The _L0BlockState dataclass should include a release_time field to accurately track the idle time of a block before it is evicted. Currently, idle_time is calculated using last_access_time, which represents the start of the last usage period, not the end of it.

When a block is marked as RELEASED, the current timestamp should be recorded as the release_time to allow for accurate idle time calculation upon eviction.

Using object as a type hint for record and then using # type: ignore[attr-defined] is not ideal for type safety. It is better to define a Protocol that specifies the expected attributes of the record object, especially since BlockAllocationRecord is defined in another module and we want to avoid tight coupling or circular dependencies.

from typing import Protocol

class _BlockAllocationRecord(Protocol):
    req_id: str
    new_block_ids: list[int]
    new_token_ids: list[int]

    def _process_record(self, record: _BlockAllocationRecord, now: float) -> None:
        """Process a single BlockAllocationRecord."""
        req_id: str = record.req_id
        block_ids: list[int] = record.new_block_ids
        token_ids: list[int] = record.new_token_ids

Initializing access_history with the current timestamp now ensures that the gap between the initial allocation and the first reuse is captured in the lmcache_mp.l0_block_reuse_gap_seconds metric. Without this, the first reuse gap is missed.

When a block transitions from RELEASED back to ACTIVE (Case 4: true reuse), the release_time should be cleared.

Similar to the initial allocation, when starting fresh after an eviction, access_history should be initialized with now to capture the first reuse gap of the new block lifecycle.

The idle_time should be calculated as the duration between the block's release and its eviction. If the block was evicted while still active (which is rare in vLLM), the idle time can be considered zero or calculated from the last access start.

Token-to-block chunking uses incorrect distribution logic

Medium Severity

_process_record splits new_token_ids across blocks using ceiling division ((len(token_ids) + num_blocks - 1) // num_blocks), but vLLM uses a fixed block size (e.g., 16 tokens per block, with only the last block potentially shorter). If the same physical block appears in a later record with a different num_blocks, the subscriber computes a different token chunk for that block, causing existing.token_ids == token_ids to fail and triggering a false eviction detection (Case 5). This would inflate eviction metrics. A safer approach would be to use the engine's actual block size for chunking rather than inferring it from the record.

 

^{Reviewed by Cursor Bugbot for commit 0f34708. Configure here.}

Tests rely on private members instead of public interface

Low Severity

The test file imports the module-private _BlockStatus and extensively accesses subscriber._shadow (a private attribute) throughout many test methods. The project rules require no cross-class private member access and that tests verify the public interface and docstring contract rather than implementation details. The OTel histogram assertions already validate behavior through the public output; the _shadow assertions couple tests to internal state.

 

^{Triggered by project rule: LMCache Code Review Style Guide}

^{Reviewed by Cursor Bugbot for commit 0f34708. Configure here.}

If sampling rate is very small in production, it's better to record "non-skipped" because that will be smaller

Skipped set stores 99% of block IDs unnecessarily

Low Severity

The _skipped set records every block ID that was not sampled. With the default 1% sample_rate, this set holds ~99% of all unique block IDs, while _shadow (which already tracks sampled blocks) holds ~1%. Since _shadow.keys() already identifies sampled blocks, the _skipped set is an inverted representation that uses ~99× more memory than needed. Checking block_id not in self._shadow instead would eliminate this set entirely.

 

^{Triggered by project rule: LMCache Code Review Style Guide}

^{Reviewed by Cursor Bugbot for commit af7d40b. Configure here.}