BufferContent

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.

Expected behavior

Short samples are routine while concurrent readers and writers touch shared pages.

When to 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 actions

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

Incident patterns

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

Diagnostic queries

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;