小白问个 nginx 和 phpFpm 问题？

刚才用sudo apt-get upgrade php后，重启nginx发现：

www@www-2:/etc/nginx$ ps -ef | egrep php
root       768     1  0 10:26 ?        00:00:00 php-fpm: master process (/etc/php/7.2/fpm/php-fpm.conf)
www-data   769   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   770   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   771   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   772   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   773   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   774   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   775   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   776   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   777   768  0 10:26 ?        00:00:00 php-fpm: pool www
www-data   778   768  0 10:26 ?        00:00:00 php-fpm: pool www
root     14201     1  0 10:31 ?        00:00:00 php-fpm: master process (/etc/php/7.3/fpm/php-fpm.conf)
www-data 14202 14201  0 10:31 ?        00:00:00 php-fpm: pool www
www-data 14203 14201  0 10:31 ?        00:00:00 php-fpm: pool www
chinaii+ 14929 27650  0 10:45 pts/0    00:00:00 grep -E --color=auto php
www@www-2:/etc/nginx$

为什么php-fpm master process有两个？？？