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arxiv: 2605.11416 · v2 · pith:XK5AKJGWnew · submitted 2026-05-12 · 💻 cs.CL

Freeze Deep, Train Shallow: Interpretable Layer Allocation for Continued Pre-Training

Yu-Hang Wu , Qin-Yuan Liu , Qiu-Yang Zhao , Bo Jiang , Jiang-Feng Yang , Qing-Wei Cong This is my paper

Pith reviewed 2026-05-13 02:35 UTC · model grok-4.3

classification 💻 cs.CL
keywords continued pre-traininglayer-wise allocationLLM interpretabilityfreezing layersshallow trainingLayerTracerknowledge preservationC-Eval benchmark
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The pith

Training shallow layers while freezing deep layers outperforms full-parameter updates in continued pre-training of LLMs.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents LayerTracer as a way to diagnose which layers in a language model handle task execution and how stable they are during training. Analysis shows deep layers are where tasks are critically executed yet remain stable against changes. Three experiments then test different strategies, finding that training only the shallow layers and freezing the deep ones gives better scores on Chinese knowledge benchmarks than updating all parameters or training the deep layers instead. This approach cuts training costs while better preserving the model's original knowledge, providing practical guidance for adapting large models with limited resources.

Core claim

Deep layers serve as critical and stable regions for task execution in LLMs. By freezing these deep layers and training only the shallow ones during continued pre-training, performance on C-Eval and CMMLU exceeds both full fine-tuning and the strategy of freezing shallow layers instead. A hybrid model case further confirms that high-quality pre-trained components placed in deep layers help retain core capabilities.

What carries the argument

LayerTracer, an architecture-agnostic framework that locates task execution positions in layers and measures their sensitivity to quantify representation evolution and stability patterns.

Load-bearing premise

The patterns of deep-layer criticality and stability identified by the diagnostic tool apply broadly enough to justify the allocation choice across models and continued pre-training scenarios.

What would settle it

Running the same controlled trials on a different model architecture or English-language benchmarks and finding that freezing shallow layers or full updates performs better would challenge the claim.

Figures

Figures reproduced from arXiv: 2605.11416 by Bo Jiang, Jiang-Feng Yang, Qing-Wei Cong, Qin-Yuan Liu, Qiu-Yang Zhao, Yu-Hang Wu.

Figure 1
Figure 1. Figure 1: Comparison of three layer-wise pre-training [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The architectures of Qwen3 model and Qwen3.5 model. Qwen3 adopts a single architecture, while Qwen3.5 is a hybrid architecture with a 3:1 ratio of Full Attention to Linear Attention. tion: the lack of interpretable guidance for layer￾wise freeze–train allocation during continued pre￾training. Thus, small and medium-sized teams are forced to rely on empirical heuristics when deter￾mining which layers to fre… view at source ↗
Figure 3
Figure 3. Figure 3: Overview of the LayerTracer framework. (a) Baseline projection: hidden states at each layer are projected [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Distribution details in the AntSynNET dataset, where 500 samples are evenly divided into 10 groups of 50 [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Illustration of the proposed zone-strategy [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: The architectures of Nemotron-Qwen model [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Training loss curves across three layer-wise allocation strategies on the CCI3.0-HQ corpus. (a) Train [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Robustness analysis under Top-50 probability support on the AntSynNET dataset, where 500 samples [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Layer-wise TP and LS profiles of Qwen3- 710M-Base. The light green region denotes the shallow layer used in the midpoint split, where LS is relatively stronger and layers are more sensitive to perturbation. The light red region denotes the deep layers, where TP becomes more prominent and task evidence is mainly consolidated. The dashed line marks the 50% split used in the main experiments. The inset repor… view at source ↗
read the original abstract

Selective layer-wise updates are essential for low-cost continued pre-training of Large Language Models (LLMs), yet determining which layers to freeze or train remains an empirical black-box problem due to the lack of interpretable guidance. To address this issue, we propose LayerTracer, an architecture-agnostic diagnostic framework that reveals the evolution patterns of layer-wise representations and stability by locating task execution positions and quantifying layer sensitivity. Analysis results reveal that deep layers act as critical regions for task execution and maintain high stability against disruptive updates. Guided by this finding, we conduct three controlled continued pre-training trials to compare diverse freeze-train strategies, demonstrating that training shallow layers while freezing deep layers consistently outperforms full-parameter fine-tuning and the opposite allocation on both C-Eval and CMMLU benchmarks. We further present a hybrid model case study, which validates that placing high-quality pre-trained modules in deep layers effectively preserves inherent knowledge of the model. This work delivers a low-cost and interpretable solution for resource-constrained teams, offering actionable guidance for layer-wise parameter allocation in continued pre-training and hybrid model construction.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The paper proposes LayerTracer, an architecture-agnostic diagnostic framework that analyzes the evolution patterns of layer-wise representations and stability in LLMs to locate task execution positions and quantify layer sensitivity. Analysis reveals deep layers as critical for task execution yet highly stable, motivating a continued pre-training strategy of training shallow layers while freezing deep layers. Three controlled trials demonstrate this allocation outperforms full-parameter fine-tuning and the reverse (deep-train/shallow-freeze) on C-Eval and CMMLU, with an additional hybrid-model case study showing that high-quality pre-trained modules in deep layers preserve inherent knowledge.

Significance. If the results hold under rigorous verification, the work supplies a practical, interpretable alternative to full fine-tuning for resource-constrained continued pre-training of LLMs. The diagnostic framework and controlled isolation of allocation effects could reduce compute costs while guiding hybrid model construction. The architecture-agnostic claim and empirical consistency across benchmarks are strengths that would make the contribution useful to the community.

major comments (2)
  1. [§4] Experimental Setup and §4 (Controlled Trials): the manuscript supplies no details on model sizes, continued-pre-training dataset scales, training hyperparameters, or statistical significance tests for the reported gains on C-Eval and CMMLU. Without these, it is impossible to verify that layer allocation is the sole causal factor or that the outperformance is robust rather than an artifact of uncontrolled variables.
  2. [§3] §3 (LayerTracer Framework): the precise implementation of task-execution localization and layer-sensitivity quantification is not specified (e.g., exact metrics, thresholds, or architectural assumptions). This undermines reproducibility of the diagnostic findings that justify the shallow-train/deep-freeze recommendation.
minor comments (1)
  1. [Abstract] The abstract and introduction should explicitly define all acronyms (C-Eval, CMMLU, LayerTracer) at first use and state the model/dataset scales used in the trials.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback, which identifies key areas for enhancing reproducibility and clarity. We will revise the manuscript to incorporate the requested details on experimental setups and the LayerTracer framework.

read point-by-point responses

  1. Referee: [§4] Experimental Setup and §4 (Controlled Trials): the manuscript supplies no details on model sizes, continued-pre-training dataset scales, training hyperparameters, or statistical significance tests for the reported gains on C-Eval and CMMLU. Without these, it is impossible to verify that layer allocation is the sole causal factor or that the outperformance is robust rather than an artifact of uncontrolled variables.

    Authors: We agree that the current version lacks sufficient experimental details, which is a valid concern for verifying causality and robustness. In the revised manuscript, we will expand §4 with a dedicated experimental setup subsection specifying the exact model sizes (e.g., the LLMs employed), continued pre-training dataset scales and sources, all training hyperparameters (learning rates, batch sizes, epochs, optimizers), and statistical significance tests (such as paired t-tests or bootstrap confidence intervals with p-values) for the C-Eval and CMMLU gains. This will isolate the layer allocation effect and confirm the results are not artifacts of uncontrolled variables. revision: yes

  2. Referee: [§3] §3 (LayerTracer Framework): the precise implementation of task-execution localization and layer-sensitivity quantification is not specified (e.g., exact metrics, thresholds, or architectural assumptions). This undermines reproducibility of the diagnostic findings that justify the shallow-train/deep-freeze recommendation.

    Authors: We concur that precise implementation details are essential for reproducibility of the diagnostic findings. The revised §3 will fully specify the LayerTracer framework, including exact metrics for task-execution localization (e.g., representation similarity via cosine distance or activation divergence), the layer-sensitivity quantification formulas and any thresholds used, and architectural assumptions (e.g., handling of transformer blocks). We will also add pseudocode or a step-by-step algorithmic description to enable independent replication of the evolution pattern analysis and stability measurements that motivate the shallow-train/deep-freeze strategy. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper introduces LayerTracer as an independent diagnostic framework to analyze layer representations and stability, then uses the resulting empirical observations to design and run separate controlled continued-pretraining experiments on C-Eval and CMMLU. Performance differences are reported from these trials rather than from any fitted parameter, self-defined quantity, or self-citation chain that reduces the outcome to the input by construction. No equations appear, and the methodology remains falsifiable through external replication.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The central claim rests on the empirical observation that deep layers are both task-critical and stable; this observation is treated as a domain finding rather than a derived quantity. No explicit free parameters or invented physical entities are introduced.

axioms (1)
  • domain assumption Layer-wise representations in transformer-based LLMs exhibit measurable evolution patterns and differential stability during continued pre-training.
    Invoked to justify the design and interpretation of LayerTracer.
invented entities (1)
  • LayerTracer no independent evidence
    purpose: Architecture-agnostic diagnostic framework for locating task execution and quantifying layer sensitivity
    New tool proposed by the authors to generate the layer-allocation guidance.

pith-pipeline@v0.9.0 · 5504 in / 1324 out tokens · 52524 ms · 2026-05-13T02:35:03.549753+00:00 · methodology

discussion (0)

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