Getting Started with QNX Neutrino - QNX Software Systems

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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
© 2009, QNX Software Systems GmbH & Co. KG. The kernel’s role The following flowchart shows the decisions that the kernel makes: Enter on a rescheduling event Find the running thread Did we just READY a higher-priority thread? Yes Remove the current thread from the RUNNING array, and run the new thread instead No Is the currently running thread running RR? No Continue running this thread until the end of its timeslice Yes Has it used more than one timeslice? No Yes Is there another READY thread at this priority? No Rest the thread's timeslice counter Yes Reshuffle the queue so that this (running) thread is at the end of the READY queue for its priority, and remove the thread from the head of the READY queue and run it Scheduling roadmap. For a multiple-CPU system, the rules are the same, except that multiple CPUs can run multiple threads concurrently. The order that the threads run (i.e., which threads get to run on the multiple CPUs) is determined in the exact same way as with a single CPU — the highest-priority READY thread will run on a CPU. For lower-priority or April 30, 2009 Chapter 1 • Processes and Threads 23
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Chapter 2 Message Passing In this c
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Message passing and client/server
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Message passing and client/server
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Multiple threads © 2009, QNX Softw
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Using message passing © 2009, QNX
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Message passing over a network © 2
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Chapter 3 Clocks, Timers, and Getti
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Using timers © 2009, QNX Software
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Advanced topics © 2009, QNX Softwa
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Chapter 4 Interrupts In this chapte
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Neutrino and interrupts © 2009, QN
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Writing interrupt handlers © 2009,
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TIME TIME Writing interrupt handler
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Summary © 2009, QNX Software Syste
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Appendix B Calling 911 In this appe
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Seeking professional help © 2009,
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Seeking professional help © 2009,
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Appendix C Sample Programs In this
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atoz.c © 2009, QNX Software System
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atoz.c © 2009, QNX Software System
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time1.c © 2009, QNX Software Syste
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tp1.c © 2009, QNX Software Systems
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Index ! /dev/null resource manager
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