/*
    ChibiOS/RT - Copyright (C) 2006,2007,2008,2009,2010 Giovanni Di Sirio.

    This file is part of ChibiOS/RT.

    ChibiOS/RT is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    ChibiOS/RT is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @page article_jitter Response Time and Jitter
 * Response time jitter is one of the most sneaky source of problems when
 * designing a real time system. When using a RTOS like ChibiOS/RT one must
 * be aware of what the jitter is and how it can affect the performance of the
 * system. A good place to start is this
 * <a href="http://en.wikipedia.org/wiki/Jitter" target="_blank">Wikipedia
 * article</a>.
 *
 * <h2>Interrupt handlers execution time</h2>
 * The total execution time of an interrupt handler includes:
 * - Hardware interrupts latency, this parameter is pretty much fixed and
 *   characteristic of the system.
 * - Fixed handler overhead, as example registers stacking/unstacking.
 * - Interrupt specific handler code execution time, as example, in a serial
 *   driver, this is the time used by the handler to transfer data from/to
 *   the UART.
 * - OS overhead. Any operating system requires to run some extra code
 *   in interrupt handlers in order to handle correct preemption and Context
 *   Switching.
 * .
 * <h2>Interrupt Response Time</h2>
 * The Interrupt Response Time is the time from an interrupt event and the
 * execution of the handler code. Unfortunately this time is not constant
 * in most cases, see the following graph:
 *
 * @dot
  digraph example {
      rankdir="LR";
      node [shape=circle, fontname=Helvetica, fontsize=8, fixedsize="true", width="0.75", height="0.75"];
      edge [fontname=Helvetica, fontsize=8];
      int [label="Interrupt"];
      busy [label="Busy"];
      served [label="Interrupt\nServed"];
      int -> served [label="Not Busy (minimum latency)"];
      int -> busy [label="Not Ready"];
      busy -> busy [label="Still Busy\n(added latency)"];
      busy -> served [label="Finally Ready"];
  }
 * @enddot
 *
 * In this scenario the jitter (busy state) is represented by the sum of:
 * - Higher or equal priority interrupt handlers execution time combined.
 *   This time can go from zero to the maximum randomly. This value can be
 *   guaranteed to be zero only if the interrupt has the highest priority in
 *   the system.
 * - Highest execution time among lower priority handlers. This value is zero
 *   on those architectures (Cortex-M3 as example) where interrupt handlers
 *   can be preempted by higher priority sources.
 * - Longest time in a kernel lock zone that can delay interrupt servicing.
 *   This value is zero for fast interrupt sources, see @ref system_states.
 * .
 * <h2>Threads Flyback Time</h2>
 * This is the time between an event, as example an interrupt, and the
 * execution of the thread that will process it. Imagine the following
 * graph as the continuation of the previous one.
 *
 * @dot
  digraph example {
      rankdir="LR";
      node [shape=circle, fontname=Helvetica, fontsize=8, fixedsize="true", width="0.75", height="0.75"];
      edge [fontname=Helvetica, fontsize=8];
      served [label="Interrupt\nServed"];
      busy [label="Busy"];
      thread [label="Thread\nAwakened"];
      served -> busy [label="Not highest Priority"];
      busy -> busy [label="Higher priority Threads\n(added latency)"];
      busy -> thread [label="Highest Priority"];
      served -> thread [label="Highest Priority (minimum latency)"];
  }
 * @enddot
 *
 * In this scenario all the jitter sources previously discussed are also
 * present and there is the added jitter caused by the activity of the
 * higher priority threads.
 *
 * <h2>Jitter Mitigation</h2>
 * For each of the previously described jitter sources there are possible
 * mitigation actions.
 *
 * <h3>Interrupt handlers optimization</h3>
 * An obvious mitigation action is to optimize the interrupt handler code as
 * much as possible for speed.<br>
 * Complex actions should never be performed in interrupt handlers.
 * An handler should just serve the interrupt and wakeup a dedicated thread in
 * order to handle the bulk of the work.<br>
 * Another possible mitigation action is to evaluate if a specific interrupt
 * handler really needs to interact with the OS, if the handler uses full
 * stand-alone code then it is possible to remove the OS related overhead.<br>
 *
 * <h3>Kernel lock zones</h3>
 * The OS kernel protects some critical internal data structure by disabling
 * (fully in simple architecture, to some extent in more advanced
 * microcontrollers) the interrupt sources. Because of this the kernel itself
 * is a jitter cause, a good OS design minimizes the jitter generated by the
 * kernel by using adequate data structures, algorithms and coding
 * practices.<br>
 * A good OS design is not the whole story, some OS primitives may generate
 * more or less jitter depending on the system state, as example the maximum
 * number of threads on a certain queue, the maximum number of nested mutexes
 * and so on. Some algorithms employed internally can have constant execution
 * time but others may have linear execution time or be even more complex.
 *
 * <h3>Higher priority threads activity</h3>
 * At thread level, the response time is affected by the interrupt-related
 * jitter but mainly by the activity of the higher priority threads and
 * contention on protected resources.<br>
 * It is possible to improve the system overall response time and reduce jitter
 * by carefully assigning priorities to the various threads and carefully
 * designing mutual exclusion zones.<br>
 * The use of the proper synchronization mechanism (semaphores, mutexes, events,
 * messages and so on) also helps to improve the overall system performance.
 * The use of the Priority Inheritance algorithm implemented in the mutexes
 * subsystem can improve the overall response time and reduce jitter but it is
 * not a magic wand, a proper system design comes first.
 */