[https://nvbugs/6330273][fix] Reserve KV cache slots for concurrent decode in V2

[Kevin-Li-2025]

Description

Fixes #15401.

KVCacheManagerV2 can under-reserve windowed pool slots when capacity planning only sees long-history requests. For small sliding windows, the stale range can leave a windowed pool with a min-slot floor of 1, which can deadlock scheduling once concurrent decode requests exceed that single slot.

This adds a generic concurrent-decode constraint to the V2 cache config: max_batch_size requests at one token block with history_length=tokens_per_block - 1. Each decode request needs one slot in every pool group, so this floors the min slots at max_batch_size without changing scheduler behavior.

The config also sets the existing StorageManager fallback typical-step explicitly, so adding the constraint does not accidentally switch ratio selection to constraint-only sizing.

Tests

• python3 -m py_compile tensorrt_llm/_torch/pyexecutor/kv_cache_manager_v2.py tests/unittest/_torch/executor/test_per_layer_head_dim.py
• Static regression check confirming the new constraint and preserved typical-step are present
• git diff --check

I attempted the targeted pytest, but local collection is blocked by a missing nvtx dependency in this checkout.

Summary by CodeRabbit

• New Features

  • Enhanced KV cache management with concurrent decoding constraints to optimize cache allocation for simultaneous requests.

• Tests

  • Added test coverage for cache configuration in concurrent decoding scenarios.

[coderabbitai]

📝 Walkthrough

Walkthrough

KVCacheManagerV2._build_cache_config now passes a constraints field and a typical_step to KVCacheManagerConfigPy. The constraints are produced by a new _build_concurrent_decode_constraint static method that returns a BatchDesc of max_batch_size KVCacheDesc entries, each sized by tokens_per_block. A unit test verifies the resulting constraint and typical_step shapes.

Changes

Concurrent decode constraint for windowed KV cache pools

Layer / File(s)	Summary
Constraint helper and cache config wiring tensorrt_llm/_torch/pyexecutor/kv_cache_manager_v2.py	Adds KVCacheDesc import, introduces the _build_concurrent_decode_constraint static method returning a BatchDesc of max_batch_size entries sized by tokens_per_block, and wires constraints plus a typical_step fallback BatchDesc(KVCacheDesc(capacity=2049, history_length=2048)) into KVCacheManagerConfigPy construction.
Unit test for constraint shape tests/unittest/_torch/executor/test_per_layer_head_dim.py	Imports GpuCacheTierConfig and adds test_build_cache_config_reserves_concurrent_decode_slots, which instantiates KVCacheManagerV2 without calling __init__, invokes _build_cache_config, and asserts constraint kv_caches length equals max_batch_size, per-entry capacity/history_length, and typical_step sizing.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested reviewers

• niukuo
• zeroepoch
• yizhang-nv
• tburt-nv

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% 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	The PR description clearly explains the issue (windowed pool min_slots underflow), the solution (generic concurrent-decode constraint), testing approach, and aligns with the template's requirements for explanation and test coverage.
Linked Issues check	✅ Passed	The code changes implement the exact solution proposed in issue #15401: adding a concurrent-decode constraint with max_batch_size requests at tokens_per_block capacity and tokens_per_block-1 history to reserve minimum slots.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to addressing issue #15401: modifying KVCacheManagerV2._build_cache_config to add constraints and setting typical_step, plus adding a regression test.
Title check	✅ Passed	The title clearly describes the main fix: reserving KV cache slots for concurrent decode in V2, which directly addresses the deadlock issue in the PR objectives.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[nvpohanh]

@lowsfer could you review this?