r/LocalLLaMA benchmark compares Qwen3.5-27B Q4 quants using KLD and size tradeoffs

A community benchmark with practical intent

The r/LocalLLaMA post Qwen3.5-27B Q4 Quantization Comparison (2026-03-03 23:50:33 UTC) reached 198 points and 73 comments by crawl time. Instead of promoting one preferred file, the author ran a broad sweep of community GGUF Q4 variants and compared each one against a BF16 baseline using a consistent metric.

The key metric is KLD (KL Divergence), used here as a proxy for how closely a quantized model’s probability distribution tracks the original BF16 weights. Lower KLD implies better faithfulness. The post evaluates models on two datasets: a custom ChatML-formatted corpus (47 chunks at context 4096, mixed science/engineering/medicine/history/finance/culture/code content) and wikitext2 test text (72 chunks at context 4096).

Notable results from the post

• Best KLD (custom dataset): unsloth_Qwen3.5-27B-UD-Q4_K_XL at 16.411 GiB with KLD 0.005087.
• Strong alternatives: bartowski Q4_K_M and unsloth Q4_K_M variants follow closely.
• Best efficiency score: bartowski_Qwen3.5-27B-IQ4_XS at 14.130 GiB and KLD 0.007062.
• Hardware and runtime: i3-12100F, 64GB DDR4-3200, RTX 3060 12GB, llama.cpp mainline build 8189.

The useful takeaway is that the “closest to BF16” choice is not necessarily the “best practical deployment” choice. For local inference users, storage and VRAM constraints can make a slightly less faithful quantization the better overall option, especially when latency and fit-on-device are hard constraints.

How to interpret this safely

This is community-generated benchmarking, not an official vendor benchmark or peer-reviewed study. Results can vary with prompt style, dataset composition, quantizer implementation, and runtime version. Even so, the post provides decision-quality signal because it compares many popular files under one measurement approach and publishes concrete tables rather than anecdotes.

Commenters generally treated it as high-value analysis, and some extended the work with additional plots to inspect size-vs-KLD trends. That collaborative validation pattern is exactly why community technical forums remain important for local LLM operations.

Sources: Reddit post (r/LocalLLaMA).