Operating Systems Questions

Q:

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?

Answer

Thrashing is caused by under allocation of the minimum number of pages required by a process, forcing it to continuously page fault. The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming. It can be eliminated by reducing the level of multiprogramming.


 


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Q:

Explain Belady's Anomaly?

Answer

Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.

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Q:

List out some reasons for process termination.

Answer

- Normal completion


- Time limit exceeded


- Memory unavailable


- Bounds violation


- Protection error


- Arithmetic error


- Time overrun


- I/O failure


- Invalid instruction


- Privileged instruction


- Data misuse


- Operator or OS intervention


- Parent termination.

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Q:

What is page cannibalizing?

Answer

Page swapping or page replacements are called page cannibalizing.

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Q:

What are the typical elements of a process image?

Answer

User data: Modifiable part of user space. May include program data, user stack area, and programs that may be modified.


User program: The instructions to be executed.


System Stack: Each process has one or more LIFO stacks associated with it. Used to store parameters and calling addresses for procedure and system calls.


Process control Block (PCB): Info needed by the OS to control processes.

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Q:

What is a binary semaphore? What is its use?

Answer

A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.

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Q:

Define latency, transfer and seek time with respect to disk I/O.

Answer

Seek time is the time required to move the disk arm to the required track. Rotational delay or latency is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any) and latency is the access time. Time taken to actually transfer a span of data is transfer time.

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Q:

What are demand-paging and pre-paging?

Answer

With demand paging, a page is brought into memory only when a location on that page is actually referenced during execution. With pre-paging, pages other than the one demanded by a page fault are brought in. The selection of such pages is done based on common access patterns, especially for secondary memory devices.

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