作者：周启超
爱可生北分团队 DBA，主要负责项目前期建设及后期疑难问题支持。做事认真，对事负责。
本文来源：原创投稿
*爱可生开源社区出品，原创内容未经授权不得随意使用，转载请联系小编并注明来源。
先介绍下背景，应用连接数据数执行任务,报 error 1135: Can’t create a new thread (errno 11) 错误日志信息如下：
2021-08-11T12:25:40.606774+08:00 0 [ERROR] [MY-000000] [connection_h] Error log throttle: 36 'Can't create thread to handle new connection'
error(s) suppressed
2021-08-11T12:25:40.606886+08:00 0 [ERROR] [MY-010249] [Server] Can't create thread to handle new connection(errno= 11)
环境版本为 SLES12SP5 ，MySQL 版本为8.0.18。
看到“Can’t create thread to handle new connection”的描述我们首先会想到操作系统对应用户的 ulimit 是否正确。和用户确认了下 ulimit 的确是正常配置的。如果幸运的话，我们可以在网上查到一些 TaskMax 设置导致 MySQL 连接异常的文章。
这次这个环境问题的确和 TaskMax 有关，观察如下截图可以看到 Tasks 达到了 limit 限制。
找到问题所在进行修复。网上有资料说 TaskMax 有两种方式进行修复，一种系统级别和一种进程级别，“系统级别不用进行 MySQL 服务重启，进程级别修改需要进行 MySQL 服务重启”。由于是生产环境，而且是专用服务器，想选择对现有服务影响最小的原则，选择了系统级别调整。
# systemctl show --property=DefaultTasksMax
DefaultTasksMax=512
# vi /etc/systemd/system.conf
#DefaultTasksMax=512
DefaultTasksMax=5120
# systemctl daemon-reexec
# systemctl show --property=DefaultTasksMax
DefaultTasksMax=5120
# systemctl status mysql
[0m mysql.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysql.service; enabled; vendor preset: disabled)
Active: active (running) since Wed 2021-08-11 12:09:49 CST; 23min ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Main PID: 2585 (mysqld)
Status: "Server is operational"
Tasks: 62 (limit: 5120)
CGroup: /system.slice/mysql.service
85 /usr/sbin/mysqld
Aug 11 12:09:48 linux systemd[1]: Starting MySQL Server...
Aug 11 12:09:49 linux systemd[1]: Started MySQL Server.
看 systemctl status mysql 输出 Tasks 一项 limit 的确更新成最新的值了。看到状态值更新了，但也不要掉以轻心。
测试环境 sysbench 模拟一下并发，发现问题依然在：
FATAL: unable to connect to MySQL server on host '10.186.63.149', port 3306, aborting...
FATAL: error 1135: Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug
FATAL: `thread_init' function failed: /usr/share/sysbench/oltp_common.lua:349: connection creation failed
重启 MySQL 服务后，状态正常：
[ 3s ] thds: 600 tps: 563.43 qps: 9803.74 (r/w/o: 8587.12/0.00/1216.62) lat (ms,95%): 1427.08 err/s: 0.00 reconn/s: 0.00
[ 9s ] thds: 600 tps: 3031.18 qps: 48139.37 (r/w/o: 42087.54/0.00/6051.82) lat (ms,95%): 831.46 err/s: 0.00 reconn/s: 0.00
[ 12s ] thds: 600 tps: 1593.91 qps: 25684.59 (r/w/o: 22491.10/0.00/3193.49) lat (ms,95%): 694.45 err/s: 0.00 reconn/s: 0.00
MySQL status 里 Task 状态可以突破原来512的限制：
# systemctl status mysql
[0m mysql.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysql.service; enabled; vendor preset: disabled)
Active: active (running) since Wed 2021-08-11 12:35:12 CST; 3min 30s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Process: 5839 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
Main PID: 5861 (mysqld)
Status: "Server is operational"
Tasks: 649 (limit: 5120)
CGroup: /system.slice/mysql.service
61 /usr/sbin/mysqld
Aug 11 12:35:09 linux systemd[1]: Starting MySQL Server...
Aug 11 12:35:12 linux systemd[1]: Started MySQL Server.
进程级别调整方式验证：
# cd /etc/systemd/system/
# mkdir mysql.service.d
# cd mysql.service.d/
# vi override.conf
[Service]
TasksMax=10000
# systemctl daemon-reload
# systemctl status mysql
[0m mysql.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysql.service; enabled; vendor preset: disabled)
Drop-In: /etc/systemd/system/mysql.service.d
erride.conf
Active: active (running) since Wed 2021-08-11 12:44:27 CST; 1min 51s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Main PID: 8229 (mysqld)
Status: "Server is operational"
Tasks: 108 (limit: 10000)
CGroup: /system.slice/mysql.service
29 /usr/sbin/mysqld
Aug 11 12:44:25 linux systemd[1]: Starting MySQL Server...
Aug 11 12:44:27 linux systemd[1]: Started MySQL Server.
重启 MySQL 服务生效（Tasks 中 limit 限制未生效，现象同系统级别调整）：
# systemctl stop mysql
# systemctl start mysql
# systemctl status mysql
[0m mysql.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysql.service; enabled; vendor preset: disabled)
Drop-In: /etc/systemd/system/mysql.service.d
erride.conf
Active: active (running) since Wed 2021-08-11 12:49:56 CST; 13s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Process: 11971 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
Main PID: 11993 (mysqld)
Status: "Server is operational"
Tasks: 50 (limit: 10000)
CGroup: /system.slice/mysql.service
993 /usr/sbin/mysqld
Aug 11 12:49:54 linux systemd[1]: Starting MySQL Server...
Aug 11 12:49:56 linux systemd[1]: Started MySQL Server.
确认系统级别参数没有更改：
# systemctl show --property=DefaultTasksMax
DefaultTasksMax=512
跑600并发 sysbench ，验证结果：
[ 3s ] thds: 600 tps: 185.49 qps: 3283.97 (r/w/o: 2868.95/0.00/415.02) lat (ms,95%): 1589.90 err/s: 0.00 reconn/s: 0.00
[ 15s ] thds: 600 tps: 9545.12 qps: 152777.72 (r/w/o: 133709.19/0.00/19068.53) lat (ms,95%): 1327.91 err/s: 0.00 reconn/s: 0.00
[ 18s ] thds: 600 tps: 1495.99 qps: 24004.40 (r/w/o: 21029.73/0.00/2974.66) lat (ms,95%): 694.45 err/s: 0.00 reconn/s: 0.00
突破原来了原来 Task 的512限制：
# systemctl status mysql
[0m mysql.service - MySQL Server
Loaded: loaded (/usr/lib/systemd/system/mysql.service; enabled; vendor preset: disabled)
Drop-In: /etc/systemd/system/mysql.service.d
erride.conf
Active: active (running) since Wed 2021-08-11 12:49:56 CST; 1min 8s ago
Docs: man:mysqld(8)
http://dev.mysql.com/doc/refman/en/using-systemd.html
Process: 11971 ExecStartPre=/usr/bin/mysqld_pre_systemd (code=exited, status=0/SUCCESS)
Main PID: 11993 (mysqld)
Status: "Server is operational"
Tasks: 649 (limit: 10000)
CGroup: /system.slice/mysql.service
993 /usr/sbin/mysqld
Aug 11 12:49:54 linux systemd[1]: Starting MySQL Server...
Aug 11 12:49:56 linux systemd[1]: Started MySQL Server.
验证结论：系统级别和进程级别设置方式2选1即可，但是都需要重启 MySQL 服务生效。