Pre*_*ton 6 mysql max-connections
什么因素决定了 mysqld 将创建的最大可能线程数？
我只关心对 mysql 的考虑，而不关心操作系统可能施加的限制。此外，我只关心 Linux，但如果您想包含特定于 Windows 的信息，请随意。
据我所知，可以为复制创建线程，innodb 的各个方面，每个客户端连接最多一个 max_connections，还有一个额外的 +1 来处理超级连接。还有什么，或者就是这样？
threads_max = max_connections
+ 1 for super
+ 2 for replication slave threads
+ ? for InnoDB ?
查看当前线程（包括所有前台和后台线程）的最佳方式是使用performance_schema. 例如：
mysql 5.7.3> select thread_id, name, type, PROCESSLIST_ID from performance_schema.threads;
+-----------+----------------------------------------+------------+----------------+
| thread_id | name | type | PROCESSLIST_ID |
+-----------+----------------------------------------+------------+----------------+
| 1 | thread/sql/main | BACKGROUND | NULL |
| 2 | thread/innodb/io_log_thread | BACKGROUND | NULL |
| 3 | thread/innodb/io_read_thread | BACKGROUND | NULL |
| 4 | thread/innodb/io_read_thread | BACKGROUND | NULL |
| 5 | thread/innodb/io_ibuf_thread | BACKGROUND | NULL |
| 6 | thread/innodb/io_read_thread | BACKGROUND | NULL |
| 7 | thread/innodb/io_read_thread | BACKGROUND | NULL |
| 8 | thread/innodb/io_write_thread | BACKGROUND | NULL |
| 9 | thread/innodb/io_write_thread | BACKGROUND | NULL |
| 10 | thread/innodb/io_write_thread | BACKGROUND | NULL |
| 11 | thread/innodb/io_write_thread | BACKGROUND | NULL |
| 14 | thread/innodb/srv_lock_timeout_thread | BACKGROUND | NULL |
| 15 | thread/innodb/srv_error_monitor_thread | BACKGROUND | NULL |
| 16 | thread/innodb/srv_monitor_thread | BACKGROUND | NULL |
| 17 | thread/innodb/srv_master_thread | BACKGROUND | NULL |
| 18 | thread/innodb/srv_purge_thread | BACKGROUND | NULL |
| 19 | thread/innodb/page_cleaner_thread | BACKGROUND | NULL |
| 20 | thread/sql/signal_handler | BACKGROUND | NULL |
| 22 | thread/sql/one_connection | FOREGROUND | 2 |
+-----------+----------------------------------------+------------+----------------+
19 rows in set (0.00 sec)
我相信后台线程的数量是固定的（实际数量将取决于一些配置设置，例如innodb_read_io_threads和数量innodb_purge_threads等。） FOREGROUND 线程是每个连接一个 + 最重要的潜力thread_cache_size。
例如在 Linux 上，以较小者为准：
grep -F 'Max process' /proc/$(/sbin/pidof mysqld)/limits|awk '{print $3}'
和：
mysql -e'show global variables like "max_connections"'
更新：正如您正确指出的，我们应该至少将后台线程添加到列表中。更多信息如下。
就您的上次更新而言，对于 InnoDB，我们有 innodb_read_io_threads 和 innodb_write_io_threads。
我刚刚在 debian 上检查了一个 MySQL 5.5.34，我们有默认值：
mysql> pager grep -i thread
PAGER set to 'grep -i thread'
mysql> show engine innodb status\G
BACKGROUND THREAD
srv_master_thread loops: 1 1_second, 1 sleeps, 0 10_second, 1 background, 1 flush
srv_master_thread log flush and writes: 2
MySQL thread id 36, OS thread handle 0xa6c32b40, query id 105 localhost root
I/O thread 0 state: waiting for i/o request (insert buffer thread)
I/O thread 1 state: waiting for i/o request (log thread)
I/O thread 2 state: waiting for i/o request (read thread)
I/O thread 3 state: waiting for i/o request (read thread)
I/O thread 4 state: waiting for i/o request (read thread)
I/O thread 5 state: waiting for i/o request (read thread)
I/O thread 6 state: waiting for i/o request (write thread)
I/O thread 7 state: waiting for i/o request (write thread)
I/O thread 8 state: waiting for i/o request (write thread)
I/O thread 9 state: waiting for i/o request (write thread)
Main thread process no. 21508, id 2758552384, state: waiting for server activity
1 row in set (0.00 sec)
mysql>
操作系统看到：
root@ubuntu:~# ps -Lp$(pidof mysqld) |wc -l
18
以 root 身份连接后，线程数保持不变（当我使另一个变得困难时，线程数增加了）：
root@ubuntu:~# mysql -proot
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 37
Server version: 5.5.34-0ubuntu0.12.04.1 (Ubuntu)
Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> \! ps -Lp$(pidof mysqld) |wc -l
18
mysql> show global status like '%thread%';
+------------------------------------------+-------+
| Variable_name | Value |
+------------------------------------------+-------+
| Delayed_insert_threads | 0 |
| Performance_schema_thread_classes_lost | 0 |
| Performance_schema_thread_instances_lost | 0 |
| Slow_launch_threads | 0 |
| Threads_cached | 0 |
| Threads_connected | 1 |
| Threads_created | 1 |
| Threads_running | 1 |
+------------------------------------------+-------+
8 rows in set (0.01 sec)
mysql> show global variables like '%thread%';
+-----------------------------------------+---------------------------+
| Variable_name | Value |
+-----------------------------------------+---------------------------+
| innodb_purge_threads | 0 |
| innodb_read_io_threads | 4 |
| innodb_thread_concurrency | 0 |
| innodb_thread_sleep_delay | 10000 |
| innodb_write_io_threads | 4 |
| max_delayed_threads | 20 |
| max_insert_delayed_threads | 20 |
| myisam_repair_threads | 1 |
| performance_schema_max_thread_classes | 50 |
| performance_schema_max_thread_instances | 1000 |
| thread_cache_size | 8 |
| thread_concurrency | 10 |
| thread_handling | one-thread-per-connection |
| thread_stack | 196608 |
+-----------------------------------------+---------------------------+
14 rows in set (0.00 sec)
所以，正如我所说，这可能是特定于版本的，但我们似乎有：
因此，如果您需要进行某种容量规划，我建议在没有用户/应用程序连接的情况下在启动后检查后台线程的数量并添加：
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