Revision as of 17:17, 6 January 2012

Intro

All synchronization mechanisms that are based on mutual exclusion suffer from priority inversion.
This is where a low-priority SO enters into a mutual-exclusion zone which it shares with a high-priority SO.
Now the low priority SO is in a mutex zone. Now, consider a medium priority thread which doesnt use the mutex zone, it preempts the low priority thread and starts performing a computationally intensive task.
So the low priority SO cant run and get out of the mutex zone. Which means, the high priority thread is blocked.
The medium-priority thread thereby indirectly blocks the progression of the high-priority thread for a potentially unbounded period of time.

RTSJ requires all system Q's to be priority ordered.
- Q's of SO's waiting for an object lock.
- SO's blocked on wait().
If more than one SO in the Q has same priority, then FIFO is followed.
Has facilities to specify priority inversion control algorithms.
- By default, RTSJ requires simple priority inheritance to occur whenever a SO is blocked waiting for a resource.

@@ Line 6: / Line 6: @@
 * So the low priority SO cant run and get out of the mutex zone. Which means, the high priority thread is blocked.
 * The medium-priority thread thereby indirectly blocks the progression of the high-priority thread for a potentially unbounded period of time.
+* Solutions ?
+** Priority Inheritance
+** Non Blocking Communication
+= Priority Inheritance =
+* If thread p is blocking thread q, then p runs with q's priority.
+== RTSJ Model ==
+* RTSJ requires all system Q's to be priority ordered.
+** Q's of SO's waiting for an object lock.
+** SO's blocked on wait().
+* If more than one SO in the Q has same priority, then FIFO is followed.
+* Has facilities to specify priority inversion control algorithms.
+** By default, RTSJ requires simple priority inheritance to occur whenever a SO is blocked waiting for a resource.
 [[Category:RealtimeJava]]