Resource Deadlocks: Conditions, Examples, and Prevention
1. What are the Four Necessary Conditions for a Resource Deadlock?
The four necessary conditions for a resource deadlock are:
- Mutual Exclusion: Each resource is either currently held by exactly one process or is free.
- Hold and Wait: Processes currently holding resources can request and wait for new resources.
- No Preemption: Resources held by a process can’t be forcibly taken away from the process.
- Circular Wait: There is a circular chain of two or more processes, each waiting for a resource held by the next member of the chain.
2. Resource Allocation and Requests Scenario
Consider the following set of resource allocations and requests:
- Process 1 holds resources A and B and is waiting for resource C.
- Process 2 holds resource D and is waiting for resource B.
- Process 3 holds resource C and is waiting for resource D.
A.) Draw a Resource Allocation Graph (RAG) for the situation given above. Use circular nodes for processes and square nodes for resources.
(Note: A visual representation of the RAG would be included here in a real-world scenario.)
B.) Is there a deadlock present in the system? If so, which processes and resources are involved? If not, give an ordering that the processes can run in to complete execution.
Yes. All three processes (i.e., 1, 2, and 3) are involved in the deadlock, including resources B, C, and D.
3. Top Energy Consumers in Laptops and Power Conservation
The display, hard disk, and CPU are the largest consumers of energy in a laptop computer. The amount of power used by the display directly correlates with the brightness setting it is being used on. The operating system (OS) can conserve energy by dimming the display after a certain period of inactivity or by dimming the display when the laptop is running on battery or in low lighting conditions. The hard disk can be spun down after a certain period of inactivity when the OS deems that the power conserved from spinning it down (as opposed to keeping it spinning) will exceed the amount of power necessary to spin it back up. Caching can help with this process, as the disk may not need to be restarted if the request can be serviced from the cache instead. The CPU can be run at a slower clock rate to conserve power during times that it becomes relatively unloaded. The power savings of running the CPU at a lower clock rate is the square of the reduction in CPU speed, so we can attain a significant power savings by running the CPU at a speed that will keep it busy instead of running alternately at full speed and turning it off when the CPU is idle.
4. Are the Four Conditions Sufficient for a Deadlock?
The four conditions (mutual exclusion, hold and wait, no preemption, and circular wait) are necessary for a resource deadlock to occur.
(a) Give an example to show that these conditions are not sufficient for a resource deadlock to occur.
Suppose that there are three processes, A, B, and C, and two resource types, R and S. Further assume that there is one instance of R and two instances of S. Consider the following execution scenario: A requests R and gets it; B requests S and gets it; C requests S and gets it (there are two instances of S); B requests R and is blocked; A requests S and is blocked. At this stage, all four conditions hold. However, there is no deadlock. When C finishes, one instance of S is released that is allocated to A. Now A can complete its execution and release R that can be allocated to B, which can then complete its execution.
(b) When are these conditions sufficient for a resource deadlock to occur?
These four conditions are sufficient if there is only one resource of each type.
5. Deadlock Possibility with Two Processes and Three Resources
A system has two processes and three identical resources. Each process needs a maximum of two resources. Is deadlock possible?
The system is deadlock-free. Suppose that each process has one resource. There is one resource free. Either process can ask for it and get it, in which case it can finish and release both resources. Consequently, deadlock is impossible.