1) The document discusses multi-tenancy in cloud computing, including virtualization techniques that allow multiple tenants to share physical hardware resources in an isolated manner. 2) It describes some of the challenges of achieving true isolation and fairness when sharing resources between multiple tenants, such as performance variability and unpredictable failures. 3) The document presents several architectural models for implementing multi-tenancy, including fully shared hardware with virtualization, and scheduling algorithms like Quincy that aim to provide fairness when distributing resources between jobs from different tenants.