Saw some errors in httpd log:
[Sat Jun 10 18:20:54.048569 2017] [core:error] [pid 5596] (24)Too many open files: [client 123.123.123.123:56399] AH00132: file permissions deny server access: /var/www/html/logo.png, referer: https://www.howtosop.com/
Please check the resource limit on open files. This includes hard limit and soft limit.
[root@test ~]# ulimit -H -n
4096
[root@test ~]# ulimit -S -n
1024
Solution
It could be too small to support current traffic volume. We should double the value of limitation.
[root@test ~]# ulimit -H -n 8192
[root@test ~]# ulimit -S -n 8192
Verify all limits are set.
[root@test ~]# ulimit -H -n
8192
[root@test ~]# ulimit -S -n
8192
Then restart httpd server. It solved.