Answer / sunitakn

The standard UNIX kernel is a nonpreemptive kernel; it does
not allow a user process to preempt a process executing in
kernel mode. Once a running process issues a system call and
enters kernel mode, preemptive context switches are disabled
until the system call is completed. Although there are
context switches, a system call may take an arbitrarily long
time to execute without voluntarily giving up the processor.
During that time, the process that made the system call may
delay the execution of a higher-priority, runnable, realtime
process.

The maximum process preemption latency for a nonpreemptive
kernel is the maximum amount of time it can take for the
running, kernel-mode process to switch out of kernel mode
back into user mode and then be preempted by a
higher-priority process. Under these conditions it is not
unusual for worst-case preemption to take seconds, which is
clearly unacceptable for many realtime applications.

I have a situation where the programme counter(PC) shows that vxworks msgQreceive function has been called by task A and Q is full, still I am not picking up the messages. There is no other task registered to receive messages from this Q. The sending task B, which is ISR in this case, is getting timedout everytime it tries posting new message in it. A's state is PEND which means I am waiting for some resource to get free but PC shows I have called vxworks msgQReceive and currently at qJobget+0x018 location. There seems to be two causes now(identified by me): either vxworks qjobget task is not able to do semTake or no messages in Q. But Q is full and semTake failure seems unlikely(though I dont have a mechanism to check so). Besides explaining root cause for above, can anyone suggest how to get my task in READY state again? I also tried flushing the Q; on doing this, more messages get posted in Q but A still doesnt pick up any :(

4 Answers  

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Write a code to connect Hardware interrupt to ISR...?

1 Answers   iNet, Wipro,

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what do u mean by Semaphore..when v wil go for using this..explain about hat.what is the difference betw..binary ,,counting and mutex semphores

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Which RTOS supports Non-Preemptive scheduling ? Why other scheduling methods are supported by such Oses?

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IN RTOS HAVING 1OKBYTES OF MEMORY AND YOUR PROGRAMM FOR ASKING FOR 5KBYTES AND YOU GOT NULL ERROR ? WHY DOES IT BEHAVES LIKE THIS

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what are the various methods to overcome it?

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what is task spawn in vxworks ?plz anyone explain me

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What is priority inversion?

10 Answers   Juniper Networks,

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hi this is srikanth i need video classes for Vxworks any body having this forward me thanking u

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Assume there is a global variable & a static global variable.Both gets memory allocated from the data segment. Then how does the visiblity of static global varible gets limited to the file alone (in which it gets declared) ???

2 Answers   Wipro,

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What is another for creating a task without using taskspawn?

2 Answers  

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