Mixing performance, configurability, density, and security at scale has, historically, been hard with PHP. Early approaches have involved CGIs, suhosin, or multiple Apache instances. Then came PHP-FPM. At Pantheon, we've taken PHP-FPM, integrated it with cgroups, namespaces, and systemd socket activation. We use it to deliver all of our goals at unheard-of densities: thousands and thousands of isolated pools per box.
Watch how it's configured and see PHP-FPM pools start real-time to serve different Drupal sites as requests come into a server.
All of our tools for this are open-source and usable on your own virtual machines and hardware.