git-svn-id: svn://svn.code.sf.net/p/chibios/svn/trunk@669 35acf78f-673a-0410-8e92-d51de3d6d3f4

1 files changed, 61 insertions, 20 deletions

diff --git a/docs/src/jitter.dox b/docs/src/jitter.dox
index 4bbedd7b7..1b4220dd4 100644
--- a/docs/src/jitter.dox
+++ b/docs/src/jitter.dox
@@ -8,15 +8,61 @@
  * A good place to start is this

  * <a href="http://en.wikipedia.org/wiki/Jitter">Wikipedia article</a>.

  *

- * <h2>Jitter Sources</h2>

- * Under ChibiOS/RT (or any other similar RTOS) there are several possible

- * jitter sources:

- * -# Hardware interrupts latency.

- * -# Interrupts service time and priority.

- * -# Kernel lock zones.

- * -# Higher priority threads activity.

+ * <h2>Interrupt Response Time</h2>

+ * This is the time from an interrupt event and the execution of the handler

+ * code.

  *

- * <h2>Jitter mitigation countermeasures</h2>

+ * @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"];

+      int -> busy [label="Not Ready"];

+      busy -> busy [label="Still Busy\n(jitter)"];

+      busy -> served [label="Finally Ready"];

+ * @enddot

+ *

+ * <h3>Jitter Sources</h3>

+ * In this scenario the jitter (busy state) is represented by the sum of:

+ * - Higher or equal priority interrupt sources 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 sources. 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 from an event, as example an interrupt, and the execution

+ * of a thread supposed to handle the event. 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="Other Threads\n(jitter)"];

+      busy -> thread [label="Highest Priority"];

+      served -> thread [label="Highest Priority"];

+ * @enddot

+ *

+ * <h3>Jitter Sources</h3>

+ * 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.

  *

@@ -26,7 +72,7 @@
  * architecture more efficient at interrupt handling, as example, the

  * ARM Cortex-M3 core present in the STM32 family is very efficient at that.

  *

- * <h3>Interrupts service time and priority</h3>

+ * <h3>Interrupts service time</h3>

  * This is the execution time of interrupt handlers, this time includes:

  * - Fixed handler overhead, as example registers stacking/unstacking.

  * - Interrupt specific service time, as example, in a serial driver, this is

@@ -41,16 +87,8 @@
  * An handler should 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 need to "speak" with the OS, if the handler uses full

+ * handler really needs to "speak" with the OS, if the handler uses full

  * stand-alone code then it is possible to remove the OS related overhead.<br>

- * On some architecture it is also possible to give to interrupt sources a

- * greater hardware priority than the kernel and not be affected by the

- * jitter introduced by OS itself (see next subsection).<br>

- * As example, in the ARM port, FIQ sources are not affected by the

- * kernel-generated jitter. The Cortex-M3 port is even better thanks to its

- * hardware-assisted interrupt architecture allowing handlers preemption,

- * late arriving, tail chaining etc. See the notes about the various

- * @ref Ports.

  *

  * <h3>Kernel lock zones</h3>

  * The OS kernel protects some critical internal data structure by disabling

@@ -67,12 +105,15 @@
  *

  * <h3>Higher priority threads activity</h3>

  * At thread level the response time is affected by the interrupt-related

- * jitter as seen in the previous subsections but also by the activity of the

- * higher priority threads and contention on protected resources.<br>

+ * jitter, as seen in the previous subsections, but also 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.

  */

 /** @} */