Getting Started with QNX Neutrino - QNX Software Systems

The kernel’s role
© 2009, QNX Software Systems GmbH & Co. KG.
Running
255
D
10
B
Priority
5
A
Blocked
0
C
Two threads on the READY queue, one blocked, one running.
This shows the kernel’s internal READY queue, which the kernel uses to decide who
to schedule next. Note that thread “C” is not on the READY queue, because it’s
blocked, and thread “D” isn’t on the READY queue either because it’s running.
Round Robin
The RR scheduling algorithm is identical to FIFO, except that the thread will not run
forever if there’s another thread at the same priority. It runs only for a system-defined
timeslice whose value you can determine by using the function
sched_rr_get_interval(). The timeslice is usually 4 ms, but it’s actually 4 times the
ticksize, which you can query or set with ClockPeriod().
What happens is that the kernel starts an RR thread, and notes the time. If the RR
thread is running for a while, the time allotted to it will be up (the timeslice will have
expired). The kernel looks to see if there is another thread at the same priority that’s
ready. If there is, the kernel runs it. If not, then the kernel will continue running the
RR thread (i.e., the kernel grants the thread another timeslice).
The rules
Let’s summarize the scheduling rules (for a single CPU), in order of importance:
• Only one thread can run at a time.
• The highest-priority ready thread will run.
• A thread will run until it blocks or exits.
• An RR thread will run for its timeslice, and then the kernel will reschedule it (if
required).
22 Chapter 1 • Processes and Threads April 30, 2009