[Core] Dedicated block pools for Mamba KV cache groups

[wenxindongwork]

Summary

Hybrid models with Mamba/linear-attention layers and prefix caching (mamba_cache_mode="align") address Mamba state by block id, so every block id in the shared pool must carry a Mamba state slot in every Mamba layer. Attention and every Mamba group draw ids from that one pool, so an attention block id reserves Mamba state it never uses and a Mamba id of one group reserves slots in the other groups' layers. On platforms that allocate one array per layer (TPU), this puts ~85% of the KV budget into idle Mamba slots for Qwen3.5-397B and collapses the prefix cache under multi-turn load.

This adds KVCacheConfig.mamba_num_blocks (from the platform hook CacheConfig.mamba_num_blocks_override): when set, every Mamba group gets its own BlockPool of that size and block ids become pool-local. Default behavior (shared pool) is unchanged.

Companion tpu-inference PR: vllm-project/tpu-inference#3508

Changes

• kv_cache_coordinator.py: per-group pools (block_pools, shared pool first); get_num_blocks_to_allocate_per_pool; cache-hit lookups go through each group's own pool, and groups sharing a spec are only batched into one lookup when they share a pool; free_blocks routes blocks back to the pool they belong to.
• kv_cache_manager.py: admission checks every pool (reserved_blocks charged to the shared pool, watermark per pool); usage = fullest pool; reset_prefix_cache / take_events cover all pools; partial-tail pins and CoW retentions are freed through the routing helper. KVCacheManager.free_blocks is the new entry point the scheduler uses for deferred frees.
• kv_cache_utils.py: get_mamba_num_blocks (honors the override only for models with Mamba groups), unify across workers, and get_max_concurrency_for_kv_cache_config bounds concurrency by the Mamba pools separately.
• config/cache.py, kv_cache_interface.py: the new fields (excluded from the compile hash).

Known limits: evict_blocks(block_ids) addresses the shared pool only (KV connectors operate on attention blocks); reserved_blocks from in-flight prefills is a sum over groups and is charged to the shared pool, which is conservative.

Tests

New: test_hybrid_mamba_align_dedicated_pools_allocate_hit_and_free, test_hybrid_mamba_align_dedicated_pool_gates_admission, test_get_max_concurrency_with_dedicated_mamba_pools. 126 passed across tests/v1/core/test_prefix_caching.py, test_mamba_align_chunk_split.py, test_single_type_kv_cache_manager.py and the capacity tests (CPU, TPU platform).

🤖 Generated with Claude Code

https://claude.ai/code/session_01Sw2sUivv1hHHcGUY8U4fyS

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[wenxindongwork]

Superseded: the split pool is now implemented entirely in tpu-inference (vllm-project/tpu-inference#3508) via a registered TPUMambaSpec/TPUMambaManager, so no vLLM change is needed.