[GG] feat(kv-offload): bound filesystem tier capacity#165
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Add an opt-in max_cache_size_bytes limit to the filesystem secondary tier. Track runtime LRU state, reserve capacity across concurrent writers, and pin lookup and queued-load sources so eviction cannot race promotion. Index and trim existing block files on startup, publish removal events before replacement stores, and document that bounded mode requires exclusive ownership of its cache namespace. Assisted-by: OpenAI Codex Signed-off-by: Derek Yates <derek.yates@live.com>
📝 WalkthroughWalkthroughThe filesystem KV tier adds an optional byte capacity limit with startup indexing, LRU eviction, pin-aware concurrent stores and loads, synchronized event handling, and expanded tests and documentation. ChangesFilesystem cache capacity
Estimated code review effort: 4 (Complex) | ~45 minutes Sequence Diagram(s)sequenceDiagram
participant Request
participant FileSystemTierManager
participant Filesystem
participant KVEvents
Request->>FileSystemTierManager: submit_store
FileSystemTierManager->>FileSystemTierManager: reserve capacity
FileSystemTierManager->>Filesystem: evict unpinned LRU blocks
FileSystemTierManager->>Filesystem: write bounded block
Filesystem-->>FileSystemTierManager: store completion
FileSystemTierManager->>KVEvents: record removal event
FileSystemTierManager-->>Request: return completed job
Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Runtime validation update (CN3, 2026-07-23): the bounded tier ran through the full cold long-context ladder while an ordered inotify monitor tracked both completed block files and in-flight temporary files. Across 41,334 filesystem events, observed physical cache high-water was 8,589,111,296 bytes against an 8,589,934,592-byte cap: 823,296 bytes under, with zero observed violations. This directly answers the pending-byte question: temporary writes did not push the namespace over capacity. The monitor emitted 7,275 coverage warnings because it was attached to an already populated dynamic directory tree; that limits the monitor formal verdict but does not change the measured ordered high-water. Together with the 71 focused tests, concurrent-writer reservation tests, startup indexing tests, and sustained runtime turnover, the pending runtime gate is complete and this PR is ready for review. |
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🧹 Nitpick comments (2)
vllm/v1/kv_offload/tiering/fs/manager.py (2)
449-484: 🚀 Performance & Scalability | 🔵 Trivial | 💤 Low valueEviction loop is O(K·N) on the startup-trim path.
list(self._cache_lru.items())is rebuilt on every outerwhileiteration and the inner loop breaks after a single eviction, so trimming K blocks from an N-entry index costs O(K·N). Runtime stores evict ~1 block each (fine), but trimming a large oversized namespace at startup can be slow. Since the LRU is oldest-first and no pins exist during initialization, you could iterate the ordered view once. Optional.🤖 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 `@vllm/v1/kv_offload/tiering/fs/manager.py` around lines 449 - 484, Optimize _evict_until_fits_locked by avoiding reconstruction of self._cache_lru.items() and restarting the scan after each eviction. Preserve oldest-first eviction and existing pinned-entry and filesystem-error handling, while using a single ordered traversal during startup trimming; retain the current failure result when no evictable entry can satisfy required_bytes.
251-260: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick winAdd
strict=to all fourzip(job_metadata.keys, job_metadata.block_ids)calls (ruff B905). These pair keys with their block offsets and are expected to be equal-length;strict=Trueclears the lint gate and turns any future length mismatch into a loud error instead of a silently short task set.
vllm/v1/kv_offload/tiering/fs/manager.py#L251-L260: addstrict=Trueto bothzip()calls insubmit_store.vllm/v1/kv_offload/tiering/fs/manager.py#L275-L275: addstrict=Trueto thezip()in the unboundedsubmit_loadbranch.vllm/v1/kv_offload/tiering/fs/manager.py#L296-L296: addstrict=Trueto thezip()in the boundedsubmit_loadbranch.Note
zip(strict=...)requires Python 3.10+; confirm the repo's declared target and rufftarget-versionallow it.🤖 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 `@vllm/v1/kv_offload/tiering/fs/manager.py` around lines 251 - 260, Update both zip calls in submit_store and the zip calls in the unbounded and bounded submit_load branches in vllm/v1/kv_offload/tiering/fs/manager.py (lines 251-260, 275, and 296) to use strict=True, preserving the existing key/block pairing. Confirm the repository Python target and Ruff target-version support Python 3.10 or newer.Source: Linters/SAST tools
🤖 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.
Nitpick comments:
In `@vllm/v1/kv_offload/tiering/fs/manager.py`:
- Around line 449-484: Optimize _evict_until_fits_locked by avoiding
reconstruction of self._cache_lru.items() and restarting the scan after each
eviction. Preserve oldest-first eviction and existing pinned-entry and
filesystem-error handling, while using a single ordered traversal during startup
trimming; retain the current failure result when no evictable entry can satisfy
required_bytes.
- Around line 251-260: Update both zip calls in submit_store and the zip calls
in the unbounded and bounded submit_load branches in
vllm/v1/kv_offload/tiering/fs/manager.py (lines 251-260, 275, and 296) to use
strict=True, preserving the existing key/block pairing. Confirm the repository
Python target and Ruff target-version support Python 3.10 or newer.
ℹ️ Review info
⚙️ Run configuration
Configuration used: Organization UI
Review profile: CHILL
Plan: Pro Plus
Run ID: 366362b8-2862-4461-8c31-82d4cafdc2d3
📒 Files selected for processing (3)
docs/features/kv_offloading_usage.mdtests/v1/kv_offload/tiering/test_fs_tier.pyvllm/v1/kv_offload/tiering/fs/manager.py
1 participant
Summary
Why
The filesystem tier currently grows without a configured retention bound. On a local NVMe tier this can fill the device and turn normal KV churn into ENOSPC failures outside the cache policy.
Bounded mode evicts the least-recently-used unpinned block before admitting a store. If every resident block is pinned by an active request or load job, that store fails with ENOSPC instead of evicting an in-use source or exceeding the configured limit. Leaving max_cache_size_bytes unset preserves the existing path.
This does not duplicate an open downstream or upstream capacity PR. The open upstream worker-transfer and CPU-policy changes address different tiers and ownership layers; none provides a byte bound for the local filesystem tier.
Forward-port proof
The current dev/gilded-gnosis versions of all three input files were byte-for-byte identical to the v19 inputs for commit d74c0a6. This commit is therefore an exact replay with no conflict resolution or semantic adaptation. The resulting manager, tests, and documentation are also byte-identical to the previously prepared outputs.
Validation on the v20 tree:
Model evaluation is not applicable because this changes filesystem cache retention, not model computation or outputs. Store/load round-trip and promotion-race behavior are covered by the focused tests.
Pending runtime gate
Before moving out of draft, run the prepared CN3 v20 acceptance gate with a small bounded NVMe tier: boot, fill beyond the configured limit, prove LRU eviction under overlapping store/load pressure, verify no in-use source is reclaimed, restart, and prove persisted-index promotion. Runtime evidence and image/source pins will be added here.
AI assistance
OpenAI Codex assisted with investigation, forward-port verification, testing, and PR drafting. The human submitter will review every changed line and complete the target-system runtime gate before requesting merge.
Summary by CodeRabbit
New Features
max_cache_size_bytes.Documentation