When GAE launched they were only charging for the actual CPU time used by the application. There was no static monthly fees on running your app on GAE. If you

When GAE launched they were only charging for the actual CPU time used by the application. There was no static monthly fees on running your app on GAE. If you are not getting money when the app is not performing tasks, you need to minimize the costs related to that state. Two ways to minimize the costs: 1)put as many apps as possible to singe machine 2)implement a fast mechanism for loading/unloading the apps based on demand. Xen like virtualization is not best choice for this purpose. Virtual machines have overhead which limits on how many of those you can put on one server. Even if the app running on VM does nothing, you still have the overhead. Also starting up the VM takes time so reacting to demand is slower. At least the original GAE would quite often shutdown the application completely and only start it when a new web request came in.