在performance_schema有如下表记录内存使用情况
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mysql> show tables like ‘%memory%summary%‘;
+-------------------------------------------------+
| Tables_in_performance_schema (%memory%summary%) |+-------------------------------------------------+
| memory_summary_by_account_by_event_name || memory_summary_by_host_by_event_name || memory_summary_by_thread_by_event_name || memory_summary_by_user_by_event_name || memory_summary_global_by_event_name |+-------------------------------------------------+
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每个内存统计表都有如下统计列：
* COUNT_ALLOC，COUNT_FREE：对内存分配和释放内存函数的调用总次数
* SUM_NUMBER_OF_BYTES_ALLOC，SUM_NUMBER_OF_BYTES_FREE：已分配和已释放的内存块的总字节大小
* CURRENT_COUNT_USED：这是一个便捷列，等于COUNT_ALLOC - COUNT_FREE
* CURRENT_NUMBER_OF_BYTES_USED：当前已分配的内存块但未释放的统计大小。这是一个便捷列，等于SUM_NUMBER_OF_BYTES_ALLOC - SUM_NUMBER_OF_BYTES_FREE
* LOW_COUNT_USED，HIGH_COUNT_USED：对应CURRENT_COUNT_USED列的低和高水位标记
* LOW_NUMBER_OF_BYTES_USED，HIGH_NUMBER_OF_BYTES_USED：对应CURRENT_NUMBER_OF_BYTES_USED列的低和高水位标记
内存统计表允许使用TRUNCATE TABLE语句。使用truncate语句时有如下行为：
* 通常，truncate操作会重置统计信息的基准数据（即清空之前的数据），但不会修改当前server的内存分配等状态。也就是说，truncate内存统计表不会释放已分配内存
* 将COUNT_ALLOC和COUNT_FREE列重置，并重新开始计数（等于内存统计信息以重置后的数值作为基准数据）
* SUM_NUMBER_OF_BYTES_ALLOC和SUM_NUMBER_OF_BYTES_FREE列重置与COUNT_ALLOC和COUNT_FREE列重置类似
* LOW_COUNT_USED和HIGH_COUNT_USED将重置为CURRENT_COUNT_USED列值
* LOW_NUMBER_OF_BYTES_USED和HIGH_NUMBER_OF_BYTES_USED将重置为CURRENT_NUMBER_OF_BYTES_USED列值
* 此外，按照帐户，主机，用户或线程分类统计的内存统计表或memory_summary_global_by_event_name表，如果在对其依赖的accounts、hosts、users表执行truncate时，会隐式对这些内存统计表执行truncate语句
简单来说，就是可以根据用户、主机、线程、账号、全局的维度对内存进行监控。同时库sys也就这些表做了进一步的格式化，可以使得用户非常容易的观察到每个对象的内存开销：
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mysql> select event_name,current_alloc from sys.memory_global_by_current_bytes limit 10;
+-----------------------------------------------------------------------------+---------------+
| event_name | current_alloc |+-----------------------------------------------------------------------------+---------------+
| memory/performance_schema/events_statements_history_long | 13.66 MiB || memory/performance_schema/events_statements_history_long.sqltext | 9.77 MiB || memory/performance_schema/events_statements_history_long.tokens | 9.77 MiB || memory/performance_schema/events_statements_summary_by_digest.tokens | 9.77 MiB || memory/performance_schema/table_handles | 9.06 MiB || memory/performance_schema/events_statements_summary_by_thread_by_event_name | 8.67 MiB || memory/sql/String::value | 6.02 MiB || memory/performance_schema/memory_summary_by_thread_by_event_name | 5.62 MiB || memory/performance_schema/events_statements_summary_by_digest | 4.88 MiB || memory/sql/TABLE | 4.35 MiB |
+-----------------------------------------------------------------------------+---------------+
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默认情况下performance_schema只对performance_schema进行了内存开销的统计。根据你的MySQL安装代码区域可能包括performance_schema、sql、client、innodb、myisam、csv、memory、blackhole、archive、partition和其他。
查看innodb相关的内存监控是否开启，默认不开启
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mysql> SELECT * FROM performance_schema.setup_instruments
-> WHERE NAME LIKE ‘%memory%‘;
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+--------------------------------------------------------------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+--------------------------------------------------------------------------------+---------+-------+
| memory/performance_schema/mutex_instances | YES | NO |
| memory/performance_schema/rwlock_instances | YES | NO |
| memory/performance_schema/cond_instances | YES | NO |
| memory/performance_schema/file_instances | YES | NO |
| memory/performance_schema/socket_instances | YES | NO |
| memory/performance_schema/metadata_locks | YES | NO |
| memory/performance_schema/file_handle | YES | NO |
| memory/performance_schema/accounts | YES | NO |
| memory/performance_schema/events_waits_summary_by_account_by_event_name | YES | NO |
| memory/performance_schema/events_stages_summary_by_account_by_event_name | YES | NO |
| memory/performance_schema/events_statements_summary_by_account_by_event_name | YES | NO |
| memory/performance_schema/events_transactions_summary_by_account_by_event_name | YES | NO |
| memory/performance_schema/memory_summary_by_account_by_event_name | YES | NO |
| memory/performance_schema/events_stages_summary_global_by_event_name | YES | NO |
| memory/performance_schema/events_statements_summary_global_by_event_name | YES | NO |
| memory/performance_schema/memory_summary_global_by_event_name | YES | NO |
| memory/performance_schema/hosts | YES | NO |
| memory/performance_schema/events_waits_summary_by_host_by_event_name | YES | NO |
| memory/performance_schema/events_stages_summary_by_host_by_event_name | YES | NO |
| memory/performance_schema/events_statements_summary_by_host_by_event_name | YES | NO |
| memory/performance_schema/events_transactions_summary_by_host_by_event_name | YES | NO |
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可以通过条件缩小范围：
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mysql> SELECT * FROM performance_schema.setup_instruments
WHERE NAME LIKE ‘%memory/innodb%‘;
+-------------------------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+-------------------------------------------+---------+-------+
| memory/innodb/adaptive hash index | NO | NO |
| memory/innodb/buf_buf_pool | NO | NO |
| memory/innodb/dict_stats_bg_recalc_pool_t | NO | NO |
| memory/innodb/dict_stats_index_map_t | NO | NO |
| memory/innodb/dict_stats_n_diff_on_level | NO | NO |
| memory/innodb/other | NO | NO |
| memory/innodb/row_log_buf | NO | NO |
| memory/innodb/row_merge_sort | NO | NO |
| memory/innodb/std | NO | NO |
| memory/innodb/trx_sys_t::rw_trx_ids | NO | NO |
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对所有可能的对象进行内存监控。因此，还需要做下面的设置：
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mysql> update performance_schema.setup_instruments set enabled = ‘yes‘ where name like ‘memory%‘;
Query OK, 306 rows affected (0.00 sec)
Rows matched: 376 Changed: 306 Warnings: 0
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但是这种在线打开内存统计的方法仅对之后新增的内存对象有效，重启数据库后又会还原设置：
如想要对全局生命周期中的对象进行内存统计，必须在配置文件中进行设置，然后重启：
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[mysqld]performance-schema-instrument=‘memory/%=COUNTED‘
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可以使用sys库下的memory_global_by_current_bytes表来查询相同的底层数据，该模式表显示了全局服务器内当前内存使用情况，按分配类型进行细分。
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mysql> SELECT * FROM sys.memory_global_by_current_bytes
WHERE event_name LIKE ‘memory/innodb/buf_buf_pool‘\G
*************************** 1. row *************************** event_name: memory/innodb/buf_buf_pool
current_count: 1
current_alloc: 131.06 MiB
current_avg_alloc: 131.06 MiB high_count: 1
high_alloc: 131.06 MiB
high_avg_alloc: 131.06 MiB
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此sys模式查询通过current_alloc()代码区域聚合当前分配的内存：
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mysql> SELECT SUBSTRING_INDEX(event_name,‘/‘,2) AS
code_area, sys.format_bytes(SUM(current_alloc))
AS current_alloc
FROM sys.x$memory_global_by_current_bytes
GROUP BY SUBSTRING_INDEX(event_name,‘/‘,2)
ORDER BY SUM(current_alloc) DESC;
+---------------------------+---------------+
| code_area | current_alloc |+---------------------------+---------------+
| memory/innodb | 843.24 MiB || memory/performance_schema | 81.29 MiB || memory/mysys | 8.20 MiB || memory/sql | 2.47 MiB || memory/memory | 174.01 KiB || memory/myisam | 46.53 KiB || memory/blackhole | 512 bytes || memory/federated | 512 bytes || memory/csv | 512 bytes || memory/vio | 496 bytes |+---------------------------+---------------+
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总结
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