Appeal 2007-2127
Reexamination Control No. 90/006,621
j. Thread scheduling and dispatching
Threads are scheduled for execution by the operating system
scheduler, which distributes processor cycles to all active threads according
to some scheduling scheme, and threads are dispatched by the dispatcher,
which institutes the thread context switch to save the state of the running
thread and load the saved state for the new thread. The scheduler,
dispatcher, and preemptive scheduling scheme are described in Nguyen,
Advanced Programmer's Guide to OS/2, page 11:
All the threads that are active within the system at any time
form a pool from which the operating system dispatches threads one
at a time, in a round-robin fashion. A dispatched thread temporarily
has the CPU all to itself. Thread dispatching is performed by a
module of the operating system called the Dispatcher/Scheduler. The
Dispatcher institutes the task switch necessary to execute the next
thread, while the Scheduler distributes processor cycles to all the
active threads within the system.
Notice that we have been speaking of active threads and
dispatched threads. These are not the same thing. At any given time
within the system there will be a number of active threads, but only
one dispatched thread. In fact it is more proper to speak of the active
threads within the system as the set of ready threads, and the
dispatched thread as the running thread. A third state in which a
thread can be found is the waiting state. A waiting thread is one
which is waiting for an external event (such as the completion of an
I/O operation). When the external event occurs (when the I/O
operation has been completed) the operating system places the waiting
thread back into the pool of ready threads. See figure 1.3 for the state
diagram for a thread.
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