Proof of concept: task eviction after snapshot for turbo-tasks-backend#91790
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The macro emitted `if !drop_partial() { reset; }` for filter_transient
inline fields, which destroyed transient residue (HasResidue branch
unconditionally cleared the field). After eviction the upper-edge to a
transient HMR root was wiped, so dirty propagation from a file change
never reached an active root and HMR didn't fire.
Replace the bool return with `DropPartialOutcome { Empty, HasResidue }`,
make `DropPartial` a pub(crate) trait that CellData also implements
(was duck-typed before), and have `TaskStorage::drop_partial` return
the outcome authoritatively — the eviction caller no longer needs a
separate `is_empty()` query. Add per-category `is_empty_data` /
`is_empty_meta` / `is_empty_transient` helpers so the final
emptiness check skips the categories the drop just cleared.
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Summary
Implements memory eviction for the turbo-tasks engine. After a persistence snapshot completes, tasks that are safe to remove are evicted from in-memory storage and transparently restored from disk on next access.
Eviction levels
current_session_clean, aggregated session-clean counts).Data and meta evictability are computed independently — if one category is modified but the other is clean, the clean category can still be dropped.
Eviction is gated behind
BackendOptions::evict_after_snapshot(off by default), and can be enabled in Next.js via theTURBO_ENGINE_EVICT_AFTER_SNAPSHOT=1env var for testing.Key changes
Orthogonal eviction decision tree (
storage_schema.rs): Data and meta evictability are computed independently. Full eviction additionally requires no meaningful transient state (session-clean flags, aggregated session-clean counts). Replaces the previous sequential bail-out approach which was too aggressive on full eviction (losing transient session state on leaf tasks) and not aggressive enough on partial eviction (blocking all eviction when only one category was modified).drop_partial()codegen (task_storage_macro.rs): New generated methods to drop datarestore_from_*()codegen changes (task_storage_macro.rs): New semantics for merging persistent data from the backend with transient data stored in memory.task_cachemoved intoStorage(storage.rs): TheCachedTaskType → TaskIddeduplication map was previously a separate field onTurboTasksBackendInner. It is now owned byStorageso eviction can remove entries when a task is fully evicted. Becausetask_cacheis a pure performance cache (entries are re-populated bytask_by_type()on miss once the task type is persisted to backing storage), evicting entries is safe. After bulk eviction the map is shrunk when it is less than half full.Parallel shard eviction (
storage.rs): Eviction iterates all storage shards in parallel after snapshot, applying the appropriate eviction level per task. Each shard is shrunk after bulk eviction to reclaim slack capacity.Design notes
current_session_cleanis set we prevent full eviction to avoid rechecking. Within a session the file-watchers are responsible for invalidations after settingcurrent_session_clean.Known limitations (proof of concept)