Operating System Scheduling Techniques

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Questions and Answers

What is the average waiting time for the processes in the FCFS scheduling?

17 (correct)

Which process has the longest waiting time in the FCFS scheduling?

All processes have the same waiting time

P3 (correct)

If the time quantum for Round Robin scheduling is set to 50 milliseconds, how might this affect the process scheduling?

Processes will be served for a maximum of 50 milliseconds each before moving to the next. (correct)

Processes with longer burst times will not be served.

Processes will be completed more quickly than in FCFS scheduling.

All processes will finish immediately regardless of burst time.

What is the waiting time for process P2 in the FCFS scheduling?

24 Signup and view all the answers

In FCFS scheduling, which statement about the order of process execution is true?

Processes are executed in the order they arrive. Signup and view all the answers

What is a defining characteristic of non-preemptive scheduling?

Processes hold the CPU until they terminate or become waiting. Signup and view all the answers

Which statement is NOT true about non-preemptive scheduling?

The operating system can forcibly take the CPU from a process. Signup and view all the answers

In non-preemptive scheduling, what occurs when a process switches to the waiting state?

The CPU remains idle until the process returns. Signup and view all the answers

What is the main advantage of non-preemptive scheduling?

It reduces context switching overhead. Signup and view all the answers

Which of the following best describes a scenario in non-preemptive scheduling?

Once a process starts its execution, it completes or waits before yielding the CPU. Signup and view all the answers

What happens to a process after its allocated time quantum expires?

It is preempted and added to the ready queue. Signup and view all the answers

How is CPU time distributed among n processes with a time quantum of q?

Each process gets 1/n of the CPU time in q time units. Signup and view all the answers

If there are 4 processes in the ready queue and a time quantum of 2, how much CPU time does each process get during one complete cycle?

2 time units Signup and view all the answers

What is the maximum duration a process can run in a single time slice?

q time units Signup and view all the answers

Which of the following correctly describes the behavior of processes in the ready queue when the time quantum is set?

Processes are given a fixed time to execute before being preempted. Signup and view all the answers

What is the primary purpose of the base and limit registers in the context of memory management?

To protect user processes from accessing each other's memory Signup and view all the answers

What characterizes external fragmentation?

Memory is available but not contiguous for allocation Signup and view all the answers

How does paging help in memory management?

By dividing memory into fixed-sized frames to avoid external fragmentation Signup and view all the answers

What is a Translation Lookaside Buffer (TLB) primarily used for?

To provide address-space protection by identifying processes uniquely Signup and view all the answers

What is a consequence of internal fragmentation in memory allocation?

Allocated memory may not be efficiently utilized, wasting space. Signup and view all the answers

What is the typical size range of entries in a Translation Lookaside Buffer (TLB)?

64 to 1,024 entries Signup and view all the answers

What occurs during a TLB miss?

The missing value is loaded into the TLB for faster future access Signup and view all the answers

Why are replacement policies important in managing TLB entries?

To determine which entry to remove when the TLB is full Signup and view all the answers

Which of the following statements is true regarding wired-down entries in a TLB?

They are meant for permanent fast access Signup and view all the answers

What is the impact of not flushing the TLB at every context switch?

Improved performance and less overhead in context switches Signup and view all the answers

Study Notes

Preemptive Scheduling

Preemptive scheduling allows the operating system to interrupt a running process and allocate the CPU to another process. This prevents a single process from monopolizing the CPU.

Non-Preemptive Scheduling

A process that receives the CPU keeps it until it completes or voluntarily releases it by transitioning to the waiting state.

First-Come, First-Served Scheduling (FCFS)

Processes are served in the order they arrive in the ready queue.
Example:
- P1 arrives first with a burst time of 24.
- P2 arrives second with a burst time of 3.
- P3 arrives third with a burst time of 3.
- P1 runs first, followed by P2 and then P3.

Round Robin (RR) Scheduling

Each process gets a small time slice (quantum) of the CPU before being preempted and moved to the end of the ready queue.
The time quantum is typically 10-100 milliseconds.
Example:
- If there are n processes in the ready queue, each process gets 1/n of the CPU time in chunks of at most q time units.

Memory Management

The Operating system manages the process of allocating and deallocating memory to processes.
It protects user processes from each other and from modifying the operating system's code and data.

Memory Management Unit (MMU)

The MMU translates logical addresses used by processes to physical addresses the computer actually uses.
The MMU uses a base register to store the smallest physical address and a limit register to define the range of logical addresses allowed for a process.

Memory Fragmentation

External Fragmentation: The total memory space exists to satisfy a request, but it is not contiguous.
Internal Fragmentation: The allocated memory may be slightly larger than requested leading to unused memory within a partition.

Paging

Paging divides physical memory into fixed-sized blocks called frames and logical memory into blocks of the same size called pages.
Pages are loaded into frames in non-contiguous physical memory.
A page table acts as a mapping between logical and physical addresses.
Paging helps in:
- Avoiding external fragmentation.
- Adapting to variable-sized memory chunks.

Translation Look-aside Buffer (TLB)

The TLB is a cache that stores recently used page translations to reduce the time spent accessing the page table.
TLB entries typically include the ASID (address-space identifier) for protection.
When a TLB miss occurs, the page translation is retrieved from the page table and loaded into the TLB.

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Description

Explore the essential scheduling techniques in operating systems, including preemptive and non-preemptive scheduling, First-Come, First-Served, and Round Robin scheduling. Understand how each method impacts CPU utilization and process execution.