Getting Started with QNX Neutrino - QNX Software Systems

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Scheduling and the real world © 2009, QNX Software Systems GmbH & Co. KG. per-thread setup required and for returning a context pointer (called ctp in the parameter lists). Note that the contents of the context pointer are entirely up to you — the library doesn’t care what you put into the context pointer. Now that the context has been created by context_alloc(), the block_func() function is called to perform the blocking operation. Note that the block_func() function gets passed the results of the context_alloc() function. Once the block_func() function unblocks, it returns a context pointer, which gets passed by the library to the handler_func(). The handler_func() is responsible for performing the “work” — for example, in a typical server, this is where the message from the client is processed. The handler_func() must return a zero for now — non-zero values are reserved for future expansion by QSS. The unblock_func() is also reserved at this time; just leave it as NULL. Perhaps this pseudo code sample will clear things up (it’s based on the same flow as shown in “Thread flow when using thread pools,” above): FOREVER DO IF (#threads < lo_water) THEN IF (#threads_total < maximum) THEN create new thread context = (*context_alloc) (handle); ENDIF ENDIF retval = (*block_func) (context); (*handler_func) (retval); IF (#threads > hi_water) THEN (*context_free) (context) kill thread ENDIF DONE Note that the above is greatly simplified; its only purpose is to show you the data flow of the ctp and handle parameters and to give some sense of the algorithms used to control the number of threads. Scheduling and the real world So far we’ve talked about scheduling algorithms and thread states, but we haven’t said much yet about why and when things are rescheduled. There’s a common misconception that rescheduling just “occurs,” without any real causes. Actually, this is a useful abstraction during design! But it’s important to understand the conditions that cause rescheduling. Recall the diagram “Scheduling roadmap” (in the “The kernel’s role” section). Rescheduling occurs only because of: • a hardware interrupt • a kernel call • a fault 76 Chapter 1 • Processes and Threads April 30, 2009
© 2009, QNX Software Systems GmbH & Co. KG. Scheduling and the real world Rescheduling — hardware interrupts Rescheduling due to a hardware interrupt occurs in two cases: • timers • other hardware Rescheduling — kernel calls The realtime clock generates periodic interrupts for the kernel, causing time-based rescheduling. For example, if you issue a sleep (10); call, a number of realtime clock interrupts will occur; the kernel increments the time-of-day clock at each interrupt. When the time-of-day clock indicates that 10 seconds have elapsed, the kernel reschedules your thread as READY. (This is discussed in more detail in the Clocks, Timers, and Getting a Kick Every So Often chapter.) Other threads might wait for hardware interrupts from peripherals, such as the serial port, a hard disk, or an audio card. In this case, they are blocked in the kernel waiting for a hardware interrupt; the thread will be rescheduled by the kernel only after that “event” is generated. If the rescheduling is caused by a thread issuing a kernel call, the rescheduling is done immediately and can be considered asynchronous to the timer and other interrupts. For example, above we called sleep(10);. This C library function is eventually translated into a kernel call. At that point, the kernel made a rescheduling decision to take your thread off of the READY queue for that priority, and then schedule another thread that was READY. There are many kernel calls that cause a process to be rescheduled. Most of them are fairly obvious. Here are a few: • timer functions (e.g., sleep()) Rescheduling — exceptions • messaging functions (e.g., MsgSendv()) • thread primitives, (e.g., pthread_cancel(), pthread_join()) The final cause of rescheduling, a CPU fault, is an exception, somewhere between a hardware interrupt and a kernel call. It operates asynchronously to the kernel (like an interrupt) but operates synchronously with the user code that caused it (like a kernel call — for example, a divide-by-zero exception). The same discussion as above (for hardware interrupts and kernel calls) applies to faults. April 30, 2009 Chapter 1 • Processes and Threads 77
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TIME TIME Writing interrupt handler
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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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Getting Started with QNX Neutrino - QNX Software Systems

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