Instances of lazy scheduling and timeslice donation in L4 implementations

Udo A. Steinberg us15 at os.inf.tu-dresden.de
Tue Jan 24 21:34:29 CET 2006


On Tue, 24 Jan 2006 19:54:29 +0100 Espen Skoglund (ES) wrote:

ES> Speaking for the Pistachio implementation: The time is accounted to
ES> the thread who currently "owns" the timeslice (thread A in this case).
ES> Each time a timer tick occurs, and there are no preemptions by higher
ES> priority threads, thread B will run on the time donated by thread A.
ES> This is according to the spec.

So far all is clear.

ES> Once the timeslice expires a new
ES> scheduling decision is made.  This may or may not cause thread B to
ES> continue running (depending on priorities, scheduling queues, etc.).

This is also clear.

The tricky case is as follows:
1) A performs a thread_switch syscall, donating its time slice to B
2) B executes and consumes part of A's time (but not all)
3) B is preempted by a higher-priority thread H
4) H blocks

How does B get to consume the remainder of A's time slice, i.e. the part
that it did not use up in 2) ?

			- Udo
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