If you have a large number of users, do not use version 1.5.6, version 1.5.6 uses your virtual server resources to the maximum
My suggestion is version 1.4.21 beta 3
My server has 9 cores!
And the number of my servers is more than 400 - My users are up to 5 thousand!
Server cores in the version reaches 1.5 to 70% - This is not really normal!
After 2 to 3 days I have to restart the server.