max_spare_servers究竟是個什麼意思？

pm.max_children=500
pm.start_servers=280
pm.min_spare_servers=50
pm.max_spare_servers=500 把max_spare_servers設置爲200後，服務器掛了一直502

#，保證空閒進程數最大值，若是空閒進程大於此值，此進行清理  。說是這個意思設置了200後關閉，服務器還出問題。。

報錯以下
● php-fpm.service - The PHP FastCGI Process Manager
Loaded: loaded (/usr/lib/systemd/system/php-fpm.service; enabled; vendor pres et: disabled)
Active: failed (Result: exit-code) since Tue 2019-03-12 10:42:56 CST; 27s ago
Process: 15964 ExecStart=/usr/sbin/php-fpm --nodaemonize (code=exited, status= 78)
Main PID: 15964 (code=exited, status=78)

Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z systemd[1]: Starting The PHP FastCGI ...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z php-fpm[15964]: [12-Mar-2019 10:42:56...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z php-fpm[15964]: [12-Mar-2019 10:42:56...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z php-fpm[15964]: [12-Mar-2019 10:42:56...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z systemd[1]: php-fpm.service: main pro...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z systemd[1]: Failed to start The PHP F...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z systemd[1]: Unit php-fpm.service ente...
Mar 12 10:42:56 iZbp16mw7ytq0ocsm6oot8Z systemd[1]: php-fpm.service failed.
Hint: Some lines were ellipsized, use -l to show in full.
[root@iZbp16mw7ytq0ocsm6oot8Z etc]# cd /php-fpm.d
-bash: cd: /php-fpm.d: No such file or directory