An imprecise computation model has the advantage of supporting overloaded conditions in dynamic real-time environments, compared to Liu and Layland's model. However, the imprecise computation model is not practical because the termination of each optional part cannot guarantee the schedulability. In order to guarantee the schedulability of the termination of the optional part, a practical imprecise computation model is presented. In the practical imprecise computation model, each task has multiple mandatory parts and optional parts to support many imprecise real-time applications. The practical imprecise computation model is supported by dynamic-priority scheduling on uniprocessors. Unfortunately, dynamic-priority scheduling is difficult to support multiprocessors. In contrast, semifixed-priority scheduling, which is part-level fixed-priority scheduling, supports only two mandatory parts so that supported imprecise real-time applications are restricted. This paper presents semi-fixed-priority scheduling with multiple mandatory parts on uniprocessors and multiprocessors respectively. In addition, this paper explains how to calculate the optional deadline of each task, which is the termination time of optional part. The schedulability analysis shows that semi-fixed-priority scheduling strictly dominates fixed-priority scheduling. Thanks to semi-fixed-priority scheduling with multiple mandatory parts, many imprecise realtime applications can be supported.