I use apache2 behind nginx on 8 core CPU 48GB RAM server. Now my system uses only 3GB of all memory, but there are high load on CPUs. How can I use my free memory instead of CPUs time?
I also use memcached and xcache.
Well, try allowing more processes to apache. Here are some helpful links:
Enable HTTP persistent connections (Keep-Alive) in Apache configuration. In theory this will reduce CPU load (caused by tcp handshaking) and increase memory consumption (in a busy server) as Apache will spawn new worker processes / threads to server new requests.
Of course this is true in a stand alone Apache installation. You have much more variables in your equation (HTTP2 connections, SSL connections, nginx proxy).