask
description	Efficient semi-structured LLM sparsification via annealing of Hessian-mask guided pruning. Achieves high sparsity with minimal accuracy loss using second-order importance estimation.
category	deep-learning
tags	["LLM","sparsification","pruning","Hessian","compression","efficiency"]
trigger	sparseforge, hessian mask, semi-structured pruning, LLM sparsification, 2:4 sparsity, model compression

SparseForge: Hessian-Mask Guided Semi-Structured Pruning

Overview

SparseForge achieves efficient semi-structured (e.g., 2:4) sparsity in LLMs by using Hessian-based second-order importance estimation with an annealing schedule to guide pruning decisions.

Core Technique

1. Hessian-Driven Importance: Compute diagonal Hessian (or Fisher information) to estimate parameter sensitivity
2. Mask Annealing: Instead of one-shot pruning, gradually anneal a binary mask over training steps
3. Semi-Structured Constraints: Enforce N:M sparsity patterns (e.g., 2 out of every 4 weights = 0) for hardware efficiency
4. Recovery Fine-Tuning: Brief fine-tuning with the annealed mask to recover accuracy

Key Benefits

• Higher accuracy at same sparsity vs. magnitude-based pruning
• Hardware-friendly 2:4 sparsity patterns compatible with Tensor Cores
• Annealing prevents catastrophic accuracy drops from aggressive one-shot pruning

Implementation Steps

1. Compute per-parameter Hessian diagonal via Fisher approximation (gradient outer product)
2. Initialize soft mask with sigmoid parametrization
3. Anneal mask temperature over training: soft → hard binary decisions
4. Apply N:M structural constraints during mask hardening
5. Fine-tune with masked weights, optionally with distillation loss from dense model

Pitfalls

• Hessian computation is expensive — use diagonal/Fisher approximation for LLM-scale models
• Annealing schedule is critical: too fast → accuracy collapse; too slow → wasted compute
• N:M constraints require specialized kernel support or careful indexing for inference speedup

Activation Keywords

sparseforge, hessian mask, semi-structured pruning, LLM sparsification, 2:4 sparsity, model compression, Fisher information, mask annealing