amass-kv 0.1.1

AMASS: decode-time sparse KV-cache attention for vLLM (quad page-score selector + DRAM value tier)

AMASS

Decode-time sparse KV-cache attention for vLLM. AMASS keeps LLM decoding fast and memory-light at long context by attending, at every decode step, to only the pages that actually carry attention mass: a compact per-page score selects a small working set (typically 5 to 10 percent of the KV cache), and a sparse paged-attention kernel reads just those pages. Prefill is untouched, so time-to-first-token is unchanged.

License note (provisional). This release is distributed under Apache-2.0 as a placeholder pending final confirmation by the authors. See LICENSE.

What is in the box

AMASS has two parts that share one selection core:

1. The quad selector (Stage A). A quadratic, Gaussian-MGF page score built from a tiny per-page summary. "quad" drops the per-key coordinates and stores only r' eigenvalues per page, which shrinks the resident selector state by 3.28x (from 15.6 percent down to 4.7 percent of the KV cache; measured 1201 MiB vs 3943 MiB) while remaining accurate enough to pick the right pages.
2. The DRAM value tier (Stage B). A pinned, device-mapped host-DRAM pool for the value cache, with a bounded resident hot buffer plus staging pool. Only the ~5 percent quad summary and the pages actually called each step stay in VRAM; the rest of V (or K+V) lives in host memory.

Two variants, one algorithm

Variant	KV residency	The play
AMASS-fast	K+V resident in VRAM	speed
AMASS-mem	V (mem-v) or K+V (mem-kv) offloaded to DRAM; only the quad summary + called pages resident	memory + speed

Install

pip install amass-kv

The import name is amass:

import amass
print(amass.__version__)

amass-kv declares torch and triton as its runtime dependencies. vLLM is an optional extra (see Dependencies); install AMASS into your existing vLLM environment, or pull a compatible engine with:

pip install "amass-kv[vllm]"     # engine + plugin
pip install "amass-kv[all]"      # + transformers (bench) + pyyaml (yaml configs)

Requires Python >= 3.10 and an NVIDIA GPU. The hot-path CUDA kernels are compiled once at first use via torch.utils.cpp_extension (targeting Hopper, sm_90a); the reference Triton kernels are the automatic fallback.

Quickstart

AMASS installs as a vLLM general plugin, so once the wheel is present vLLM auto-registers it at engine startup with no code change: AMASS patches itself in wherever vLLM would have chosen the FlashAttention backend. Configuration is a single AmassConfig, sourced from the AMASS_CONFIG environment variable (an inline JSON string, or a path to a JSON/YAML file):

# Run any vLLM entry point (serve / offline) with AMASS active.
export AMASS_CONFIG='{"variant": "fast", "coverage": 0.95}'
vllm serve meta-llama/Llama-3.1-8B-Instruct

# The memory play: offload V to DRAM, keep K resident.
export AMASS_CONFIG='{"variant": "mem-v", "coverage": 0.95}'

# Turn AMASS off (stock FlashAttention / FullKV reference line):
export AMASS_DISABLE=1

Key AmassConfig fields: variant (fast / mem-v / mem-kv), coverage (adaptive per-head nucleus coverage over exact page masses; default 0.95) or budget (a fixed selected-page fraction), score (quad default, or the r8 low-rank fallback), and use_cuda (hand-CUDA hot path vs Triton reference).

Headline results (measured)

• 3.28x smaller selector state. The quad summary is 4.7 percent of the KV cache vs 15.6 percent for the low-rank r8 baseline (1201 vs 3943 MiB measured), with a cheaper tail-free hot-path kernel.
• Near-lossless on LongBench. quad at a 5 percent per-step budget lands within noise of FullKV (LongBench-v1 delta about -0.22 for quad vs -0.38 for r8 at the same budget; about -0.29 at coverage 0.95 across 14 subsets).
• Faster in the regime that matters. AMASS-fast beats dense FlashAttention-3 in the long-context, high-batch decode regime by up to about 2x, and is at parity at batch size 1 (the hot path is bandwidth-bound and only pulls ahead once the dense KV read dominates). It does not claim >=2x everywhere.
• 76 to 92 percent VRAM saved in the mem variant, by holding only the quad summary plus the called pages resident and serving the rest of V from DRAM.

Numbers are per-kernel and end-to-end reproducible with amass-bench (below). See the paper for the full protocol, benchmarks (RULER, LongBench v1/v2, SCBench, a reasoning suite), and ablations.

• Paper: see the project page linked in this repository.

Benchmarking: amass-bench

The wheel ships a benchmarker so the latency claims are reproducible on any Hopper box with one command:

amass-bench kernels --quick    # per-kernel: r8_score / topb / decode, CUDA vs Triton vs dense-FA
amass-bench e2e --ctx 16384    # end-to-end decode-step TPOT in a real vLLM run
amass-bench all --json out.json
python -m amass.bench kernels  # identical to the console entry point

amass-bench e2e needs the bench + vllm extras (it tokenizes a real model and spins up an engine). Timing is CUDA-event based with warmup and medians, and reports both eager and CUDA-graph-replay speedups plus an HBM roofline for the bandwidth-bound score kernel. Full flag reference: amass-bench --help.

Dependencies

Dependency	Why	How it is declared
torch	tensors + runtime CUDA (load_inline) kernels	hard (>=2.4)
triton	reference / fallback kernels	hard (>=3.0)
vllm	plugin host + attention backend	extra [vllm] (>=0.8)
transformers	amass-bench e2e tokenizer (lazy import)	extra [bench]
pyyaml	YAML AMASS_CONFIG files (JSON needs no dep)	extra [yaml]

numpy is not a direct dependency: nothing under amass/ imports it; it arrives transitively through torch. vLLM is an extra rather than a hard floor because it is a heavy, CUDA-ABI-sensitive wheel that production deployments pin themselves, and a hard floor could silently upgrade a pinned engine.

License

Apache-2.0 (provisional; see the note at the top). See LICENSE for the full text.