Text Generation
GGUF
English
quant
target_bpw
experimental
conversational

Experimental global target bits‑per‑weight quantization of ibm-granite/granite-4.1-8b

Using non-standard (forked) LLaMA C++ release b9358 for quantization.

Original model: ibm-granite/granite-4.1-8b

From the original model creators:

mof-class3-qualified

Granite-4.1-8B

Model Summary: Granite-4.1-8B is a 8B parameter long-context instruct model finetuned from Granite-4.1-8B-Base using a combination of open source instruction datasets with permissive license and internally collected synthetic datasets. Granite 4.1 models have gone through an improved post-training pipeline, including supervised finetuning and reinforcement learning alignment, resulting in enhanced tool calling, instruction following, and chat capabilities.

Supported Languages: English, German, Spanish, French, Japanese, Portuguese, Arabic, Czech, Italian, Korean, Dutch, and Chinese. Users may finetune Granite 4.1 models for languages beyond these languages.

Intended use: The model is designed to follow general instructions and can serve as the foundation for AI assistants across diverse domains, including business applications, as well as for LLM agents equipped with tool-use capabilities.


⚠️ PLEASE READ THIS BEFORE USING THESE EXPERIMENTAL VERSIONS! ⚠️

An area of personal interest is finding ways to optimize the inference performance of LLMs when deployed in resource-constrained environments like commodity hardware, desktops, laptops, mobiles, edge devices, etc. There are many approaches to accomplish this, including architecture simplification and knowledge distillation, but my focus has been primarily on quantization and pruning.

The method to produce these experimental versions involves using a custom version of llama-imatrix to generate an imatrix that includes tensor statistics, and a custom version of llama-quantize, which computes a per-tensor quantization error, to automatically select the lowest error quantization recipe that achieves a global target bits‑per‑weight (bpw). More details on the implementation and test results here

There are two pull requests (#14891 & #15550) to merge these changes back into the core llama.cpp project. This may or may not ever happen so, until then, the modified versions will be available on GitHub.

For testing and comparison, I use models produced by Bartowski (see credits below) and Unsloth (Daniel and Michael Han do some really interesting stuff!) but when they don't provide versions of the required model, tests and comparisons are against standard quantization obtained by simply running llama-quantize with no further optimizations.

All experimental versions were generated using an appropriate imatrix created from datasets available at eaddario/imatrix-calibration. In llama.cpp, an imatrix is a calibration file derived from running representative text through the model and collecting activation statistics. It is used to weight quantization error so that error in more “important” directions (as estimated from activations) is penalized more heavily.

The process to generate these models is roughly as follows:

  1. Convert the original model's safetensors to GGUF F16*
  2. Estimate the Perplexity score for the F16 model (baseline) using the wikitext-2-raw-v1 dataset, and save the logits
  3. Generate an imatrix from the most appropriate calibration dataset
  4. Quantize the baseline model targeting a bpw average (e.g. llama-quantize --target-bpw 4.5678 --state-file --imatrix imatrix.gguf baseline-model-F16.gguf 12)
  5. Calculate Perplexity, KL Divergence, ARC (Easy+Challenge), GPQA-Diamond, HellaSwag, MMLU-Redux, Truthful QA and WinoGrande scores for each quantized model
  6. Keep version with the best 𝜌PPL and μKLD scores
  7. Repeat until all desired quants are created

*BF16 would be preferred, but F16 performs better on Apple's GPUs

Advantages and disadvantages of the global target bits‑per‑weight quantization process

Advantages

  1. Target arbitrary size models

    • When specifying --target-bpw 4.5678 for instance, the algorithm will produce a model (nearly) exactly of that size, which is very useful for maximizing VRAM usage. In a system with 24GB VRAM and a 70B model, standard quants might produce a 16.8GB file (too small, quality left on table) or a 24.1GB file (won't fit). This approach can generate a 23.85GB file to utilize the hardware fully.
  2. Data-driven mixed precision often can improve quality at fixed size

    • Instead of using hardcoded heuristics (e.g. make attn_v Q5_K for a 70B model), that may be sub‑optimal for a given architecture or size, the quantization mix is determined by the actual error sensitivity of the specific model's weights. This, in practice, often yields a better quality/size trade-off, especially in aggressive quantization scenarios (1.5 to 3.5 bpw), or for unusual architectures.

    • Please note: llama.cpp’s heuristics have been tuned across many models and are highly optimized; although the target bpw method produces better quality often (>75% based on tests with 130 models from 11 different families), it can also lose in surprising cases.

  3. Allows better like-for-like comparisons between models and families

    • Standard llama.cpp quantization uses hardcoded rules like: "use Q4_K_M, except bump some tensors up/down, except fall back if incompatible, except keep some tensors unquantized..." and for that reason, two different models quantized with the same Q4_K_M type can end up with very different bpw (e.g. 4.75 and 4.30).

    • All things being equal, the performance of a model is usually proportional to its overall bpw size; models with a higher bpw tend to perform better than lower bpw models. Since model A has simply been given more bits, it will typically perform better (lower perplexity, better eval scores, etc.) even if the underlying quantization method is identical. That makes comparing the performance not a controlled experiment, because the comparison is between models with different effective compression ratios.

    • --target-bpw tries to address that by making the experiment more controlled: each model gets quantized to land on (approximately) the same global byte budget, so that the models' performance differences are more attributable to architecture/training differences, quantization error behaviour at the same compression ratio, optimizer’s allocation decisions, etc.

Disadvantages

  1. Quantization process is significantly slower than standard

    • This approach can take 5x-10x longer as it quantizes a sample of most tensors into 15 different formats, dequantizes them back to floats, computes error diffs, and selects the best size/error option that fits the global bpw budget.

    • However, the --state-file option will save/use the above-mentioned computations so that future quantizations, for the same model, can be generated at normal speed. It also allows to interrupt the computation process and resume it at a later time.

  2. The optimization target is only a proxy for the model's performance quality

    • The process minimizes a per-tensor estimated error computed from sampled rows, not actual perplexity or divergence of output distributions (a future version may address this). Since errors interact nonlinearly across layers, there are no guarantees it will select the best possible quantization recipe subject to the bpw size constraint.
  3. An imatrix with activations data is required for best results

    • Activation data is required to compute the bias factor (i.e. the systematic error projected onto activation directions). If the imatrix file does not contain activation data, the --target-bpw option will refuse to run.

Models

Bits per weight, size, perplexity and KL Divergence scores

Model BPW Size (GB) μPPL 𝜌PPL μKLD Same Top-P
granite-4.1-8b-F16 16.0006 17.6 8.691178 ±0.065443 100% N/A N/A
granite-4.1-8b-Q2_K 1.7500 1.93 87.318832 ±0.781580 57.61% 2.889523 ±0.005948 34.309 ±0.125
granite-4.1-8b-Q2_K 2.5000 2.75 12.534216 ±0.095606 86.12% 0.644965 ±0.002755 67.231 ±0.124
granite-4.1-8b-Q3_K 3.5000 3.85 9.381594 ±0.070128 96.18% 0.173887 ±0.001079 82.732 ±0.100
granite-4.1-8b-Q4_K 4.4999 4.95 8.867438 ±0.067303 98.88% 0.047917 ±0.000392 90.937 ±0.076
granite-4.1-8b-Q5_K 5.4999 6.05 8.766150 ±0.066421 99.48% 0.018940 ±0.000165 94.120 ±0.062
granite-4.1-8b-Q6_K 6.4998 7.15 8.755199 ±0.066400 99.74% 0.007326 ±0.000066 96.165 ±0.051
granite-4.1-8b-Q7_K 7.4998 8.25 8.751241 ±0.066500 99.82% 0.003568 ±0.000040 97.235 ±0.043
granite-4.1-8b-Q8_0 8.4988 9.34 8.749119 ±0.066517 99.85% 0.002052 ±0.000024 97.749 ±0.039

ARC, GPQA-Diamond, HellaSwag, MMLU-Redux, Truthful QA, and WinoGrande scores

Scores generated using llama-perplexity with 750 tasks per test, and a context size of 1024 tokens.

For the test data used in the generation of these scores, follow the appropriate links: ARC Challenge, Truthful QA, GPQA-Diamond, HellaSwag, MMLU-Redux, WinoGrande

Model ARC Challenge GPQA-Diamond HellaSwag MMLU-Redox Truthful QA WinoGrande Avg Score
granite-4.1-8b-Q1_L 36.5333 ±1.7594 19.1919 ±2.8058 36.00 27.2000 ±1.6260 28.9333 ±1.6569 52.5333 ±1.8246 33.40
granite-4.1-8b-Q2_K 60.4000 ±1.7870 29.7980 ±3.2586 70.00 59.2000 ±1.7958 33.4667 ±1.7242 65.2000 ±1.7405 53.01
granite-4.1-8b-Q3_K 62.0000 ±1.7736 21.7172 ±2.9377 79.33 69.2000 ±1.6869 39.6000 ±1.7870 71.7333 ±1.6453 57.26
granite-4.1-8b-Q4_K 66.9333 ±1.7190 23.2323 ±3.0089 79.73 71.4667 ±1.6500 38.9333 ±1.7816 73.4667 ±1.6132 58.96
granite-4.1-8b-Q5_K 66.4000 ±1.7259 22.7273 ±2.9858 79.87 72.1333 ±1.6382 38.5333 ±1.7783 73.4667 ±1.6132 58.86
granite-4.1-8b-Q6_K 67.0667 ±1.7172 24.7475 ±3.0746 80.13 72.6667 ±1.6284 38.2667 ±1.7759 73.7333 ±1.6080 59.44
granite-4.1-8b-Q7_K 66.4000 ±1.7259 26.7677 ±3.1544 80.27 72.1333 ±1.6382 38.5333 ±1.7783 73.6000 ±1.6106 59.62
granite-4.1-8b-Q8_0 66.8000 ±1.7207 26.7677 ±3.1544 80.53 72.4000 ±1.6334 38.4000 ±1.7771 73.2000 ±1.6184 59.68

Tokens per second benchmarks

Scores generated using llama-bench. Standard (llama-quantize with no optimization) Q4_K_M quantization included for comparison.

model size params backend threads test t/s
granite-4.1-8b-Q1_L 1.79 GiB 8.79 B BLAS,MTL 12 pp512 783.11 ±0.52
granite-4.1-8b-Q1_L 1.79 GiB 8.79 B BLAS,MTL 12 tg128 68.68 ±0.17
granite-4.1-8b-Q1_L 1.79 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 108.35 ±1.28
granite-4.1-8b-Q2_K 2.56 GiB 8.79 B BLAS,MTL 12 pp512 728.97 ±10.22
granite-4.1-8b-Q2_K 2.56 GiB 8.79 B BLAS,MTL 12 tg128 68.76 ±0.21
granite-4.1-8b-Q2_K 2.56 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 108.98 ±0.24
granite-4.1-8b-Q3_K 3.58 GiB 8.79 B BLAS,MTL 12 pp512 733.45 ±9.51
granite-4.1-8b-Q3_K 3.58 GiB 8.79 B BLAS,MTL 12 tg128 63.63 ±1.20
granite-4.1-8b-Q3_K 3.58 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 94.51 ±1.15
granite-4.1-8b-Q4_K 4.61 GiB 8.79 B BLAS,MTL 12 pp512 771.63 ±0.97
granite-4.1-8b-Q4_K 4.61 GiB 8.79 B BLAS,MTL 12 tg128 66.33 ±1.24
granite-4.1-8b-Q4_K 4.61 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 105.98 ±4.76
granite-4.1-8b-Q5_K 5.63 GiB 8.79 B BLAS,MTL 12 pp512 673.26 ±34.19
granite-4.1-8b-Q5_K 5.63 GiB 8.79 B BLAS,MTL 12 tg128 51.29 ±3.09
granite-4.1-8b-Q5_K 5.63 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 83.45 ±2.31
granite-4.1-8b-Q6_K 6.65 GiB 8.79 B BLAS,MTL 12 pp512 703.41 ±23.92
granite-4.1-8b-Q6_K 6.65 GiB 8.79 B BLAS,MTL 12 tg128 52.12 ±1.38
granite-4.1-8b-Q6_K 6.65 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 87.04 ±0.22
granite-4.1-8b-Q7_K 7.68 GiB 8.79 B BLAS,MTL 12 pp512 614.53 ±0.48
granite-4.1-8b-Q7_K 7.68 GiB 8.79 B BLAS,MTL 12 tg128 49.47 ±0.59
granite-4.1-8b-Q7_K 7.68 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 83.45 ±0.24
granite-4.1-8b-Q8_0 8.70 GiB 8.79 B BLAS,MTL 12 pp512 800.32 ±0.73
granite-4.1-8b-Q8_0 8.70 GiB 8.79 B BLAS,MTL 12 tg128 46.66 ±0.04
granite-4.1-8b-Q8_0 8.70 GiB 8.79 B BLAS,MTL 12 pp1024+tg1024 77.87 ±0.30

Metrics used

Perplexity: one of the key metrics used in NLP evaluation. It measures the quality of a language model by evaluating how well it predicts the next token given a particular sequence of words. A PPL of 1 indicates an exact match between predicted and actual, whereas values greater than one indicate a degree of "surprise" the generated token differs from the expected.

Kullback–Leibler (KL) Divergence: a statistical measure of how much a probability distribution differs from another. When quantizing models (or altering the original tensors in any way for that matter), the closest we can preserve the weights' probability distribution to the original model the better, thus the closest to 0 the better.

AI2 Reasoning Challenge (ARC): a benchmark to evaluate the ability of AI models to answer complex science questions that require logical reasoning beyond pattern matching.

GPQA-Diamond: a challenging dataset of 448 multiple-choice questions written by domain experts in biology, physics, and chemistry.

HellaSwag: the Harder Endings, Longer contexts, and Low-shot Activities for Situations With Adversarial Generations (bit of a mouthful!) is a benchmark designed to test commonsense natural language inference. It requires the model to predict the most likely ending of a sentence.

MMLU: the Massive Multitask Language Understanding evaluates LLMs’ general knowledge and problem-solving abilities across 57 subjects, including elementary mathematics, US history, computer science, and law.

Truthful QA: evaluates how well LLMs generate truthful responses to questions. It identifies whether AI models can avoid generating false or misleading information, particularly in areas where human knowledge is prone to misconceptions.

Winogrande: based on the Winograd Schema Challenge, is a natural language understanding task requiring models to resolve ambiguities in sentences involving pronoun references.

Credits

LLaMa C++ has a large and vibrant community of contributors (~1,600 last time I checked) that actively maintain and extend its functionality, adding new models and architectures almost as fast as they appear. Considering the breakneck speed at which the AI/ML field is advancing, this alone is a remarkable feat!

While I'm grateful to all contributors, I want to recognise three in particular:

  • Colin Kealty (Bartowski), for the many contributions and for being one of the best sources of high quality quantized models available on Hugging Face
  • Georgi Gerganov for his amazing work with llama.cpp and the ggml/gguf libraries
  • Iwan Kawrakow for being one of the key authors behind the many quantization algorithms and the imatrix functionality.
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