"Multi-threading" refers to the use of multiple processors for parallel processing by a single application.
The multi-threading support is currently limited to certain runtime intensive placement algorithms in ISE 11. In ISE 12, the routing algorithms will also support multi-threading.
Architecture support is currently limited to Virtex-6, Virtex-5 and Spartan-6 architectures. There are no plans to add support for older architectures.
In ISE 11, multi-threading supports the use of a second processor only. In ISE 12, support for more than two processors is planned. The application will determine how many processors can be used based on the user options, the number of processors available and the load on those processors.
In ISE 11, there is no messaging to indicate that multi-threading is in use and how many processors are involved. A CR is being evaluated to add some messaging to report this information.