How to increase the number of open files allowed for Apache on CentOS 7

Introduction

When trying to start or stop Apache you may see errors like "Error: Too many open files" and needs to be increased.

Procedure

On a server using systemd to manage services do the following:

mkdir /usr/lib/systemd/system/httpd.service.d

Then create a file with a .conf extension like mkdir /usr/lib/systemd/system/httpd.service.d/limits.conf and add the following:

[Service]
LimitNOFILE=40000

Then run these to reload the change and restart Apache:

systemctl daemon-reload
/scripts/restartsrv_httpd --hard

If this doesn't work then directly modify /usr/sbin/apachectl and comment out the following section:

#if [ "x$ULIMIT_MAX_FILES" != "x" ] ; then
# $ULIMIT_MAX_FILES
#fi

Then just after that add this line with the limit needed:

ulimit -n 40000

and then restart Apache again with:

/scripts/restartsrv_httpd --hard

You can use the following article to check what limits are set for Apache processes:

How to check the configured number of open files allowed for Apache