Try this :

echo “10240” > /proc/sys/net/core/somaxconn

or set (to make permanent)

net.core.somaxconn = 10240 (or any number over what you are using as max connections)

in

/etc/sysctl.conf

then run

/sbin/sysctl -p /etc/sysctl.conf

Also set your local ulimit to

ulimit -n 65535

just to have some overhead in case you want to increase the concurrent connections in ab. Let me know if that helps. If you want my thoughts on why I think this fixes things (assuming it helps you) let me know

Also if you move to Worker MPM (you will have to redo php to be threadsafe if you use php) it might lower you CPU usage for a higher RAM usage depending on what your running off your server.