[PD][Feature] Add KV consumer partial-group caching for hybrid Mamba models

[underfituu]

What this PR does

This PR fixes the 0% D-side prefix-cache hit rate issue for hybrid Mamba models under Prefill-Decode (PD) disaggregation with align mode, as reported in #42547, by excluding the Mamba group from D-side KV cache hit rate accounting.

Why we need it

For PD remote-prefill requests, vLLM allocates D-side KV slots based on the computed cache hits before pulling the KV cache from the P-side. The current hybrid coordinator requires a common hit length across all cache groups. As reported in #42554, since the connector only transfers the running state in the Mamba group, the D-side hit rate drops to 0%.

As a result, the D-side treats the reusable attention KV as missing, allocates extra KV slots, and pulls the same prefix KV cache again. This increases block pressure and limits decode concurrency, leading to severe performance degradation.

Design

We introduce a per-request flag, skip_mamba_align, in the kv_cache_manager derived from request.kv_transfer_params. When enabled, the hit-length computation bypasses the Mamba group, allowing full attention prefix blocks to be successfully reused during the decode phase.

Test

• Model: Qwen3.5-35B-A3B (Mamba hybrid model)
• Hardware: 2 × NVIDIA H20
• Transport: NIXL (NixlConnector, kv_role=kv_both, kv_parallel_size=2)
• Proxy: tests/v1/kv_connector/nixl_integration/toy_proxy_server.py
• Server Flags (P and D sides):

  --enforce-eager
  --mamba-cache-mode align
  --max-num-batched-tokens 16384
  --max-num-seqs 256
  --block-size 128
  --no-disable-hybrid-kv-cache-manager
  --max-model-len 24576
  --async-scheduling
  --gpu-memory-utilization 0.90

• Prefix caching set:

  • P side: --enable-prefix-caching
  • D side: toggled --enable-prefix-caching (ON, with PR) vs --no-enable-prefix-caching (OFF, baseline)

• vllm bench serve:

  --dataset-name prefix_repetition
  --prefix-repetition-output-len 1024

Results

OFF: D-side prefix caching disabled.
ON: D-side prefix caching enabled with this PR.
P-side prefix caching is enabled in both cases.

prefix_len	suffix_len	num_prefix	num_prompts	concurrency	OFF TTFT (ms)	ON TTFT (ms)	ΔTTFT	OFF Output TPS	ON Output TPS	ΔTPS
16,384	1,024	2	512	256	278,653	47,264	-83.0%	401.4	804.4	+100.4%
16,384	1,024	4	512	256	302,969	45,333	-85.0%	399.1	713.9	+78.9%
16,384	1,024	8	512	256	301,226	63,055	-79.1%	391.7	748.8	+91.2%
16,384	1,024	2	256	48	10,801	2,953	-72.7%	429.8	436.5	+1.6%
4,096	1,024	2	512	256	103,409	64,204	-37.9%	891.4	939.8	+5.4%

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request introduces support for KV consumer partial-group caching in disaggregated inference, ensuring Mamba layers do not interfere with prefix-caching block alignment. It updates the KV cache coordinator, manager, and scheduler to handle VllmConfig and adjusts logic for KV consumers. The reviewer suggested centralizing the KV consumer check by adding a property to VllmConfig to reduce code duplication.

[gemini-code-assist]

The logic to determine if the instance is a KV consumer is duplicated in multiple places within this pull request. This increases maintenance overhead and risk of inconsistencies.

Specifically, the check vllm_config.kv_transfer_config is not None and vllm_config.kv_transfer_config.is_kv_consumer (or a variation of it) is present in:

• vllm/model_executor/models/config.py
• vllm/v1/core/kv_cache_coordinator.py
• vllm/v1/core/kv_cache_utils.py (this file)
• vllm/v1/core/sched/scheduler.py

To improve maintainability, consider centralizing this logic. For example, you could add a property to the VllmConfig class:

# In vllm/config/config.py
class VllmConfig:
    ...
    @property
    def is_kv_consumer(self) -> bool:
        return (self.kv_transfer_config is not None and
                self.kv_transfer_config.is_kv_consumer)

This would allow you to replace the repeated checks with a cleaner vllm_config.is_kv_consumer.

[NickLucche]

Thanks for contributing @underfituu .
We dont have consumers with PD disagg (check #42554 out), as nixl is using kv_both.

[underfituu]

Thanks for contributing @underfituu . We dont have consumers with PD disagg (check #42554 out), as nixl is using kv_both.

Thanks for pointing that out! We actually overlooked the kv_both scenario. Let us head back to the drawing board and rethink our approach.

[underfituu]

Thanks for contributing @underfituu . We dont have consumers with PD disagg (check #42554 out), as nixl is using kv_both.

Hey @NickLucche , the latest commit introduces a per-request flag for skip_mamba_align, making it fully compatible with NIXL. We’d highly appreciate it if you could test this patch in your environment to see if the D-side cache hit is fully restored.

[NickLucche]

Hey @underfituu sorry for being late on this.
Would you mind elaborating your idea a bit more? I think I am reading it wrong, but are you trying to skip prefix caching on P..?

[NickLucche]

I feel we're spiling request-level pd checking into the kv cache manager. Let's discuss the proposed change a bit more to see if/where we can move this

[lHrHenry233]

I feel we're spiling request-level pd checking into the kv cache manager. Let's discuss the proposed change a bit more to see if/where we can move this

I agree with this concern. However, without changing the hit accounting path, the D node has no way to reuse the gated-attention KV cache independently of the Mamba state. We considered this flag-based approach the smallest change we could make:

• it does not change P-side prefix caching;
• it does not change normal non-PD hybrid Mamba prefix-cache behavior;
• it only changes the D-side hit-length calculation for requests that are actually using remote prefill;
• it avoids changing the KV transfer protocol, block layout, or Mamba state representation.

I agree that the exact location and naming of the flag can be improved. We would be happy to further discuss with you and the community on where this flag should live and how it should be named. But from our current understanding, some change to the hybrid cache-hit accounting is necessary; otherwise the valid gated-attention KV hit on the D side will continue to be discarded by the Mamba group's 0 hit.

[lHrHenry233]

Hey @underfituu sorry for being late on this. Would you mind elaborating your idea a bit more? I think I am reading it wrong, but are you trying to skip prefix caching on P..?

Thanks @NickLucche, let me clarify the intent here.

No, we are not trying to skip prefix caching on the P side. In our setup, prefix caching on the P node remains enabled and unchanged.

The issue we are trying to fix is on the D side for PD-disaggregated hybrid Mamba models. After the D side pulls the remote prefill result from P, the attention KV blocks can be available/reusable on D, but the Mamba/SSM state is not represented as normal prefix-cache blocks in the D-side block table. Then HybridKVCacheCoordinator computes the cache hit length by min-reducing across KV groups. Since the Mamba group reports 0 cached blocks, it collapses the valid attention / gated-attention KV hit length to 0 as well.

So the idea is only to change the D-side cache-hit accounting for PD-pull requests: when the request is actually using remote prefill, we skip the Mamba/SSM group in the hybrid hit-length min-reduction, so the D node can reuse Qwen3.5's gated-attention KV cache independently of the Mamba state. The Mamba state is still treated as not locally prefix-cached on D.

[NickLucche]

Thanks for elaborating and for the work @lHrHenry233 @underfituu , I think this is an interesting approach.

I would like to pick one solution between #42547 and this PR.
To make changes even less invasive and avoiding having to thread skip_mamba_align through various layers, I think we should build on top #43874 (and RFC #43807).

This way we could apply this D-specific fix by checking for role at config time once kv_both is deprecated.
The changes would then be local to HybridKVCacheCoordinator, which could check whether current instance is consumer/D, and set skip_mamba_align once accordingly.

Also, I assume you're running based on #42554 right?
Ow one will run into

(EngineCore pid=618132) ERROR 06-01 13:44:14 [core.py:1161]   File "/home/NickLucche/llmd/vllm/vllm/distributed/kv_transfer/kv_connector/v1/nixl/worker.py", line 2338, in _apply_prefix_caching
(EngineCore pid=618132) ERROR 06-01 13:44:14 [core.py:1161]     assert num_local_blocks == num_remote_blocks

[NickLucche]

what do you mean with "Mamba state is not transferred", that's not right we transfer the temporal/Conv states.

[lHrHenry233]

what do you mean with "Mamba state is not transferred", that's not right we transfer the temporal/Conv states.

Here the state means the cahe aside from the running state. The annotation is indeeded confusing. We will improve it.

[NickLucche]

cruft, this crashes at runtime

[lHrHenry233]

cruft, this crashes at runtime

Sorry, this derives from the first commit and we have deleted it.

[lHrHenry233]

Thanks for elaborating and for the work @lHrHenry233 @underfituu , I think this is an interesting approach.

I would like to pick one solution between #42547 and this PR. To make changes even less invasive and avoiding having to thread skip_mamba_align through various layers, I think we should build on top #43874 (and RFC #43807).

This way we could apply this D-specific fix by checking for role at config time once kv_both is deprecated. The changes would then be local to HybridKVCacheCoordinator, which could check whether current instance is consumer/D, and set skip_mamba_align once accordingly.

Also, I assume you're running based on #42554 right? Ow one will run into

(EngineCore pid=618132) ERROR 06-01 13:44:14 [core.py:1161]   File "/home/NickLucche/llmd/vllm/vllm/distributed/kv_transfer/kv_connector/v1/nixl/worker.py", line 2338, in _apply_prefix_caching
(EngineCore pid=618132) ERROR 06-01 13:44:14 [core.py:1161]     assert num_local_blocks == num_remote_blocks

Thanks for your feedback! Yes, you are right—our worker is running based on #42554. We will also update the PR description later with more details to make this clear.

[ZhanqiuHu]

Thanks! @NickLucche Here an example alternative for reference showing the "per-group" approach we discussed offline: #44243

[underfituu]

Thanks! @NickLucche Here an example alternative for reference showing the "per-group" approach we discussed offline: #44243

Thanks for sharing this. We're currently reviewing the approach and will share our thoughts on the 'per-group' logic shortly. Stay tuned.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @underfituu.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork