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LWLock: BufferContent

Waiting to access a data page in memory
PostgreSQL wait event dossier
ClassLWLock EventBufferContent VersionsPG 13-18 Evidence3 source location(s)

Official description

Waiting to access a data page in memory

PG 13 PG 14 PG 15 PG 16 PG 17 PG 18

Trigger mechanism

A backend has found the shared buffer it needs but cannot yet take that buffer descriptor’s content lock. Heap and index code acquire this lock before reading or changing the in-memory page, so many workers touching one page can serialize here.

Normal or trouble?

  • Normal: Short samples are routine while concurrent readers and writers touch shared pages.
  • Investigate: Repeated samples on many foreground sessions usually point to a hot heap/index page, a right-growing index, or concurrent maintenance on the same blocks.

Diagnostic SQL

Sessions waiting on LWLock/BufferContent
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 = 'BufferContent'
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 = 'BufferContent'
ORDER BY a.pid, l.granted, l.locktype, l.mode;

Response

  1. Find the relations and statements shared by the waiters.
  2. Use page/index evidence to confirm a hotspot; do not infer one from the wait name alone.
  3. Spread hot keys, batch writes, or move maintenance away from the peak before considering capacity changes.

Source evidence

Typical incident pattern

A monotonically increasing key concentrates concurrent B-tree inserts on the rightmost leaf page; BufferContent rises with insert latency.