vLLM Ascend KV Compression

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An independently packaged TriAttention KV-cache compression plugin for the upstream-aligned vLLM-HUST and vLLM-Ascend-HUST stacks. Version 0.6 precomputes phase coefficients across sampled layers and generates model-matched calibration artifacts itself, without changing either host repository.

Status: experimental release candidate. Package lifecycle, Ascend kernel smoke, three cold 16K engineering-control pairs, and bounded public LongBench-v2/LongBench quality checks pass. The full V4.6 official baseline, production-corpus calibration, repeated public performance runs, and stability matrix remain release gates. See Validation.

Ownership and maintenance

• School: Huazhong University of Science and Technology (HUST)
• Group: CGCL
• Advisor: Prof. Yao Wan (万瑶)
• Project lead: Sichen Liu (刘思辰), @Seas0
• Maintainers: Jiawan Zhang (张家万), @Jiawan23; Ruohao Wei (韦若皓), @kotoriqaq0; and @Seas0

The team agrees to maintain compatibility with vLLM-HUST and vLLM-Ascend-HUST and to publish this work through the vLLM-HUST Extension Manager. This is a CGCL-maintained plugin, not code built into either host.

Source and redistribution scope

The scoring method adapts TriAttention commit a4bc3c8f and the paper TriAttention: Efficient Long Reasoning with Trigonometric KV Compression. The Ascend paged-KV runtime was rewritten and does not copy the reference CUDA kernels.

Repository code and documentation are Apache-2.0. Model weights, datasets, raw logs, and calibration statistics are not included in the wheel or sdist. The development-only committed statistics have incomplete model/data provenance and must not be redistributed merely under this repository's license. See NOTICE, ownership and licensing, and calibration artifacts.

Compatibility

The supported topology is one Ascend NPU, the v1 scheduler and NPUModelRunner, one full-attention KV group, block size 128, and dense BF16/FP16 K/V. Unsupported combinations fail during startup. The manifest uses the stable vllm.general_plugins discovery entry point; current hosts do not expose a frozen native KV-lifecycle API, so the narrow host range and contract tests intentionally guard the internal integration seams.

Install, enable, disable, and uninstall

Install the matched host stack first, then the released wheel. The plugin wheel deliberately does not declare vllm or vllm-ascend as Python package dependencies: those hardware-specific packages must be provisioned as one tested runtime lock, while host compatibility is enforced by the Extension Manager manifest and startup checks. This also prevents plugin installation from asking pip to re-resolve an already provisioned host environment.

For the currently supported vLLM 0.28 / vLLM-Ascend 0.25 line, do not mix in Triton-Ascend 3.2.2. That older wheel pins NumPy 1.26.4, whereas the supported vLLM line requires opencv-python-headless>=4.13, whose available wheels require NumPy 2. Downgrading OpenCV to 4.9 violates the vLLM requirement. Use the matched Triton-Ascend 3.6 host stack instead.

After the host and Extension Manager are present, install the plugin without changing host packages:

python -m pip install --no-deps vllm-ascend-kvcompress-hust==0.6.0

For a fresh environment, install vllm-hust-ext separately from its approved index before the command above. Source/plugin-test installs should likewise use python -m pip install --no-deps . after provisioning the locked host stack.

Copy examples/triattention.json and set stats_path to the artifact location. If it does not exist, the enabled plugin generates model-matched statistics before vLLM loads its serving weights, atomically saves them, releases the temporary model, and continues startup. For production, set calibration_input_path to a licensed representative UTF-8 corpus; the bundled text is a bootstrap default. Then run:

vllm-hust-ext extension validate org.vllm-hust.ascend-kvcompress
vllm-hust-ext extension configure org.vllm-hust.ascend-kvcompress \
  --file /absolute/path/triattention.json
vllm-hust-ext extension enable org.vllm-hust.ascend-kvcompress
vllm-hust-ext extension status org.vllm-hust.ascend-kvcompress

export VLLM_PLUGINS=ascend,ascend_kvcompress
vllm-hust-ext run -- vllm serve /path/to/model \
  --block-size 128 --no-enable-prefix-caching --no-async-scheduling

If VLLM_PLUGINS is unset, vLLM discovers all installed plugins. The package remains inert unless the manager or direct enable flag activates it. Stop all host processes before changing state:

vllm-hust-ext extension disable org.vllm-hust.ascend-kvcompress
vllm-hust-ext extension forget org.vllm-hust.ascend-kvcompress
python -m pip uninstall vllm-ascend-kvcompress-hust

These operations only change the plugin package and manager state. Detailed source/environment instructions are in the installation guide, and the isolated wheel workflow is in packaging and release.

For development without Extension Manager:

export VLLM_PLUGINS=ascend,ascend_kvcompress
export VLLM_ASCEND_KVCOMPRESS_ENABLED=1
export VLLM_ASCEND_KVCOMPRESS_CONFIG=/absolute/path/triattention.json
vllm serve /path/to/model --block-size 128 --no-enable-prefix-caching

Unset both VLLM_ASCEND_KVCOMPRESS_* variables and restart to disable it.

Runtime and long-context optimization

After explicit activation, the adapter validates and hooks the current scheduler, KV-cache manager, concrete Ascend block table, and NPUModelRunner. Compression is committed only after a synchronous model step: semantic RoPE positions stay unchanged, physical attention/slot indices use a per-request offset, and old blocks are released at the next scheduler barrier.

Version 0.6 prepares RoPE phase coefficients once for all sampled layers and reuses them in direct paged-K scoring. On Ascend 910B2, the median amortized scoring cost was 0.270 ms per sampled layer versus 0.460 ms for the former in-kernel phase path, a 41.3% reduction. The validated stride is 8 (six of 48 layers for selection); every K/V layer is still materialized. Persistent copy, fused aggregation, device-resident offsets, dynamic JIT lengths, and the short-output bypass remain. A fused Triton K/V copy experiment was removed after measuring 14.504 ms versus 0.334 ms for the retained workspace path.

In three cold engineering-control pairs on Qwen2.5-14B, each with four fixed 16,384+1,024 requests at 0.4 RPS/concurrency 4, all 24 requests completed with the expected output length. Compression retained 32/128 blocks per request (75% physical KV reduction). Median total throughput was 1431.3 versus 1129.5 tok/s (+26.7%); mean TPOT was 39.16 versus 52.43 ms (-25.3%); mean E2E was 44.37 versus 57.81 s (-23.2%). This is engineering evidence against a same-host compatibility control, not the required official V4.6 B0 claim.

A separate public A3 sweep used all eligible non-truncated cases from LongBench-v2 (116), LongBench passage_retrieval_en (200), and LongBench qasper (93). With the 8K/stride-8 candidate, LongBench-v2 accuracy was unchanged, request throughput improved 2.81%, and mean E2E fell 2.72%; Qasper F1 returned to the exact B0 value and throughput was neutral (+0.03%). Retrieval stayed at 100% and deliberately bypassed compression because its output cap is 32 tokens. B1 recorded 127/127 scheduler/worker commits and reduced physical blocks 60.67% over compressed cases. See the full public benchmark record, including the rejected 4K Qasper result and single-run limitations.

Conflict matrix

Configuration and validation

The example uses an 8192-token budget, 1024-token recompute window, 512-token protected recent window, 8192-token scoring chunks, and every eighth scoring layer. It bypasses compression below 64 requested output tokens. KV-related token counts must be positive multiples of block size 128.

• Current validation record
• V4.6-derived requirements
• Benchmark protocol
• Public long-context benchmarks
• Packaging and release
• Method extension API
• Calibration artifacts

VLLM_PLUGINS='' TORCH_DEVICE_BACKEND_AUTOLOAD=0 python -m pytest -q
python -m ruff check src tests scripts
python -m ruff format --check src tests scripts

An NPU release candidate must also pass numerical kernel smoke and the declared long-context quality/performance/HBM matrix on the exact host stack.