[bzsuni]

Overview

Fix speculative decode block reservations for asynchronous KV loads with
multiple KV cache groups. Under KV memory pressure, under-reserving these
blocks can leave a request admitted for an asynchronous KV load but unable
to start decoding once the load completes.

Claims

• Account for every KV cache group using its actual allocation block size.
• Extend the existing promotion-margin regression test to cover single-group,
  multi-group, and mixed-block-size configurations.

Validation

.venv/bin/python -m pytest -q \
  tests/v1/kv_connector/unit/test_remote_prefill_lifecycle.py \
  tests/v1/kv_connector/unit/test_remote_decode_lifecycle.py \
  tests/v1/kv_connector/unit/test_kv_load_failure_recovery.py

32 passed. Pre-commit checks and manual Python 3.12 mypy passed.

Regression controls replace only the reservation method in the test process:

Implementation	Single group	Multiple groups	Different block sizes
Original	Pass	Fail	Fail
Original × group count	Pass	Pass	Fail
This fix	Pass	Pass	Pass

The broader scheduler/decode/recovery run showed no additional failures:
both the patched implementation and the original reservation method produced
171 passes and the same 46 failing test IDs. Observed environment limitations
included unavailable offline model configurations and insufficient visible
GPUs for parallel tests.

Validation exercises the scheduler and KV allocation paths with simulated
remote completion. Live NIXL P/D and model-level evaluations were not run.

Details

The existing calculation converts the speculative token margin to blocks
using the scheduler block size once. However, each KV cache group allocates
independently from the shared block pool, and groups may use different
allocation block sizes.

Sum the reservation across cache managers using each manager's block size.
Simply multiplying the old result by the group count is insufficient for
mixed-block-size configurations. The existing conservative reservation
approach is preserved.

The regression test checks admission and subsequent scheduling rather than
asserting the helper's arithmetic directly.

---

Pull Request Checklist

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• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

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• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

[claude]

Claude Code Review

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[snadampal]

one suggestion :num_spec_tokens, num_lookahead_tokens, and each manager's block_size are constant for a given deployment, but the function _spec_decode_step_blocks is called once per in-flight prefill on every scheduling step which is unnecessary.
this can be fixed either in this PR or a new one.

[bzsuni]

one suggestion :num_spec_tokens, num_lookahead_tokens, and each manager's block_size are constant for a given deployment, but the function _spec_decode_step_blocks is called once per in-flight prefill on every scheduling step which is unnecessary. this can be fixed either in this PR or a new one.

Thanks, updated to cache the value with cached_property

[njhill]

Thanks @bzsuni, the fix looks correct: summing per-group cdiv(N, B_g) across managers is a sound upper bound, and the parametrized test exercises the deadlock directly.

Over-reservation for Mamba groups. Summing over all single_type_managers also reserves cdiv(..., mamba.block_size) = 1 block per Mamba group, but Mamba never needs it: in non-align mode its spec blocks are already counted by MambaManager.get_num_blocks_to_allocate (and its block size is huge), and in align mode lookahead tokens are explicitly ignored. For hybrid GDN/Mamba + MTP models this over-reserves ~1 block per Mamba group for every in-flight async load, which adds up on a P/D decode instance.

Could we make this more precise by letting each manager account for its own lookahead need? E.g. in _request_remaining_blocks, pass num_tokens = full_num_tokens + 1 + num_spec_tokens + num_lookahead_tokens (with num_tokens_main_model = full_num_tokens) to coordinator.get_num_blocks_to_allocate rather than adding _spec_decode_step_blocks on top. That handles Mamba/SWA/etc. correctly per group, and the manual per-manager sum would then only be needed for the admitting request's own margin (where it could skip managers whose allocation doesn't depend on token count).

A few nits:

1. Since kv_cache_manager and the token limits are fixed at __init__, it'd be simpler to compute this once as a plain attribute in __init__ rather than turning the method into a @cached_property.
2. Please keep a short form of the original test docstring explaining the deadlock (two loads fill the pool, the head can't be promoted, nothing is running so no block is ever freed). It's the reason the test exists.
3. The docstring line "The token limits and allocation block sizes are fixed at initialization." can be dropped (it only justifies the caching).

[bzsuni]

Thanks @bzsuni, the fix looks correct: summing per-group cdiv(N, B_g) across managers is a sound upper bound, and the parametrized test exercises the deadlock directly.

Over-reservation for Mamba groups. Summing over all single_type_managers also reserves cdiv(..., mamba.block_size) = 1 block per Mamba group, but Mamba never needs it: in non-align mode its spec blocks are already counted by MambaManager.get_num_blocks_to_allocate (and its block size is huge), and in align mode lookahead tokens are explicitly ignored. For hybrid GDN/Mamba + MTP models this over-reserves ~1 block per Mamba group for every in-flight async load, which adds up on a P/D decode instance.

Could we make this more precise by letting each manager account for its own lookahead need? E.g. in _request_remaining_blocks, pass num_tokens = full_num_tokens + 1 + num_spec_tokens + num_lookahead_tokens (with num_tokens_main_model = full_num_tokens) to coordinator.get_num_blocks_to_allocate rather than adding _spec_decode_step_blocks on top. That handles Mamba/SWA/etc. correctly per group, and the manual per-manager sum would then only be needed for the admitting request's own margin (where it could skip managers whose allocation doesn't depend on token count).

A few nits:

1. Since kv_cache_manager and the token limits are fixed at __init__, it'd be simpler to compute this once as a plain attribute in __init__ rather than turning the method into a @cached_property.
2. Please keep a short form of the original test docstring explaining the deadlock (two loads fill the pool, the head can't be promoted, nothing is running so no block is ever freed). It's the reason the test exists.
3. The docstring line "The token limits and allocation block sizes are fixed at initialization." can be dropped (it only justifies the caching).

Thanks for the detailed review! Addressed in 616f596b94:

• In-flight reservations now account for speculative/lookahead tokens through the coordinator, keeping num_tokens_main_model unchanged. The admitting request's margin excludes Mamba groups.
• Replaced cached_property with an attribute computed in __init__, restored the deadlock explanation in the test docstring, and removed the caching-only docstring.
• Added admission and promotion regression tests for Mamba none and align modes. Both cases fail with the previous logic and pass with the fix.

[njhill]

/ci run

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❌ This PR is 1 commit behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[bzsuni]

/ci run

[github-actions]

❌ @bzsuni, A reviewer with write access must run /ci run, approve the PR, or add the ready label first.

[njhill]

/ci run

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❌ This PR is 2 commits behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

[github-actions]

✅ @bzsuni, CI is now available for this PR.

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[njhill]

/ci run

[github-actions]

✅ Triggered Buildkite CI #93959 for commit f211aa933f3a.

[njhill]

Thanks @bzsuni