第一个问题
limits.conf的限制在/proc/pid/limits中未生效

# cat /proc/3606/limits
Limit                     Soft Limit           Hard Limit           Units
Max processes             31202                31202                processes
Max open files            1024                 4096                 files

在Centos7 & ubuntu 系统中，使用Systemd替代了之前的SysV。/etc/security/limits.conf文件的配置作用域缩小了。
/etc/security/limits.conf的配置，只适用于通过PAM认证登录用户的资源限制，它对systemd的service的资源限制不生效。因此登录用户的限制，通过/etc/security/limits.conf与/etc/security/limits.d下的文件设置即可。

对于systemd service的资源设置，则需修改全局配置，
全局配置文件放在/etc/systemd/system.conf和/etc/systemd/user.conf，
同时也会加载两个对应目录中的所有.conf文件/etc/systemd/system.conf.d/.conf和/etc/systemd/user.conf.d/.conf
system.conf是系统实例使用的，user.conf是用户实例使用的。

vim /etc/systemd/system.conf
DefaultLimitNOFILE=100000
DefaultLimitNPROC=65535

修改并重启即可

# cat /proc/3613/limits
Limit                     Soft Limit           Hard Limit           Units
Max processes             65535                65535                processes
Max open files            100000               100000               files

第二个问题
在服务里面设置LimitNOFILE=infinity为什么不是无穷大？
在服务里面设置LimitNOFILE=infinity 后，通过查看pid的limit发现openfile是65536 ，而不是无穷大
查看服务配置

[root@iZwz98aynkjcxvtra0f375Z ~]# cat /etc/systemd/system/multi-user.target.wants/docker.service |grep -vi "^#"|grep -vi "^$"
[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
BindsTo=containerd.service
After=network-online.target firewalld.service containerd.service
Wants=network-online.target
Requires=docker.socket
[Service]
Type=notify
ExecStart=/usr/bin/dockerd -H fd://
ExecStartPost=/usr/sbin/iptables -P FORWARD ACCEPT
ExecReload=/bin/kill -s HUP $MAINPID
TimeoutSec=0
RestartSec=2
Restart=always
StartLimitBurst=3
StartLimitInterval=60s
LimitNOFILE=infinity
LimitNPROC=infinity
LimitCORE=infinity
TasksMax=infinity
Delegate=yes
KillMode=process
[Install]
WantedBy=multi-user.target

查看配置效果

# cat /proc/11019/limits
Limit                     Soft Limit           Hard Limit           Units
Max cpu time              unlimited            unlimited            seconds
Max file size             unlimited            unlimited            bytes
Max data size             unlimited            unlimited            bytes
Max stack size            8388608              unlimited            bytes
Max core file size        0                    unlimited            bytes
Max resident set          unlimited            unlimited            bytes
Max processes             31202                31202                processes
Max open files            65536                65536                files
Max locked memory         65536                65536                bytes
Max address space         unlimited            unlimited            bytes
Max file locks            unlimited            unlimited            locks
Max pending signals       31202                31202                signals
Max msgqueue size         819200               819200               bytes
Max nice priority         0                    0
Max realtime priority     0                    0
Max realtime timeout      unlimited            unlimited            us

这个是systemd的bug，低于240的版本需要手动设置才可以生效
LimitNOFILE=102400
https://github.com/systemd/systemd/issues/6559

第三个问题
为什么openfile不能设置为unlimited

[root@iZwz98aynkjcxvtra0f375Z ~]# ulimit -n
65535
[root@iZwz98aynkjcxvtra0f375Z ~]# ulimit -n unlimited
-bash: ulimit: open files: cannot modify limit: Operation not permitted

原因是 centos7里 openfile不能大于nr_open

[root@iZwz98aynkjcxvtra0f375Z ~]# cat /proc/sys/fs/nr_open
1048576
[root@iZwz98aynkjcxvtra0f375Z ~]# ulimit -n 1048577
-bash: ulimit: open files: cannot modify limit: Operation not permitted
[root@iZwz98aynkjcxvtra0f375Z ~]# ulimit -n 1048576
[root@iZwz98aynkjcxvtra0f375Z ~]# ulimit -n
1048576

关于file-max nr_open file_nr的解释 可参考
https://www.kernel.org/doc/Documentation/sysctl/fs.txt

第四个问题
使用supervisor管理进程（测试环境ubuntu 1604）启动进程后，maxfile是1024
需要修改配置文件

#cat  /etc/supervisor/supervisord.conf
[supervisord]
logfile=/var/log/supervisor/supervisord.log ; (main log file;default $CWD/supervisord.log)
pidfile=/var/run/supervisord.pid ; (supervisord pidfile;default supervisord.pid)
childlogdir=/var/log/supervisor            ; ('AUTO' child log dir, default $TEMP)

下面这两行
minfds=655350                  ; min. avail startup file descriptors; default 1024
minprocs=65535                 ; min. avail process descriptors;default 200

# cat /proc/2423/limits 
Limit                     Soft Limit           Hard Limit           Units     
Max cpu time              unlimited            unlimited            seconds   
Max file size             unlimited            unlimited            bytes     
Max data size             unlimited            unlimited            bytes     
Max stack size            8388608              unlimited            bytes     
Max core file size        0                    unlimited            bytes     
Max resident set          unlimited            unlimited            bytes     
Max processes             65535                65535                processes 
Max open files            655350               655350               files          ------------修改成功
Max locked memory         65536                65536                bytes     
Max address space         unlimited            unlimited            bytes     
Max file locks            unlimited            unlimited            locks     
Max pending signals       61946                61946                signals   
Max msgqueue size         819200               819200               bytes     
Max nice priority         0                    0                    
Max realtime priority     0                    0                    
Max realtime timeout      unlimited            unlimited            us    

外部文档
https://www.cnblogs.com/zengkefu/p/5635153.html
https://blog.csdn.net/google0802/article/details/52304776
http://blog.cloud.360.cn/post/tuning-your-system-for-high-concurrency.html
https://blog.csdn.net/qq_38165374/article/details/104881340