Pruning LLMs Like a Physicist: Block Removal as an Ising Optimization Problem

來源原標題

Pruning LLMs Like a Physicist: Block Removal as an Ising Optimization Problem

機構

HuggingFace

原始連結

https://huggingface.co/blog/MultiverseComputingCAI/pruning-llms-like-a-physicist-block-removal-as-an

摘要原文

Back to Articles Pruning LLMs Like a Physicist: Block Removal as an Ising Optimization Problem Team Article Published September 21, 2026 Upvote 12 +6 Antonio Tiene AntonioTN Follow MultiverseComputingCAI Ali Hashemi ali-hashemi Follow MultiverseComputingCAI David Jansen Jansenhbar Follow MultiverseComputingCAI Roman Rausch spinflip Follow MultiverseComputingCAI One of the cheapest ways to make a large language model faster is also one of the bluntest: delete whole transformer blocks. Because the model literally gets shorter, block removal (also called depth pruning) buys predictable inference speedups on top of the memory savings, and it stacks cleanly with quantization, low-rank compression, and other techniques. The hard part is deciding which blocks to cut. Remove the wrong ones and the model collapses; and the effect of removing any one block depends on which others you remove alongside it, so the choices interact. That makes it a combinatorial problem, not a ranking problem, and combinatorial problems with interacting binary variables are exactly what the physics of spin systems was built to describe. Our latest paper, LLM Compression by Block Removal with Constrained Binary Optimization , takes that correspondence literally. We reformulate block selection as a constrained binary optimization (CBO) problem that maps directly onto an Ising glass , a disordered spin system with all-to-all interactions and a fixed number of "up" spins. The energy of that spin system turns out to be a strong, cheap proxy for how well the pruned model will actually score on benchmarks, which means we can rank a huge number of candidate configurations without benchmarking any of them, and hand the hard instances to the same classical and quantum-inspired solvers we use elsewhere at Multiverse. The payoff in the deep-compression regime is large: at 50% compression of Llama-3.3-70B-Instruct, we gain almost 23 percentage points on MMLU over the best competing block-removal method. Why picking blocks is a many-body problem Most existing block-removal methods score each block on its own, then remove the ones that look least important, using magnitude, sensitivity, or "block influence" heuristics. In physics terms these are mean-field methods: they treat each block as if its contribution were independent of the others, the way mean-field theory replaces a spin's neighbors with a single averaged field. A related shortcut is to only ever remove a single consecutive run of blocks, which keeps the problem small but throws away most of the search space. The trouble is that blocks are not independent, any more than spins in a real magnet are. Whether removing block 20 hurts the model depends on whether you also removed block 19 or block 24, an interaction, or coupling , between the two decisions. As models get deeper and more heterogeneous, ignoring those couplings leaves quality on the table, especially when you want to remove a lot of blocks at once. What you really want is to search over combinations of blocks while accounting for how they interact, but the number of combinations grows exponentially, so brute force looks hopeless. This is precisely the regime, exponentially large configuration spaces with pairwise couplings, where the tools of statistical physics earn their keep. The idea: turn block selection into an energy-minimization problem We attach a binary variable to each transformer block: 0 means keep it, 1 means remove it, just like a spin that can point down or up. Then we do a second-order Taylor expansion of the model's loss with respect to those variables, which produces an (approximate) Hessian matrix. The diagonal of that Hessian is how much each block matters on its own; the off-diagonal entries are exactly the pairwise couplings between blocks, the many-body physics that mean-field methods throw away. That reformulation turns "which blocks should I remove?" into a clean optimization: find the set of M blocks whose removal minimizes the energy xᵀH⁰x , subject to removing exactly M of the N blocks. Mathematically this is a constrained binary optimization problem; physically it is an Ising glass, an all-to-all coupled spin system with conserved magnetization (the fixed number of removed blocks plays the role of a fixed total spin). The key property we establish is that this energy is a strong proxy for downstream quality: low-energy states of the spin system correspond to high-performing pruned models. Minimizing energy and maximizing benchmark score become the same search. Block selection becomes a constrained binary optimization problem, equivalent to finding low-energy states of an Ising glass; each solution says which M of N blocks to delete. Right: the coupling variable α we insert into each block's residual path to build the Hessian. Source: paper Figure 1. The reason this is practical is cost. The Hessian, i.e. the full set of couplings, is computed just once, from forward and backward passes on a small calibration dataset. After that, evaluating any candidate configuration is a single cheap energy calculation, no need to run the actual model, let alone benchmark it. And because the couplings don't depend on the compression target, the same Hessian can be reused to solve for many different values of M . Solving it: exact when you can, quantum or quantum-inspired when you can't For most models the configuration space is large but still checkable. Because computing one energy is so cheap, we brute-force it on a single GPU, checking up to tens of billions of spin configurations. A few million take seconds; the hardest tractable case here, removing 8 of Llama-3.3-70B's 80 blocks (about 29 billion configurations), took roughly two days. Beyond that the exact approach breaks down, and this is where casting the problem as an Ising glass pays off a second time. In its equivalent QUBO form (the constraint absorbed into a penalty term), the exact same task can be handed to the highly optimized classical, quantum, and quantum-inspired solvers built for this class of Hamiltonian, the machinery of quantum annealing, QAOA, tabu search, and specialized branch-and-bound. We find that an open-source tabu solver reliably reaches the lowest-energy states in seconds , even on the hardest cases we can verify against brute force. So the method scales to models where enumerating configurations is out of the question, using solvers that are squarely in Multiverse's domain. There's a subtle but important point here, and it runs against the usual grain of optimization. Normally a CBO or annealing solver is judged by whether it finds the true ground state. We don't actually need the ground state. What we need is a fast way to generate a handful of good low-energy states, and that is a far easier bar, which is why lightweight solvers work so well for us and why we can afford to run several of them. Why the whole low-energy spectrum matters The energy is a strong proxy for quality, but not a perfect one, so the single lowest-energy state isn't always the best model. This turns out to be a feature, not a bug: once the Hamiltonian is set up, reading off the ground state and the low-lying excited states is essentially free, giving a spectrum of high-quality candidate prunings to try rather than one fragile answer. Exploring excited states, not just the ground state, is itself an area of active physics research, and it maps neatly onto what practitioners actually need here. A concrete example: for Llama-3.1-8B-Instruct at 16/32 blocks removed, most of the top states cut blocks toward the end of the model, as prior work would expect. But the 17th excited state is the first to propose removing a block near the beginning of the model, and after light retraining that configuration outperforms the ground state across several benchmarks.