Lists of Long Descriptive type Questions that may be asked in Written Exams.
- (1) What is Deadlock?
- (2) Explain preempt able and non-preempt able resource with example
- (3) List the conditions that lead to deadlock.
Question-1 what is Deadlock?
- A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.
- Suppose process D holds resource T and process C holds resource U.
- Now process D requests resource U and process C requests resource T but none of process will get this resource because these resources are already hold by other process so both can be blocked, with neither one be able to proceed, this situation is called deadlock.
Resource allocation graphs. (a) Holding a resource. (b) Requesting a resource. (c) Deadlock.
- As shown in figure Above, resource T assigned to process D and resource U is assigned to process C.
- Process D is requesting / waiting for resource U and process C is requesting / waiting for resource T.
- Processes C and D are in deadlock over resources T and U.
Question-2 Explain preempt able and non-preempt able resource with example.
- Resources come in two types: preemptable and non-preemptable.
- A preemptable resource is one that can be taken away from the process holding it with no ill effects.
- Consider, for example, a system with 32 MB of user memory, one printer, and two 32-MB processes such that each process wants to print something.
- Process A requests and gets the printer, then starts to compute the values to print.
- Before it has finished with the computation, it exceeds its time quantum and is swapped out.
- Process B now runs and tries, unsuccessfully, to acquire the printer.
- Potentially, we now have a deadlock situation, because A has the printer and B has the memory, and neither can proceed without the resource held by the other.
- Fortunately, it is possible to preempt (take away) the memory from B by swapping it out and swapping A in. Now A can run, do its printing, and then release the printer. No deadlock occurs.
- A non-preemptable resource, in contrast, is one that cannot be taken away from its current owner without causing the computation to fail.
- If a process has begun to burn a CD-ROM, suddenly taking the CD recorder away from it and giving it to another process will result in a garbled CD, CD recorders are not preemptable at an arbitrary moment.
- In general, deadlocks involve non-preemptable resources.
Question-3 List the conditions that lead to deadlock.
Conditions that lead to deadlock
There are four conditions that must hold for deadlock:
1) Mutual exclusion condition
- Each resource is either currently assigned to exactly one process or is available.
2) Hold and wait condition
- Process currently holding resources granted earlier can request more resources.
3) No preemption condition
- Previously granted resources cannot be forcibly taken away from process.
4) Circular wait condition
- There must be a circular chain of 2 or more processes. Each process is waiting for resource that is held by next member of the chain.
All four of these conditions must be present for a deadlock to occur.
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