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HyQuant is a research method that keeps selected attention positions and recent context in full precision while storing or computing most other attention states in low-bit formats. Its authors report decode-kernel speedups of up to 3.58× over FlashAttention-2 on tested context lengths, but end-to-end decode gains were smaller—1.04× to 1.17×—and benchmark accuracy stayed close to the full-precision baseline in the reported tests. These are results on a bounded set of models, tasks, and hardware, not a general production guarantee.

How HyQuant allocates precision

HyQuant is built around the observation that attention is not distributed evenly across key positions: some positions attract attention persistently across queries. The method identifies these important “vertical-line” positions, keeps them in full precision, and also protects a recent local window. Other context positions are handled in lower precision.

The authors’ analysis of Llama-3.1-8B and Qwen3-8B found that the top 5% of key positions together with a 128-token local window covered 85.63% and 82.53% of attention mass, respectively. Those measurements describe these two models and the authors’ analysis; they should not be assumed to hold for other models.

During prefill

For prefill, HyQuant computes selected vertical-line positions and the local sliding window in full precision, while computing the remaining context in low precision.

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During decode

For decode, most keys and values in the KV cache are stored in 4-bit formats, while selected positions are retained in full precision. HyQuant fuses dequantization with attention rather than first expanding the entire cache into a full-precision copy. The paper’s tested implementation uses Key-4bit and Value-4bit for the remaining KV positions.

What the reported accuracy evidence shows

The paper evaluates Qwen3-8B, Qwen3-32B, Llama-3.1-8B-Instruct, and GLM-4-9B-0414. It reports LongBench v1 results for long-context tasks, plus GSM8K and MATH500 results for mathematical reasoning.

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On the Qwen3-8B thinking-mode LongBench v1 table, HyQuant’s reported average across 11 tasks is 45.04, versus 44.59 for the full-precision FlashAttention-2 baseline. On Llama-3.1-8B-Instruct, the reported averages are 46.73 and 46.63, respectively. These small differences are measured benchmark outcomes, not evidence that quantization makes the underlying model more capable; the authors characterize small gains of this kind as normal evaluation variance.

Operator error is not the same as task accuracy

In a separate Qwen3-8B analysis, the authors compare intermediate attention-output mean squared error with full-precision FlashAttention. Keeping the top 1% or 5% of high-score positions in full precision and quantizing the rest to 4-bit brought measured error toward the uniform 8-bit error level across tested sequence lengths from 1K to 32K. This is evidence about attention-operator error, not a guarantee of downstream task scores.

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Speedups: kernel results versus end-to-end decoding

On an NVIDIA H100, the authors report the following decode speedups relative to FlashAttention-2. Kernel-level improvements grow with prefix length, while end-to-end gains remain much smaller:

Prefix length Decode-kernel speedup End-to-end decode speedup
1,024 tokens 1.32× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)
2,048 tokens 2.40× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)
4,096 tokens 3.06× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)
8,192 tokens 3.36× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)
16,384 tokens 3.52× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)
32,768 tokens 3.58× 1.04× to 1.17× across tested prefix lengths; exact per-length value not stated in the cited summary (HyQuant authors, 2026)

The kernel figures isolate the decode kernel; end-to-end decoding includes more of the inference path, so its improvement is the more relevant number for estimating overall serving impact. The reported range does not establish how a particular serving stack will perform.

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Memory and runtime trade-offs

  • Position-selection overhead: The authors attribute 3%–5% of total runtime to identifying vertical-line positions.
  • Extra cache for retained precision: At the reported 5% retention setting, keeping vertical-line tokens in full precision increases non-window KV-cache size by about 15% versus strict 4-bit quantization. The total cache cost also depends on local-window size.
  • Accuracy versus memory: Increasing the retained-token ratio generally lowers quantization error and improves accuracy in the authors’ tests, but increases the high-precision memory budget. A larger full-precision window also slightly improved accuracy in the reported ablation.
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How far the findings generalize

The reported evidence covers the four named models, the listed benchmark families, the tested context lengths, and experiments on an NVIDIA H100. It does not establish independent replication, compatibility across serving systems, or performance for every model architecture and workload. A like-for-like evaluation should keep the model, context length, low-bit format, retained-position ratio, local-window size, hardware, and latency measurement boundary consistent; kernel latency alone is not an end-to-end deployment comparison.

The paper’s current arXiv record lists an initial submission on 28 August 2026, version 3 revised on 16 September 2026, and the comment “EMNLP 2026 Main.” The record links to the authors’ implementation at arXiv:2608.22153 and the HyQuant GitHub repository.

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