[Bugfix][KV Offload] Track cache recency once per request

[mindungil]

Purpose

The offloading scheduler re-touched the full cached prefix while a request was decoding. Besides doing repeated work, this made frequency-based policies such as ARC count one request as many accesses. Transfer-pinned LRU blocks also re-entered the evictable set in completion order, which could make an early prefix block the eviction victim and leave an unusable cached suffix.

Fix

• Remove scheduler-driven cache-policy touch calls.
• Record the keys actually loaded or offered for store in ReqContext, including their end-token positions.
• Apply one logical head-to-tail cache access in on_request_finished, independently of later transfer completion order.
• Keep new ARC entries in T1, count each ready reused entry once per request, and adapt B1/B2 ghost hits once before insertion.
• Treat tiering cascade reads as internal pins rather than additional request accesses. Primary recency is committed when the request finishes; secondary cleanup still waits until no more cascades can be submitted.
• Give LRU a logical recency rank separate from physical pinning, with bounded lazy-heap compaction and atomic batch eviction.
• Merge token positions across hybrid KV groups so tails are evicted before heads globally, not only within each group.

The existing touch API remains available for compatibility, and the new policy finalization hook has a default implementation for external policies.

Request access accounting is scoped to keys observed by the offloading manager: ready resident keys passed through prepare_load() or prepare_store() count once per request. A key that remains purely GPU-local and never reaches the CPU manager does not change CPU recency. New writes, speculative lookup() calls, and internal tiering cascade reads are not frequency hits.

Test Plan

python -m pytest -q tests/v1/kv_offload/cpu/test_manager.py
python -m pytest -q tests/v1/kv_offload/tiering/test_tiering_offloading.py
python -m pytest -q tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py
python -m pytest -q tests/v1/kv_offload/cpu/policies/test_factory.py
pre-commit run --files $(git diff --name-only origin/main...HEAD)

Test Result

• Related unit tests: 303 passed
  • CPU manager: 50 passed
  • Tiering manager: 56 passed
  • Offloading scheduler: 189 passed
  • Policy factory: 8 passed
• Pre-commit, including ruff and mypy for Python 3.10: passed
• GPU E2E (Qwen2.5-0.5B-Instruct on an NVIDIA RTX PRO 6000 Blackwell, 64-block CPU LRU cache with 16-block replacement): 1/1 passed; replay observed 768 cached tokens (expected 768), and generated output matched the baseline.
• GPU environment note: the test used nightly image base 1970f3ed4; the PR Python changes were overlaid, with the scheduler diff applied to that image base to account for one-day API drift.
• Model evaluation: N/A; this changes CPU-cache eviction bookkeeping only.

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• The purpose of the PR, such as the issue it resolves.
• The test plan and locally executed test results.
• Documentation update considered; no user-facing documentation change is needed for this internal behavior fix.

[claude]

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[orozery]

Thanks @mindungil !
This overlaps with some thoughts I had in mind on removing touch.
The OffloadingManager should use the req_context in prepare_store and in on_request_finished to maintain the order of blocks heads to tail.
This will also solve the current issue with frequency based evictions (such as ARC) which currently are inaccurate due to the fact that keys are re-touched while the request is decoding.

[mindungil]

Thanks, that makes sense. To confirm the intended design: I plan to remove scheduler-driven touch, track ordered accesses per request through ReqContext during load/store, and commit the final head-to-tail ordering in on_request_finished, while allowing late transfer completions to consume that ordering. For ARC, existing blocks would count as one access per request, while newly stored blocks would remain in T1. Does that match what you had in mind?

[orozery]

Thanks, that makes sense. To confirm the intended design: I plan to remove scheduler-driven touch, track ordered accesses per request through ReqContext during load/store, and commit the final head-to-tail ordering in on_request_finished, while allowing late transfer completions to consume that ordering. For ARC, existing blocks would count as one access per request, while newly stored blocks would remain in T1. Does that match what you had in mind?

Yep!

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Improved KV-cache eviction order when requests complete out of sequence or transfers arrive late.
  • Preserved cache recency across CPU, GPU, and secondary tiers during cascaded reads and delayed store operations.
  • Prevented internal cascade reads from incorrectly changing cache recency.
  • Improved LRU and ARC behavior for reuse, eviction, request completion, and cache reset scenarios.
  • Corrected store-threshold handling so ready resident keys retain appropriate recency.

• Tests

  • Added comprehensive coverage for request ordering, eviction, tier propagation, and cache policy behavior.

Walkthrough

The change records offload-key positions in request contexts, removes scheduler touch calls, tracks request-scoped cache access, and updates LRU and ARC policies to apply recency at request completion. Tests cover ordering, eviction, tiered transfers, reset behavior, and ARC reuse.

Changes

KV cache recency and eviction

Layer / File(s)	Summary
Offload position tracking vllm/v1/kv_offload/base.py, vllm/distributed/kv_transfer/kv_connector/v1/offloading/scheduler.py, tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py	ReqContext stores key positions. The scheduler records boundary positions and no longer calls manager.touch. Tests validate positions and removed touch calls.
Request access finalization vllm/v1/kv_offload/cpu/manager.py, vllm/v1/kv_offload/cpu/policies/base.py, vllm/v1/kv_offload/tiering/manager.py, tests/v1/kv_offload/tiering/test_tiering_offloading.py	The CPU manager groups request accesses and finalizes them through cache policies. Tiered cascade reads do not update request recency.
Heap-based LRU eviction vllm/v1/kv_offload/cpu/policies/lru.py, tests/v1/kv_offload/cpu/test_manager.py	LRU uses ranked entries and a lazy-invalidating heap. Eviction handles protected keys, stale entries, deduplication, and heap compaction.
ARC request adaptation vllm/v1/kv_offload/cpu/policies/arc.py, tests/v1/kv_offload/cpu/test_manager.py	ARC applies insertion and reuse semantics at request completion. Ghost-hit adaptation and T1/T2 transitions are covered by regression tests.

Estimated code review effort: 4 (Complex) | ~60 minutes

Merge Risk: ⚪ Minimal · up to 49f5c

This change makes KV offload cache recency request-scoped and updates LRU/ARC eviction behavior accordingly. No merge-blocking correctness, security, availability, or data-integrity risk remains identified.

Sequence Diagram(s)

sequenceDiagram
  participant OffloadingScheduler
  participant ReqContext
  participant CPUOffloadingManager
  participant CachePolicy
  OffloadingScheduler->>ReqContext: Record offload-key positions
  CPUOffloadingManager->>ReqContext: Read request access positions
  CPUOffloadingManager->>CachePolicy: Finalize grouped request access
  CachePolicy-->>CPUOffloadingManager: Update LRU or ARC recency

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 27.91% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 86 functions across 10 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly summarizes the primary change: fixing KV offload cache recency tracking so each request contributes one access.
Description check	✅ Passed	The description is directly related to the changeset and explains the purpose, implementation, and test results in sufficient detail.

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Inline comments:
In `@vllm/v1/kv_offload/cpu/manager.py`:
- Around line 255-266: Update prepare_store() to partition the original offered
keys into keys_to_store and ready_existing_keys before applying store_threshold,
ensuring ready resident keys recreated by _record_accesses() remain eligible for
reused_keys and CachePolicy.on_request_finished(). Apply the threshold only to
keys_to_store, and increment stores_skipped_in_current_batch for keys removed by
that filtering.

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• tests/v1/kv_connector/unit/offloading_connector/test_scheduler.py
• tests/v1/kv_offload/cpu/test_manager.py
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• vllm/distributed/kv_transfer/kv_connector/v1/offloading/scheduler.py
• vllm/v1/kv_offload/base.py
• vllm/v1/kv_offload/cpu/manager.py
• vllm/v1/kv_offload/cpu/policies/arc.py
• vllm/v1/kv_offload/cpu/policies/base.py
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• vllm/v1/kv_offload/tiering/manager.py

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[mindungil]

Follow-up design audit completed against the direction confirmed above. The current implementation removes scheduler-driven touch, records load/store observations in ReqContext, commits one head-to-tail access in on_request_finished, keeps ARC insertions in T1 while counting ready reuse once per request, and prevents late transfer/cascade completion from overwriting logical recency.

I clarified the access boundary in the PR description: accounting is scoped to keys observed by the offloading manager. A key that remains purely GPU-local and never reaches the CPU manager does not update CPU recency; speculative lookup, new writes, and internal cascade pins are not frequency hits.

Revalidation remains green: 303 related unit tests (50 CPU manager, 56 tiering, 189 scheduler, 8 policy factory), plus the GPU E2E replay with 768/768 expected cached tokens and matching output.

[mergify]

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

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

[orozery]

Thanks @mindungil ! I went over the base.py changes and it looks good.
I actually think this aligns nicely with #49413.
@Change72 WDYT?

[Change72]

Yes, this aligns well with #49413. Event work can reuse the key-position mapping in ReqContext.

The scopes are complementary: this PR handles cache recency and eviction, while #49413 handles event provenance and removal.

[mergify]

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

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