The User-Mode Thread Model refers to one SPE thread managing a set of user-level functions running in parallel.
The user-level functions are called microthreads (and also user threads and user-level tasks) . The SPE thread is supported by the operating system. The microthreads are created and supported by user software; the operating system is not involved. However, the set of microthreads can run across a set of SPUs.
The SPU application schedules tasks in shared memory, and the tasks are processed by available SPUs. For example, in game programming, the tasks can refer to scene objects that need updating. Microthreads can complete at any time, and new microthreads can be spawned at any time.
One advantage of this programming model is that the microthreads, running on a set of SPUs under the control of an SPE thread, have predictable overhead. A single SPE cannot save and restore the MFC commands queues without assistance from the PPE.