LWLock: SerializablePredicateList

Official description

Waiting to access the list of predicate locks held by serializable transactions

Trigger mechanism

pgstat_report_wait_start at src/backend/storage/lmgr/lwlock.c:739 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/storage/lmgr/predicate.c:2220. The instrumented operation is: Waiting to access the list of predicate locks held by serializable transactions. LWLockAcquire could not immediately take the lightweight-lock tranche displayed as SerializablePredicateList. The generic LWLock reporter publishes the tranche name while the backend sleeps on the internal shared-memory resource.

Normal or trouble?

• Normal: A brief sample is normal around short internal critical sections.
• Investigate: Investigate when the same tranche affects at least 10% of active foreground sessions in three consecutive one-second samples; treat 25% or waits beyond five seconds as urgent.

Diagnostic SQL

SELECT pid, backend_type, usename, datname, application_name,
       state, now() - query_start AS query_age,
       now() - xact_start AS xact_age,
       wait_event_type, wait_event,
       pg_blocking_pids(pid) AS blocking_pids,
       left(query, 160) AS query
FROM pg_stat_activity
WHERE wait_event_type = 'LWLock'
  AND wait_event = 'SerializablePredicateList'
ORDER BY query_age DESC NULLS LAST;

SELECT wait_event, count(*) AS waiting_sessions,
       count(*) FILTER (WHERE state = 'active') AS active_waiters,
       max(now() - query_start) AS oldest_query
FROM pg_stat_activity
WHERE wait_event_type = 'LWLock'
GROUP BY wait_event
ORDER BY waiting_sessions DESC, wait_event;

SELECT a.pid, l.locktype, l.mode, l.granted, l.fastpath,
       d.datname, n.nspname, c.relname,
       l.page, l.tuple, l.virtualxid, l.transactionid,
       l.classid, l.objid, l.objsubid
FROM pg_stat_activity AS a
LEFT JOIN pg_locks AS l ON l.pid = a.pid
LEFT JOIN pg_database AS d ON d.oid = l.database
LEFT JOIN pg_class AS c
  ON c.oid = l.relation
 AND l.database = (
       SELECT oid FROM pg_database WHERE datname = current_database()
     )
LEFT JOIN pg_namespace AS n ON n.oid = c.relnamespace
WHERE a.wait_event_type = 'LWLock'
  AND a.wait_event = 'SerializablePredicateList'
ORDER BY a.pid, l.granted, l.locktype, l.mode;

Response

1. Repeat the snapshot and isolate one hot tranche.
2. Correlate it with the protected resource and current workload phase.
3. Reduce the specific contention source; adding concurrency can make a shared-memory hotspot worse.

Source evidence

• PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:739 — pgstat_report_wait_start
• PostgreSQL 18.6 · src/backend/storage/lmgr/predicate.c:2220 — SerializablePredicateList
• PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:336 — SerializablePredicateList

Related controls and signals

• GUCs: None directly.
• Metrics: [metric:waiting_sessions] · [metric:wait_event_share]