# LWLock: XactSLRU

> 等待访问事务状态 SLRU 缓存。
---

<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>XactSLRU</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}

等待访问事务状态 SLRU 缓存。

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

## 触发机制 {#mechanism}

事务状态 SLRU 锁保护 pg_xact 缓存元数据与页面，PostgreSQL 读取或更新事务提交状态时会经过这里。对未缓存或很老 XID 的可见性检查会把这条路径带到前台。

## 正常还是麻烦？ {#normal-vs-trouble}

- <span class="severity-normal">**正常:**</span> 事务结束与偶发 pg_xact 缓存未命中会带来短暂等待。
- <span class="severity-watch">**需要调查:**</span> 持续等待可能表示事务状态缓存抖动、访问很老的元组，或 pg_xact 路径存在存储延迟。

## 诊断 SQL {#diagnostic-sql}

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

## 处置建议 {#response}

1. 检查老事务以及 vacuum/freeze 健康度。
2. 关联 XactBuffer 与 pg_xact I/O 等待。
3. 先消除可见性边界阻塞者并恢复清理进度，再考虑容量调整。

## 源码证据 {#source}

- [PostgreSQL 18.6 · src/backend/storage/lmgr/lwlock.c:178](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/storage/lmgr/lwlock.c#L178) — `XactSLRU`
- [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:403](https://github.com/postgres/postgres/blob/REL_18_6/src/backend/utils/activity/wait_event_names.txt#L403) — `XactSLRU`

## 关联控制项与信号 {#related}

- **GUC:** [[guc:autovacuum_freeze_max_age]](/zh/guc/autovacuum-freeze-max-age/) · [[guc:vacuum_freeze_table_age]](/zh/guc/vacuum-freeze-table-age/)
- **指标:** [[metric:waiting_sessions]](/zh/metric/waiting-sessions/) · [[metric:wait_event_share]](/zh/metric/wait-event-share/) · [[metric:oldest_transaction_age]](/zh/metric/oldest-transaction-age/) · [[metric:xact_slru_io]](/zh/metric/xact-slru-io/)

## 典型事故模式 {#incident}

长生命周期快照阻止清理，同时查询反复访问冷的旧元组版本，引发 pg_xact 缓存抖动与 XactSLRU 竞争。

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反链：

- [LWLock](/zh/lwlock/)