1. A method for executing an application on two or more processors comprising: determining a first portion of one or more modules of the application; determining a second portion of one or more remaining modules of the application not included in said first portion; executing the first portion of modules on each of the two or more processors; selecting a first of said two or more processors as an active processor; and executing the second portion of modules on the active processor and not on the other ones of said two or more processors which are designated as passive processors with respect to said second portion of modules, wherein said two or more processors are included in a same system wherein the application is a quality of service application that controls performance of the system for a plurality of I/O classes of I/O requests in accordance with performance goals including an I/O request processing rate for each of the I/O classes said first portion of modules including a control engine that executes only on said active processor and adjusts, to obtain the performance goals, one or more values used by each of the two or more processors in connection with controlling resources allocated to servicing received I/O requests of each of the I/O classes.