Module tiering 

Enhanced temporal scoring with EMA + popcount + recency, hysteresis, and budgeted maintenance (ADR-020).

Scoring Formula

score = w_ema * ema_rate
      + w_pop * (popcount(access_window) / 64)
      + w_rec * exp(-dt / tau)

Where dt = now - last_access and tau is the recency decay constant.

Hysteresis

To prevent tier oscillation, upgrades require the score to exceed the threshold by the hysteresis margin, and downgrades require the score to fall below the threshold by the same margin. A minimum residency period further dampens churn.

Types

The types BlockKey, BlockMeta, and Tier are defined here for self-containment while store.rs is developed in parallel. Once crate::store lands, replace these definitions with:

use crate::store::{BlockKey, BlockMeta, Tier};

Structs

BlockKey

Opaque block identifier.

BlockMeta

Per-block metadata tracked by the tiered store.

MaintenanceResult

Result of a maintenance tick operation.

MigrationCandidate

Candidate for tier migration during maintenance.

ScoredPartition

Result of scoring and partitioning blocks into tier buckets.

TierConfig

Enhanced tier policy with EMA + popcount + recency scoring.

Enums

Tier

Storage tier for a block.

Functions

bits_for_tier

Get the quantization bit width for a tier.

choose_tier

Choose the target tier for a block, applying hysteresis and residency.

choose_tiers_batch

Compute tier decisions for many blocks at once.

compute_score

Compute the composite score for a block.

compute_scores_batch

Compute scores for many blocks at once.

score_and_partition

Score blocks and partition into hot/warm/cold/evict buckets based on raw score thresholds.

select_candidates

Select blocks that need tier migration.

tick_decay

Decay EMA for blocks not accessed in the current tick.

top_k_coldest

Find the k blocks with the lowest scores (useful for eviction).

touch

Record an access event on a block’s metadata.