RE: SMP Scheduling

> 
>  - Threads have priorities. Threads in one priority class are served
>    round-robin. Higher priority queues are served first.

That's correct except for the UP-IPC path, where the scheduler is shortcut
in favor of a timeslice donation scheme. The SMP-IPC, path again uses the
scheduler, however, since synchronization (e.g., via IPIs) is needed anyway.

 
>  - A thread has a timeslice size and a "total quantum". Each time the
>    thread is preempted (or calls L4_Yield), the time it has consumed is
>    subtracted from the total quantum until it is depleted. At this point
>    the thread's scheduler gets a preemption message.
>  - Whether the preemption message is delivered to the scheduler or not,
>    a thread's quantum is reset to its initial value and the thread is
>    runnable again.
>  - Because preemption messages are sent with timeout 0, one would need
>    at least one thread per CPU that handles preemption
>    messages. Otherwise one could lose messages when the scheduler is
>    busy.

That's true with respect to the X.2 spec. The current implementation doesn't
use timeout 0, however, as you can see in tcb_t::send_preemption_ipc(u64_t
time) in kernel/src/api/v4/thread.cc. Afaik no-one has really used
preemption messages in the original L4KA:Pistachio. I know of several
prototypes addressing scheduling problems in L4, but no official version is
available (yet). 
 
Regards,
-Jan

--
Jan Stoess
System Architecture Group
University of Karlsruhe
Phone: +49 (721) 608-4056
Fax: +49 (721) 608-7664
eMail: [email protected]