arXiv Computation and Language

FlashPrefill V2: Block-Sparse Prefill Attention for Long-Context LLM Serving

arXiv:2608. 19758v1 Announce Type: new Abstract: Long-context modeling is a pivotal capability for Large Language Models, yet the quadratic complexity of attention remains a critical bottleneck, particularly during the compute-intensive prefilling phase.

arXiv Computation and Language
Sep 28

UniPrefill: Universal Long-Context Prefill Acceleration via Block-wise Dynamic Sparsification

UniPrefill is a prefill‑acceleration framework that works with virtually any model architecture, directly speeding up token‑level computation. It is implemented as a continuous‑batching operator and extends vLLM’s scheduling to support prefill‑decode co‑processing and tensor parallelism. The method delivers up to a 2.1× speedup in Time‑to‑First‑Token, with gains growing as concurrent requests increase.

By Qihang Fan, Huaibo Huang, Zhiying Wu, Bingning Wang, Ran He
arXiv Machine Learning
Sep 30

Block Sparse Flash Attention

Block Sparse Flash Attention (BSFA) is a drop‑in replacement for FlashAttention that speeds up long‑context inference by pruning about 50% of computation and memory transfers. It selects the top‑k most important value blocks for each query using exact query‑key similarities and calibrated per‑layer, per‑head thresholds, requiring only a one‑time training‑free calibration. On Llama‑3.1‑8B, BSFA delivers up to 1.13× speedup on LongBench with a 1.1% accuracy drop and up to 1.24× on Needle‑in‑a‑Haystack retrieval with a 1% drop, while the attention kernel itself accelerates by up to 1.38×.

By Daniel Ohayon, Itay Lamprecht, Itay Hubara, Israel Cohen, Daniel Soudry, Noam Elata
arXiv AI
Sep 2

Faster Than Flash: Exploiting Attention Sparsity for Efficient Long-Context Decoding

The paper introduces Faster Flash Decoding (FFD), a hardware‑algorithm co‑design that fuses the selector and computation into a single kernel to eliminate memory‑bandwidth bottlenecks in long‑context decoding. By replacing metadata indices with low‑bit quantized, content‑aware scanning and employing a top‑delta strategy for dynamic block filtering, FFD achieves up to 11.6× kernel‑level speedup and scales to 256K context length while preserving model accuracy. The approach is training‑free, plug‑and‑play, and demonstrates significant throughput gains on benchmarks such as RULER and LongBench.

By Zhigeng Liu, Zhiyuan Ning, Ruixiao Li, Xiaoran Liu, Yuerong Song, Min Zhang, Ziwei He, Xipeng Qiu
arXiv Machine Learning
Aug 20

FlashAttention for Scalable Vector Architectures

FlashAttention-V is a blocked FlashAttention implementation optimized for scalable vector architectures, designed to reduce the memory bandwidth bottleneck of transformer attention on CPUs. By fusing operations, exploiting parallelism across attention heads, and inter‑head packing, it improves vector register utilization and memory locality, enabling efficient scaling from short to very long vectors. Benchmarks on TinyLlama, Llama 3.2, Qwen2.5, and Pythia‑410M show 22×–42× speedups over scalar FlashAttention in prefill and 8×–11× in decode on a Banana Pi BPI‑F3, while also revealing quantization‑related bottlenecks that limit long‑vector scalability.

By Sonia Rani Gupta, Nikela Papadopoulou, Miquel Peric\`as
arXiv Machine Learning
Sep 21

Elastic Threshold Attention: Learned Contextual Sparsity for Long-Context Decoding

Elastic Threshold Attention (ETA) is a trainable attention mechanism that dynamically predicts contextual thresholds from query representations, enabling selective pruning of KV cache tokens during long‑context decoding. By multiplicatively suppressing sub‑threshold logits during training, ETA avoids representation collapse and eliminates localized attention sinks, allowing a 1.45B model to match dense attention performance at roughly 85% training sparsity and 38% active decode density. At inference, a custom Triton kernel achieves up to 2.5× faster decoding on sequences up to 512K tokens, and an offline calibration step can further reduce compute by 27% by freezing per‑head thresholds.

By Themistoklis Haris, Henry Li, Maryam Karimzadehgan
arXiv Machine Learning
Sep 16

LLM Inference in a Flash!

The paper "LLM Inference in a Flash!" proposes an integer‑only quantization scheme and a dictionary‑based KV cache compression technique to enable large language model inference on compute‑in‑flash (CIF) devices. By eliminating floating‑point operations and reducing KV cache traffic through sparse dictionary coding, the authors achieve minimal accuracy loss while cutting dynamic KV cache traffic by 15× on Llama‑3.1‑8B and Qwen‑2.5‑7B models.

By Sebastian Zhao, Minseo Kim, Coleman Hooper, Luca Manolache, Michael W. Mahoney, Yakun Sophia Shao, Kurt Keutzer, Amir Gholami