Operating System (OS) Interview Questions And Answers - Part IX.

41. What Is A PCB? Explain The Structure Of Process Control Block?


Process Control block is used for storing the collection of information about the processes and this is also called as the Data Structure which Stores the information about the process. The information of the Process is used by the CPU at the Run time.

A process control block in an operating system will at least have the following information:

• Process State: Information about the current state of the process, from new, ready, running, terminated.
• Program Counter: States the location for the next command to be run for the same process.
• CPU Register: State information on various types of registers is stored during interrupts for continuity to occur correctly when process resumes.
• Memory Management Information: State data on memory is stored such as page tables, limit registers, or segment tables.
• Accounting Information: Stores state data on values of CPU and real time utilized, time limits, account numbers, job or process numbers.
• I/O Status Information: Data on list of I/O devices apportioned to the process, a list of open files etc.

All major operating systems should at least have the above structure tailored to their unique process control block architectures.


42. Define & Differentiate Between Swapping & Spawning?

Swapping concept comes in terms of process scheduling. Swapping is basically implemented by Medium term scheduler. Medium term scheduler removes process from CPU for duration and reduces the degree of multiprogramming. And after some time these process can again be reintroduced into main memory. Process execution will again be resumed from the point it left CPU. This scheme is called swapping. More generally we can say swapping is removing of process from memory to secondary memory and again back to main memory.

Process of creating child thread from parent thread is called spawning. When the OS at the explicit request of another process creates a process, this action is called process spawning.


43. What Is Disk Scheduling? What Are The Different Disk Scheduling Techniques?

The processes running on a machine may have multiple outstanding requests for data from the disk. In what order should requests be served? Disk Scheduling is the answer for this issue.

File systems must be accessed in an efficient manner, especially with hard drives, which are the slowest part of a computer. In multiprogramming systems, many processes may be generating requests for reading and writing disk records. As a computer deals with multiple processes over a period of time, a list of requests to access the disk builds up. For efficiency purposes, all requests (from all processes) are aggregated together.

The technique that the operating system uses to determine which requests to satisfy first is called disk scheduling. Disk scheduling involves a careful examination of pending requests to determine the most efficient ways to service the waiting requests.

Types of disk scheduling techniques are:

• First Come-First Serve (FCFS)
• Shortest Seek Time First (SSTF)
• Elevator (SCAN)
• Circular SCAN (C-SCAN)
• LOOK
• C-LOOK

44. Give Several Reasons Why Dead Line Scheduling Is Complex?

In deadline scheduling certain jobs are scheduled to be completed within a specific time or deadline. These jobs may have very high value if delivered on time and may be worthless if delivered later than the deadline. The user is often willing to pay a premium to have the system ensure on-time consumption.

Deadline scheduling is complex for many reasons.

• The user must supply the resource requirements of the job in advance. Such information is rarely available.
• The system must run the deadline job without severely degrading service to other users.
• The system must plan its resource requirements through to the deadline because new jobs may arrive and place unpredictable demands on the system.
• If many deadline jobs are to be active at once, scheduling could become so complex.
• The intensive resource management required by deadline scheduling may generate substantial overhead.

45. Distinguish Between Scheduling Policy & Scheduling Mechanisms.

Scheduling Mechanism gives us the ability to perform an action; while Scheduling Policy decides what we do with the mechanism. Most automobiles have the power to travel at speeds of over 150 kilometers per hour (the mechanism), but legal speed limits are usually set well below that (the policy).


ALSO READ:

36. What two advantages and disadvantages do threads have over multiple processes? Suggest one application that would benefit from the use of threads, and one that would not.
37. Explain the terms time-sharing and multi-programming?
38. What is spooling and what are the benefits? Is it useful on a single-user system?
39. Why is the process table needed in a timesharing system? Is it also needed in personal computer systems in which only one process exists, that process taking over the entire machine until it is finished?
40. Differentiate between a process and a program?

41. What Is A PCB? Explain The Structure Of Process Control Block?
42. Define & Differentiate Between Swapping & Spawning?
43. What Is Disk Scheduling? What Are The Different Disk Scheduling Techniques?
44. Give Several Reasons Why Dead Line Scheduling Is Complex?
45. Distinguish Between Scheduling Policy & Scheduling Mechanisms.

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