Redhat 6 Process Limit a Error 11
When upgrading from MySQL 5.1 to MariaDB 5.5 you may encounter a process limit problem with your MariaDB installation. Firstly this is not MariaDB at fault. The issue is likely triggered with high connection counts to the database and possibly exasperated by not using Thread pooling. You will encounter SQLSTATE[HY000]  Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug. when trying to connect. There is a very good explanation and troubleshooting with a work around on the Percona MySQL Performance Blog
Review your connections and thread consumption usage prior to doing an upgrade to avoid this being a problem.