# LWLock: BufferMapping

> 等待将数据块关联到 buffer pool 中的 buffer。
---

<div class="event-kicker">PostgreSQL 等待事件档案</div>
<div class="event-identity">
  <span>类别<strong>LWLock</strong></span>
  <span>事件<strong><code>BufferMapping</code></strong></span>
  <span>版本<strong>PG 13-18</strong></span>
  <span>证据<strong>3 个源码位置</strong></span>
</div>

## 官方描述译文 {#official-description}

等待将数据块关联到 buffer pool 中的 buffer。

| PG 13 | PG 14 | PG 15 | PG 16 | PG 17 | PG 18 |
| :---: | :---: | :---: | :---: | :---: | :---: |
| ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |

## 触发机制 {#mechanism}

缓冲区管理器把 relation/fork/block 标签哈希到由 BufferMappingLock 保护的分区。查找、插入、淘汰和重新分配标签都会短暂取得该分区锁；大规模并发未命中或缓冲区抖动会增加碰撞。

## 正常还是麻烦？ {#normal-vs-trouble}

- <span class="severity-normal">**正常:**</span> 页面进入或离开共享缓冲区时，短暂等待属于正常现象。
- <span class="severity-watch">**需要调查:**</span> 持续出现通常意味着工作集反复冲刷 shared buffers、大量并行扫描，或访问集中映射到少数分区。

## 诊断 SQL {#diagnostic-sql}

```sql {title="正在等待 LWLock/BufferMapping 的会话"}
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 = 'BufferMapping'
ORDER BY query_age DESC NULLS LAST;
```

```sql {title="当前 LWLock 等待分布"}
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;
```

```sql {title="这些会话的锁与关系上下文"}
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 = 'BufferMapping'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## 处置建议 {#response}

1. 按数据库与语句关联缓冲命中率和读取量。
2. 检查是否出现新扫描、缓存过小或并发突增。
3. 先降低并发扫描扇出或修正访问路径，再决定是否扩大 shared_buffers。

## 源码证据 {#source}

- [PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:152](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/storage/lmgr/lwlock.c#L152) — `BufferMapping`
- [PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:739](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/storage/lmgr/lwlock.c#L739) — `pgstat_report_wait_start`
- [PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:377](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L377) — `BufferMapping`

## 关联控制项与信号 {#related}

- **GUC:** [[guc:shared_buffers]](/zh/guc/shared-buffers/) · [[guc:max_parallel_workers_per_gather]](/zh/guc/max-parallel-workers-per-gather/)
- **指标:** [[metric:waiting_sessions]](/zh/metric/waiting-sessions/) · [[metric:wait_event_share]](/zh/metric/wait-event-share/) · [[metric:buffer_hit_ratio]](/zh/metric/buffer-hit-ratio/) · [[metric:blocks_read]](/zh/metric/blocks-read/)

## 典型事故模式 {#incident}

执行计划退化引发大量并发大扫描，缓冲映射分区变热，同时有用页面被挤出缓存。

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反链：

- [LWLock](/zh/lwlock/)