feat(kv-cache): support replicated MLA indexer with sharded CKV

[FujitsuPolycom]

Summary

Add lockstep KV allocation for mixed MLA groups and allow the target sparse-indexer KV cache to be replicated while the large main CKV remains DCP-sharded.

This closes the allocator/model gap that previously made replicated-indexer KV incompatible with the sharded MLA cache: the groups keep distinct physical page geometry but share one logical block-ID lifecycle and global token span.

Scope

• group compatible replicated and sharded MLA specs separately
• account their combined bytes per logical block
• allocate, cache, promote, and free matching block IDs in lockstep
• propagate per-group DCP ownership into block tables and attention metadata
• gate target indexer replication behind VLLM_DCP_REPLICATE_INDEXER_CACHE
• keep draft/MTP cache policy separate
• reject unsupported external KV loads in lockstep mode

The feature defaults off.

Validation

Validated as part of the public TP4/DCP4/MTP3 integration stack on 4x RTX PRO 6000 Blackwell 96 GB:

• 3 focused mixed-DCP allocator tests passed
• vLLM policy/workspace suite: 160 passed, 18 skipped
• mypy Python 3.10-3.13 and static integration checks passed
• exact long-context target + MTP startup and CUDA graph capture passed

Design

GLM-5.2 Sparse CKV under Decode Context Parallelism

This PR is independently useful and is the allocation prerequisite for the later sparse-CKV decode work.

Credits

The broader design builds on Koush's sparse-CKV work and Luke Alonso's B12X/Sparkinfer infrastructure.

Implementation, tests, and documentation were developed with OpenAI Codex assistance and manually reviewed and GPU-validated on the target host.

Summary by CodeRabbit

• New Features

  • Added an option to replicate DeepSeek sparse-indexer KV caches across decode context parallel partitions.
  • Added lockstep allocation for compatible MLA KV-cache groups, improving shared block usage and memory planning.

• Bug Fixes

  • Improved replicated-cache indexing, metadata, top-k handling, and sequence-length calculations.
  • Corrected KV-cache block hashing, allocation, prefix caching, and slot mapping for mixed replicated and sharded layouts.

• Reliability

  • Expanded coverage across supported parallelism configurations and cache-management scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

This change adds configurable DCP replication for DeepSeek sparse-indexer caches, updates indexer metadata handling for global replicated state, and introduces lockstep MLA KV-cache allocation with synchronized block IDs, tensor pools, prefix caching, and expanded validation.

Changes

DCP indexer replication

Layer / File(s)	Summary
Replicated indexer configuration and runtime wiring vllm/envs.py, vllm/model_executor/models/deepseek_v2.py, vllm/model_executor/layers/sparse_attn_indexer.py, tests/models/..., tests/model_executor/...	Adds the replication environment flag, validates supported configurations, propagates replication into DeepSeek and sparse-indexer construction, and verifies B12X skips DCP top-k merging.
Replicated indexer metadata and sequence lengths vllm/v1/attention/backends/mla/indexer.py, tests/v1/attention/test_indexer_dcp_localize.py	Sizes replicated metadata against global context and avoids DCP localization when replicated state is active.
Lockstep MLA grouping and allocation vllm/v1/core/kv_cache_utils.py, vllm/v1/core/kv_cache_coordinator.py, vllm/v1/core/block_pool.py, tests/v1/core/..., tests/v1/worker/...	Adds lockstep MLA grouping, packed tensor allocation, synchronized block management, cache-hash handling, and coverage for allocation, prefix caching, reshaping, and slot mappings.

Estimated code review effort: 5 (Critical) | ~120 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Environment
  participant DeepseekV32IndexerCache
  participant DeepseekV32IndexerMetadataBuilder
  participant SparseAttnIndexer
  Environment->>DeepseekV32IndexerCache: Configure replicated indexer cache
  DeepseekV32IndexerCache->>DeepseekV32IndexerMetadataBuilder: Provide replicated KV-cache spec
  DeepseekV32IndexerMetadataBuilder->>SparseAttnIndexer: Build global metadata without DCP localization
  SparseAttnIndexer-->>DeepseekV32IndexerMetadataBuilder: Produce global top-k decode metadata

Loading

Possibly related PRs

• local-inference-lab/vllm#4: Updates related DeepSeek indexer DCP-local sequence-length and decode metadata handling.
• local-inference-lab/vllm#37: Addresses related B12X DCP global-top-k warmup and merge behavior.

Suggested reviewers: lukealonso, jeejeelee, zjy0516

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 19.48% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: replicated MLA indexer support alongside sharded CKV caches.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@tests/v1/core/test_prefix_caching.py`:
- Line 242: Update the zip call in the block-hash/block-ID pairing loop to pass
strict=True explicitly, preserving the existing iteration while making
mismatched input lengths raise an error.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a6c85652-b86c-4c37-a254-382f61a8652a

📥 Commits

Reviewing files that changed from the base of the PR and between 6a2edcf and 4535bee.

📒 Files selected for processing (14)

• tests/model_executor/layers/test_sparse_attn_indexer_b12x.py
• tests/models/test_dcp_shard_draft_defaults.py
• tests/v1/attention/test_indexer_dcp_localize.py
• tests/v1/core/test_kv_cache_utils.py
• tests/v1/core/test_prefix_caching.py
• tests/v1/worker/test_attn_utils.py
• tests/v1/worker/test_gpu_block_table.py
• vllm/envs.py
• vllm/model_executor/layers/sparse_attn_indexer.py
• vllm/model_executor/models/deepseek_v2.py
• vllm/v1/attention/backends/mla/indexer.py
• vllm/v1/core/block_pool.py
• vllm/v1/core/kv_cache_coordinator.py
• vllm/v1/core/kv_cache_utils.py

[coderabbitai]

📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Add explicit strict= to zip().

Flagged by Ruff (B905). Lengths match today, but strict=True makes the block-hash/block-id pairing fail loudly if they ever diverge.

Proposed fix

-    for block_hash, block_id in zip(request.block_hashes, target_ids):
+    for block_hash, block_id in zip(request.block_hashes, target_ids, strict=True):

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change

	for block_hash, block_id in zip(request.block_hashes, target_ids):
	for block_hash, block_id in zip(request.block_hashes, target_ids, strict=True):

🧰 Tools

🪛 Ruff (0.15.21)

[warning] 242-242: zip() without an explicit strict= parameter

Add explicit value for parameter strict=

(B905)

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@tests/v1/core/test_prefix_caching.py` at line 242, Update the zip call in the
block-hash/block-ID pairing loop to pass strict=True explicitly, preserving the
existing iteration while making mismatched input lengths raise an error.

Source: Linters/SAST tools

[voipmonitor]

Independent GG validation on 8x RTX PRO 6000 Blackwell, GLM-5.2 NVFP4, TP8/DCP8/MTP0/A16:

Indexer mode	64k prefill	400k prefill	KV tokens
Sharded	3,279 tok/s	2,989.6 tok/s	4,174,848
Replicated (#159)	5,683 tok/s	3,934.1 tok/s	3,062,784

Replication improved DCP8 by 73.3% at 64k and 31.6% at 400k by removing the eight-rank indexer candidate merge. The 64k result is within 1.5% of our TP8/DCP4 result, but replication costs 26.6% of DCP8 KV capacity and remains 22.5% behind DCP4 at 400k.

The exact installed overlay passed 254 focused tests. This confirms the implementation is useful as an explicit capacity/performance profile, but the measured VRAM trade-off argues against making full replication an unconditional default.