"kargahi"kargahi@moe.or.ir writes:
Would you please say me why Fiasco is a real-time u-kernel & what are the real-time properties of Fiasco?
For Fiasco, "Real-Time" means that Fiasco implements static ``hard'' thread priorities, and the worst-case time to react to an event such as an interrupt or a timeout are bound. To keep the bound low, Fiasco is is fully preemtible in kernel mode, and it guarantees that ``independent thread groups'' are always executable no matter what is going on in the kernel.
After that, please say me how the priorities affect the timeslice of a task and where the timeslice of a task is allocated to it.
The scheduler uses two variables: Priorities determine the order in which threads are executed (thread_t::schedule()). The time-slice length determines when the kernel considers running a different thread of the same priority.
The kernel has the concept of a "current time slice" which is the number of clock ticks the current thread can execute without being preempted. The current time slice (timeslice_ticks_left) is checked in thread_timer_interrupt(), and it is initialized in thread_t::schedule(). Please not that it is not set on every kernel-context switch -- only when a thread is preempted or voluntarily goes to sleep (that is, only in thread_t::schedule(), not in thread_t::switch_to()).
Different L4 kernels handle scheduling differently. Jean Wolter has a short paper describing the current state of the art for some L4 implementations that I'm sure he will make available for the asking.
Michael