Answer / achal ubbott

Priority inversion is a serious problem if it is
unbounded.A high priority task is said to undergo PI ,if it
has to wait for a resource,already held by a low priority
task. the LPT cannot release it because it gets preemptied
by tasks of intermediate priorities. solution to this is
priority inheritance .

Answer / venkatgopu

Priority inversion is a situation where in lower priority
tasks will run blocking higher priority tasks waiting for
resource (mutex).

priority: A(High) B(Medium) C(Low)

C was ready to run. So C starts running. locks mutex

A is ready to run. Bcz of A is higher priority than C, A
gets the CPU (otherwords C relinguishes CPU).
While executing "A" task, it's need of resource(mutex) but
it locked by lower priority task "C". so task "A" going to
be waiting state(priority inversion)

B is ready to run. Bcz B is Higher priority than C, B get
the CPU frm C.

For task "B", its no need of any resource(mutex). so it
wont be waiting for any task to get resource. it will be
finished first.

Then, Task "C" gets CPU from task "B" and finished its
task, and reliese the resource(mutex).

Task C (waiting task for resource)then gets mutex finish
execution..

Here:

B(M) task finished exe first
C(L) task finished exe second
A(H) task finished exe last even its have higher priority
than both B and C

The solution to priority inversion is Priority inheritance

Answer / saleem

Priority inversion is a situation where in lower priority
tasks will run blocking higher priority tasks waiting for
resource (mutex). For ex: consider 3 tasks. A, B and C, A
being highest priority task and C is lowest. Look at
sequence of context swaps

A goes for I/O . unlocks mutex.
C was ready to run. So C starts running. locks mutex
B is ready to run. Swaps out C and takes mutex.
A is ready to run. but A is blocked as mutex is locked by B.
but B will never relinqishes the mutex as its higher
priority than C.

The solution to priority inversion is Priority inheritance.

Answer / sunita

Priority inheritance and Priority ceiling can be used to
avoid Priority Inversion depending upon what RTOS support is
available.

Answer / sriniv

Take three tasks A,B and C.

A - High.
B - Medium.
C - Low.

C is scheduled and running and locks a resource which is
shared with A. Now B is scheduled and taked the CPU time so
it starts running keeping C in wait state.

Now A is scheduled which is the higher priority task than B
and C and gets the CPU time and it starts running. But it
needs the shared resource which is locked by task C. So A
waits for that resource.

Now CPU starts servicing task B as it has higher priority
than C. A is suppose to wait till task B is finished even
though it has highest priority than B. To avoid this
situation exchange the priority levels of A and C. After
this C will become highest priority and gets the CPU time
by suspending task B. Task C will be serviced and releases
resource shared with task A. As soon as task C is released
the shared resource change the prority levels of A and C
again. Now A will become highest priority than C and CPU
starts running Task A.

This is called Proirity inversion.

Answer / manojkumar

In scheduling, priority inversion is the scenario where a
low priority task holds a shared resource that is required
by a high priority task. This causes the execution of the
high priority task to be blocked until the low priority task
has released the resource, effectively "inverting" the
relative priorities of the two tasks.

The MARS Pathfinder problem is a good example to this
question. In this problem low priority jobs held a
system-wide important resource, in this case a
mutex on the data bus. The path finder suffered from
*priority inversion problem*.

Answer / pandiarajan.

A and C have shared resource so they use mutex. but B is not
using share resource of A and C. so B can swap out c and
start run. now A is ready to run but resource is locked by
C. after B finished C will continue..and B wise versa.. A
still wait to run.

Answer / balu

Whenever the highest priority task that is
otherwise enabled is not executing
1 Usual cause – contention for limited quantity
resource with a lower priority task
2 For example, a lower priority task has locked a
resource while in a critical section

Answer / ahmed hamdy

When using priority scheduling, it is
important that the RTOS has a sufficient number of
priority levels, so that applications with stringent
priority requirements can be implemented [13].
Unbounded priority inversion occurs when a higher priority
task must wait on a low priority task
to release a resource while the low priority task is waiting
for a medium priority task. The RTOS
can prevent priority inversion by giving the lower
priority task the same priority as the higher
priority task that is being blocked (called priority
inheritance). In this case, the blocking task can
finish execution without being preempted by a medium
priority task. The designer must make
sure that the RTOS being used prevents unbounded priority
inversion

Answer / boopathi

"A goes for I/O . unlocks mutex.
C was ready to run. So C starts running. locks mutex
B is ready to run. Swaps out C and takes mutex.
A is ready to run. but A is blocked as mutex is locked by B.
but B will never relinqishes the mutex as its higher
priority than C."

From the above, I understand the task B swaps C and takes
mutex. After this task A becomes ready and sees the mutex is
locked by B. Why cant A again swap B (Because A has higher
priority than B)? Im confused here, Please reply.

what is task spawn in vxworks ?plz anyone explain me

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