141 lines
5.5 KiB
ReStructuredText
141 lines
5.5 KiB
ReStructuredText
Fibers
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######
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A fiber is an execution thread and a lightweight alternative to a task. It can
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use nanokernel objects but not microkernel objects. A runnable fiber will
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preempt the execution of any task but it will not preempt the execution of
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another fiber.
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Defining Fibers
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***************
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A fiber is defined as a routine that takes two 32-bit values as
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arguments and returns a void within the application, for example:
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.. code-block:: c
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void fiber ( int arg1, int arg2 );
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.. note::
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A pointer can be passed to a fiber as one of the parameters but it
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must be cast to a 32-bit integer.
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Unlike a nanokernel task, a fiber cannot be defined within the project
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file.
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Fibers can be written in assembly. How to code a fiber in assembly is
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beyond the scope of this document.
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Starting a Fiber
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****************
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A nanokernel fiber must be explicitly started by calling
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:c:func:`fiber_fiber_start()` or :c:func:`task_fiber_start()` to create
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and start a fiber. The function :c:func:`fiber_fiber_start()` creates
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and starts a fiber from another fiber, while
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:c:func:`task_fiber_start()` does so from a task. Both APIs use the
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parameters *parameter1* and *parameter2* as *arg1* and *arg2* given to
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the fiber . The full documentation on these APIs can be found in the
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:ref:`code`.
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When :c:func:`task_fiber_start()`is called from a task, the new fiber
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will be immediately ready to run. The background task immediately stops
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execution, yielding to the new fiber until the fiber calls a blocking
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service that de-schedules it. If the fiber performs a return from the
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routine in which it started, the fiber is terminated, and its stack can
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then be reused or de-allocated.
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Fiber Stack Definition
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**********************
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The fiber stack is used for local variables and for calling functions or
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subroutines. Additionally, the first locations on the stack are used by
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the kernel for the context control structure. Allocate or declare the
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fiber stack prior to calling :c:func:`fiber_fiber_start()`. A fiber
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stack can be any sort of buffer. In this example the fiber stack is
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defined as an array of 32-bit integers:
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.. code-block::cpp
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int32_t process_stack[256];
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The size of the fiber stack can be set freely. It is recommended to
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start with a stack much larger than you think you need, say 1 KB for a
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simple fiber, and then reduce it after testing the functionality of the
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fiber to optimize memory usage. The number of local variables and of
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function calls with large local variables determine the required stack
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size.
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Stopping a Fiber
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****************
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There are no APIs to stop or suspend a fiber. Only one API can influence
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the scheduling of a fiber, :c:func:`fiber_yield()`. When a fiber yields
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itself, the nanokernel checks for another runnable fiber of the same or
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higher priority. If a fiber of the same priority or higher is found, a
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context switch occurs. If no other fibers are ready to execute, or if
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all the runnable fibers have a lower priority than the currently
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running fiber, the nanokernel does not perform any scheduling allowing
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the running fiber to continue. A task or an ISR cannot call
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:c:func:`fiber_yield()`.
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If a fiber executes lengthy computations that will introduce an
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unacceptable delay in the scheduling of other fibers, it should yield
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by placing a :c:func:`fiber_yield()` call within the loop of a
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computational cannot call :c:func:`fiber_yield()`.
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Scheduling Fibers
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*****************
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The fibers in the Zephyr Kernel are priority-scheduled. When several fibers
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are ready to run, they run in the order of their priority. When more
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than one fiber of the same priority is ready to run, they are ordered
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by the time that each became runnable. Each fiber runs until it is
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unscheduled by an invoked kernel service or until it terminates. Using
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prioritized fibers, avoiding interrupts, and considering the interrupts
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worst case arrival rate and cost allows the kernel to use a simple
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rate-monotonic analysis techniques with the nanokernel. Using this
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technique an application can meet its deadlines.
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When an external event, handled by an ISR, marks a fiber runnable, the
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scheduler inserts the fiber into the list of runnable fibers based on
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its priority. The worst case delay after that point is the sum of the
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maximum execution times between un-scheduling points of the earlier
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runnable fibers of higher or equal priority.
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The nanokernel provides three mechanisms to reduce the worst-case delay
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for responding to an external event:
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Moving Computation Processing to a Task
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=======================================
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Move the processing to a task to minimize the amount of computation that
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is performed at the fiber level. This reduces the scheduling delay for
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fibers because a task is preempted when an ISR makes a fiber that
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handles the external event runnable.
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Moving Code to Handle External Event to ISR
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===========================================
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Move the code to handle the external event into an ISR. The ISR is
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executed immediately after the event is recognized, without waiting for
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the other fibers in the queue to be unscheduled.
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Adding Yielding Points to Fibers
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================================
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Add yielding points to fibers with :c:func:`fiber_yield()`. This service
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un-schedules a fiber and places it at the end of the ready fiber list
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of fibers with that priority. It allows other fibers at the same
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priority to get to the head of the queue faster. If a fiber executes
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code that will take some time, periodically call
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:c:func:`fiber_yield()`. Multi-threading using blocking fibers is
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effective in coding hard real-time applications.
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