# 检查内存使用情况

要在performance\_schema中启用内存监测，请启用 <mark style="color:blue;">**memory**</mark> 类的插桩。启用后就可以查看MySQL内部结构如何使用内存的详细信息。

```sql
UPDATE setup_instruments
SET enabled = 'YES'
WHERE name LIKE 'memory/%'
  AND enabled = 'NO';
```

## 直接使用performance schema

Performance Schema将内存使用统计信息存储在摘要表中，摘要表的名称以 <mark style="color:blue;">**memory\_summary\_**</mark> 前缀开头。内存使用聚合统计，其参数如下表所示：

<table><thead><tr><th width="423">聚合表</th><th>描述</th></tr></thead><tbody><tr><td><strong>memory_summary_global_by_event_name</strong></td><td>按事件名全局聚合</td></tr><tr><td><strong>memory_summary_by_thread_by_event_name</strong></td><td>按线程聚合：包括后台线程和用户线程</td></tr><tr><td><strong>memory_summary_by_account_by_event_name</strong></td><td>按用户账号聚合</td></tr><tr><td><strong>memory_summary_by_host_by_event_name</strong></td><td>按主机聚合</td></tr><tr><td><strong>memory_summary_by_user_by_event_name</strong></td><td>按用户名聚合</td></tr></tbody></table>

例如，要找到占用大部分内存的 InnoDB 结构，可以执行以下查询：

```sql
SELECT
    event_name,
    current_number_of_bytes_used / 1024 / 1024 AS current_mb,
    high_number_of_bytes_used / 1024 / 1024    AS high_mb
FROM memory_summary_global_by_event_name
WHERE event_name LIKE 'memory/innodb/%'
ORDER BY current_number_of_bytes_used DESC
LIMIT 10;
```

## 使用sys schema

使用sys schema中的视图可以更好地获取内存统计信息，可以按 host、user、thread 或 global 进行聚合。

```sql
SHOW TABLES FROM sys LIKE 'memory_%';

+---------------------------------+
|Tables_in_sys (memory_%)         |
+---------------------------------+
|memory_by_host_by_current_bytes  |
|memory_by_thread_by_current_bytes|
|memory_by_user_by_current_bytes  |
|memory_global_by_current_bytes   |
|memory_global_total              |
+---------------------------------+
```

<mark style="color:blue;">**memory\_global\_total**</mark> 视图包含一个单独的值，显示被监测内存的总量：

```sql
SELECT * FROM sys.memory_global_total;

+---------------+
|total_allocated|
+---------------+
|143.44 MiB     |
+---------------+
```


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