After switching to fcgi I got a very serious problem which is going to be more and more after reach up to 500 Virtual Servers. Many PHP customers who are using fcgi php have this problem that after a while the php pages won't serve anymore and the error page "Service Temporary Unavailable" shown. meanwhile other virtual hosts php pages serving well with fcgi. I discovered that this problem only sovled when I restart httpd. Also while this error happened in the error_log I got this :
[warn] mod_fcgid: can't apply process slot for /home/domain/public_html/modules.php
How can I solve the problem ? Any suggestion ?