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issue: #46540 Empty timetick is just used to sync up the time clock between different component in milvus. So empty timetick can be ignored if we achieve the lsn/mvcc semantic for timetick. Currently, some components need the empty timetick to trigger some operation, such as flush/tsafe. So we only slow down the empty time tick for 5 seconds. <!-- This is an auto-generated comment: release notes by coderabbit.ai --> - Core invariant: with LSN/MVCC semantics consumers only need (a) the first timetick that advances the latest-required-MVCC to unblock MVCC-dependent waits and (b) occasional periodic timeticks (~≤5s) for clock synchronization—therefore frequent non-persisted empty timeticks can be suppressed without breaking MVCC correctness. - Logic removed/simplified: per-message dispatch/consumption of frequent non-persisted empty timeticks is suppressed — an MVCC-aware filter emptyTimeTickSlowdowner (internal/util/pipeline/consuming_slowdown.go) short-circuits frequent empty timeticks in the stream pipeline (internal/util/pipeline/stream_pipeline.go), and the WAL flusher rate-limits non-persisted timetick dispatch to one emission per ~5s (internal/streamingnode/server/flusher/flusherimpl/wal_flusher.go); the delegator exposes GetLatestRequiredMVCCTimeTick to drive the filter (internal/querynodev2/delegator/delegator.go). - Why this does NOT introduce data loss or regressions: the slowdowner always refreshes latestRequiredMVCCTimeTick via GetLatestRequiredMVCCTimeTick and (1) never filters timeticks < latestRequiredMVCCTimeTick (so existing tsafe/flush waits stay unblocked) and (2) always lets the first timetick ≥ latestRequiredMVCCTimeTick pass to notify pending MVCC waits; separately, WAL flusher suppression applies only to non-persisted timeticks and still emits when the 5s threshold elapses, preserving periodic clock-sync messages used by flush/tsafe. - Enhancement summary (where it takes effect): adds GetLatestRequiredMVCCTimeTick on ShardDelegator and LastestMVCCTimeTickGetter, wires emptyTimeTickSlowdowner into NewPipelineWithStream (internal/util/pipeline), and adds WAL flusher rate-limiting + metrics (internal/streamingnode/server/flusher/flusherimpl/wal_flusher.go, pkg/metrics) to reduce CPU/dispatch overhead while keeping MVCC correctness and periodic synchronization. <!-- end of auto-generated comment: release notes by coderabbit.ai --> --------- Signed-off-by: chyezh <chyezh@outlook.com>