# LWLock: OldSnapshotTimeMap

> 等待读取或更新旧快照控制信息。
---

<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>OldSnapshotTimeMap</code></strong></span>
  <span>版本<strong>PG 13-16</strong></span>
  <span>证据<strong>2 个源码位置</strong></span>
</div>

## 官方描述译文 {#official-description}

等待读取或更新旧快照控制信息。

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

## 触发机制 {#mechanism}

`pgstat_report_wait_start` 位于 `src/backend/storage/lmgr/lwlock.c:750`，是经 grep 核验的报告路径。公共身份/资源映射位于 `src/backend/utils/time/snapmgr.c:1758`。探针覆盖的操作是：等待读取或更新旧快照控制信息。LWLockAcquire 无法立即取得显示为 OldSnapshotTimeMap 的轻量级锁 tranche；后端在内部共享内存资源上休眠时，通用 LWLock 报告器发布该 tranche 名称。

## 正常还是麻烦？ {#normal-vs-trouble}

- <span class="severity-normal">**正常:**</span> 短暂内部临界区周围的零星样本属于正常。
- <span class="severity-watch">**需要调查:**</span> 若同一 tranche 在连续三次一秒采样中影响至少 10% 的活动前台会话，应当调查；达到 25% 或持续超过五秒按紧急处理。

## 诊断 SQL {#diagnostic-sql}

```sql {title="正在等待 LWLock/OldSnapshotTimeMap 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 = 'OldSnapshotTimeMap'
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 = 'OldSnapshotTimeMap'
ORDER BY a.pid, l.granted, l.locktype, l.mode;
```

## 处置建议 {#response}

1. 重复采样并锁定一个热点 tranche。
2. 把它与受保护资源及当前负载阶段关联。
3. 降低具体竞争源；增加并发可能让共享内存热点更糟。

## 源码证据 {#source}

- [PostgreSQL 16.15 · src/backend/storage/lmgr/lwlock.c:750](https://github.com/postgres/postgres/blob/REL_16_15/src/backend/storage/lmgr/lwlock.c#L750) — `pgstat_report_wait_start`
- [PostgreSQL 16.15 · src/backend/utils/time/snapmgr.c:1758](https://github.com/postgres/postgres/blob/REL_16_15/src/backend/utils/time/snapmgr.c#L1758) — `OldSnapshotTimeMap`

## 关联控制项与信号 {#related}

- **GUC:** 无直接关联项。
- **指标:** [[metric:waiting_sessions]](/zh/metric/waiting-sessions/) · [[metric:wait_event_share]](/zh/metric/wait-event-share/)