Why nios ii program execution time, sometimes fast, sometimes slow?

so didn't say much about the implementation of your multi processor system, but could ist be that the variantion comes from any kind of arbitration logic that controls the access of the different cores and puts one into waiting for a resource while another one is using it ?

In this system inside, a total of three processors, two of which cpu the program memory sram is external, and the bus is independent, and the other a cpu program memory using fpga internal sram.

and what about data memories or any other ports or registered shared between these cores ? is there anything they share ?

In this system, there are three cpu, we call xcpu ycpu and zcpu, xcpu and ycpu between the dual-port ram sharing, ycpu and zcpu are also among the shared dual-port ram.

Now xcpu require real-time highly, 100us interrupt time, the program run time-critical, can not exceed a certain time

does this cpu has some kind of caches ?

Thank you,

THANKS