Explain briefly Deadlock Avoidance and Detection?

Deadlocks are a serious issue that needs to be avoided since it can cause the whole system to hang or crash.

What is Deadlock Avoidance?

- Avoiding a deadlock is possible only if certain information regarding the processes is available with the operating system.
- This information has to be made available to the OS just before the resources are allocated to the processes.
- These are the processes that are to be consumed by the process in its lifetime.
- For every resource request made by the process, any potential threats are checked by the system i.e., whether granting the request of the process will send it in to an unsafe zone or not.
- If it is so then there are possibilities that the system could enter a deadlock.
- Therefore, only those requests are granted by the process that will ensure a safe state of the process.
- It is important for the system to determine whether the next level of the process will be safe or unsafe.
- There are 3 things that the operating system must know at any before or after the execution of the process:
1. The currently available resources.
2. The resources currently allocated to the processes.
3. Resources to be required and released in the future by these processes.

- It is possible that a process might be in an unsafe state but still may not cause a deadlock.
- By the notion of the safe and unsafe state of the process we refer to the system’s ability of entering in to a deadlock.
An example will make it clearer:
- Consider a resource A requested by a process which would make the process state unsafe.
- At the same time it releases another resource say B preventing the circular wait of the resources.
- In such a situation, the system is said to be in an unsafe state though not necessarily in a deadlock.
- There are various algorithms that have been designed for deadlock avoidance and one such is the banker’s algorithm.
- To use this algorithm knowledge about the resource usage limit is required in advance.
-  It is impossible for most of the systems to know what a process will request for in advance.
- This only implies that the deadlock avoidance is also not possible here.
- There are other two algorithms for achieving this task namely wound/ wait and wait/ die algorithms.
- Each of them makes use of a symmetry breaking technique.

What is Deadlock Detection?

- Deadlocks are free to occur under the implementation of this concept.
- Then through the state of the system, the occurrence of the deadlock is confirmed and subsequently mended.
- Here, the resource allocation activities are tracked along with the process states by certain algorithms.
- After this, the algorithm is used for removing the deadlock.
- Deadlock detection is quite easy since the OS scheduler knows about the resources that have been locked by the processes.
- Model checking is one of the techniques used for deadlock detection.
- In this a finite state model is created up on which a progress analysis of the process is carried out and all the terminal sets of the model are found.
- Each of these stands for a deadlock.
- Correction of the deadlock can be done by any of the below mentioned methods after the deadlock has been detected:
1. Process termination: This is about aborting one or more of the processes that cause the deadlock thus ensuring a certain and speedy removal of the deadlock. But this method might prove to be a little expensive because of the loss of the partial computations.
2. Resource preemption: This is about a successive preemption of the allocated resources until the breakdown of the deadlock.

What is the cause of thrashing? How does the system detect thrashing? Once it detects thrashing, what can the system do to eliminate this problem?

- Thrashing takes place when the sub-system of the virtual memory of the computer system is involved in a state of paging constantly.

- It rapidly exchanges data in the memory with the data available on the disk excluding level of processing of most of the applications. 

- Thrashing leads to the degradation of the performance of the computer or may even cause it to collapse. 

- The problem may further worsen until the issue is identified and addressed. 

- If there are not enough pages available for the job, it becomes very likely that your system will suffer from thrashing since it’s an activity involving high paging. 

- This also leads to high rate of page fault. 

- This in turn cuts down the utilization of the CPU. 

- Modern systems utilize the concept of the paging system for executing many programs.

- However, this is what makes them prone to thrashing. 

- But this occurs only if the system does not have at present sufficient memory as required by the application or if the disk access time is too long. 

- Thrashing is also quite common in the communication systems where the conflicts concerning the internal bus access is common. 

- The order of magnitude or degree by which the latency and throughput of a system might degrade depends up on the algorithms and the configuration that is being used. 

- In systems making use of virtual memory systems, workloads and programs presenting insufficient locality of reference may lead to thrashing. 

- Thrashing occurs when the physical memory of the system is not able to contain in itself the workload or the program. 

- Thrashing can also be called as the constant data swapping.

- Older systems were low end computers i.e., the RAM they had was insufficient to be employed in modern usage patterns. 

- Thus, when their memory was increased they became noticeably faster. 

- This happened because of the availability of more memory which reduce the amount of swapping and thus increased the processing speed. 

- IBM system/ 370 (mainframe computer) faced this kind of situation. 

- In it a certain instruction consisted of an execute instruction pointing over to another move instruction. 

- Both of these instructions crossed the page boundary and also the source from which the data has to be moved and the destination where it was to be placed both crossed the page boundary. 

- Thus, this particular instruction altogether required 8 pages and that too at the same time in memory. 

- Now if the operating system allocated less than 8 pages, a page fault is sure to occur. 

- This page fault will lead to thrashing of all the attempts of restarting the failing instruction. 

- This may even reduce the CPU utilization to almost zero!

How can a system handle thrashing?

For resolving the problem of thrashing, the following things can be done:

1. Increasing the amount of main memory i.e., the RAM in the system. This is the best ever solution for this and will be helpful for a long term also.

2. Decreasing the number of programs to be executed by the system.

3. Replacing the programs that utilize heavy memory with their less memory utilizing equivalents.

4. Making improvements in the spatial locality.

- Thrashing can also occur in cache memory i.e., the faster storage space that is used for speeding up the data access. 

- Then it is called cache thrashing. 

- It occurs when the cache is accessed in a way that it leaves it of no benefit. 

- When this happens many main memory locations compete with each other for getting the same cache lines that it turn leads to a large number of cache misses.