MultiXactMemberSLRU

Mechanism

This lock protects the SLRU cache for pg_multixact/members, which stores the transaction members of multitransaction IDs used by shared row locks. Concurrent row-lock creation and old member lookups meet here.

Expected behavior

Brief waits occur in workloads that use SELECT FOR SHARE/KEY SHARE or create multixacts through foreign-key checks.

When to investigate

Sustained contention points to heavy shared row locking, multixact churn, lagging freeze, or slow pg_multixact storage.

Diagnostic actions

• Find statements and tables creating many shared row locks.
• Check multixact age and autovacuum progress.
• Reduce lock fan-out and unblock multixact freeze; investigate member SLRU I/O if paired with buffer waits.

Incident patterns

A high-fan-out foreign-key workload locks many parent rows while a long transaction delays multixact cleanup, driving member-cache contention.

Diagnostic queries

Sessions waiting on LWLock/MultiXactMemberSLRU

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 = 'MultiXactMemberSLRU'
ORDER BY query_age DESC NULLS LAST;

Current LWLock cohort

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;

Lock and relation context for these sessions

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 = 'MultiXactMemberSLRU'
ORDER BY a.pid, l.granted, l.locktype, l.mode;