This paper focuses on semi-partitioned cyclic executives for mixed-criticality multiprocessor systems in the case of which a job can be split and execute on different processors, this strategy incorporates most of the advantages of the fully partitioned scheduling while further maximising the total processor utilization. We propose algorithms in order to improve the schedulability of such a system. The length of each frame can be reduced to an optimal (minimum) value, which is a necessary schedulability condition for any schedule under this system model (i.e. every processor's capacity is fully utilized within each frame so that the frame cannot be further shortened), whilst all timing requirements of the system are satisfied. By conducting extensive experiments on a very large number of randomly generated job sets, it is demonstrated that our proposed algorithms significantly reduce the required length of each frame; it is possible for almost 100% of the job sets in the experiments to obtain their optimal values with the exception of a very limited number of specific situations. We also prove that when the complete times of some specific jobs are fixed given values, the strategy to shorten the frame presented in this paper is optimal.