Questions about how operating systems decide which process to execute at which time.

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Which process comes first?

Let's say we have 3 processes: Process 1 service time 6 arrival time 0 Process 2 service time 2 arrival time 1 Process 3 service time 5 arrival time 3 We know that: after 1 time, process 2 will ...
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40 views

Can you explain this exercise solution to me? [closed]

A process running on CTSS (Compatible Time Sharing System) needs 30 quanta to complete. How many times must it be swapped in, including the very first time (before it has run at all)? • Answer : The ...
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Highest Response Ratio Next (Scheduling Algorithm)

In CPU scheduling HRRN(Highest Response Ratio Next) algorithm chooses the next process to be scheduled using the formula (W+S)/S where W-> waiting time & S->Service Time. Can someone share their ...
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Problems of cyclic executive approach for scheduling of real time systems

I have a doubt on the cyclic executive approach for scheduling on real time systems. From Alan Burns's book Real-time Systems and Programming Languages. This final property represents one of the ...
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Operating system processes

I'm a bit confused about this question: "Multiple jobs can run in parallel and finish faster than if they had run sequentially. Suppose that two jobs, each needing 10 minutes of CPU time, start ...
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57 views

User level threads are transparent to the kernel?

Find whether given statement True or False? Explain. User level threads are transparent to the kernel? My attempt : False. Since, user level threads are managed by a user level library ...
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1answer
47 views

How does the OS determine the CPU burst time of a process?

This question is related to OS. After having solved many problems in CPU scheduling I was wondering how does the CPU know the burst time of all the processes in the main memory before executing them? ...
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2answers
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Is the program in secondary memory can be a process?

Let us consider the following scenario, The cpu started executing a program, which is in secondary memory with n lines of code, so, it brings m ( < n ) lines of code in to main memory and started ...
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Processes running in Distributed Systems (Advanced Operating System)

I had a test today, there was a question that was a bit confusing. Anyway I am asking the question, from memory, and explaining what I answered. It would be great if someone could explain if I did ...
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Difficulty understanding pre-emptive vs non-preemptive CPU scheduling

I'm having difficulty understanding what my book is trying to say in regards to preemptive and non-preemptive CPU scheduling. It says the following: CPU Scheduling decisions may take place under ...
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1answer
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Starvation Condition in CPU/Process Scheduling?

Let's say OS is using preemptive CPU scheduling. A process P1 gets a chance to run for some time and then next higher priority process comes and preempts P1. After that higher priority processes are ...
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1answer
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Can the processor just turn off the interrupt request line?

I was doing some of the interrupts question online and found this Multiple choice question How can the processor ignore other interrupts when it is servicing one a) By turning off the interrupt ...
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Optimality of EDF

For Earliest Deadline First, I have read that if utilization is less or equal to 1, the task set is schedulable under EDF. But does this criteria expresses the optimality for EDF? If not, how can we ...
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Time quantum assignment to processes

In a book I am reading it is stated that an input/output intensive process should receive a higher time quantum whereas a computing intensive process on the other hand should receive a lower one. Here ...
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Processes concurrency

I am getting ready for my end of year exams and I got stuck on this question. Consider the following 3-process concurrent program which uses semaphores S1, ...
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44 views

Efficiency of incorporating enhancement

I've been having problems finding the efficiency of an enhanced processor, here is the question: Suppose that we want to enhance the processor used for Web serving. The new processor is 10 ...
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1answer
143 views

What is a student process?

According to Galvin and Silberschatz, 5 queues are maintained in multilevel queue scheduling, each for: System Process Interactive Process Interactive Editing Process Batch process Student ...
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325 views

Can Shortest Job First Scheduling be subject to convoy effect?

Look at the below table for example : Look at the Non-Preemptive SJF table Suppose if the burst time of process $P_1$ was a very large number compared to $7$, the processes $P_3,P_2$ and $P_4$ have ...
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Switching between user-level and kernel-level threads

After reading difference between user-level threads and kernel-level threads ,now I am in doubt of context switching.Context switching is managed by OS when switching between processes/threads but ...
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105 views

Round-Robin Scheduler - What exactly is the quantum time used for?

Suppose you have three stages for a job: Creation stage = the job gets created Executing stage = the job is executed/served Finishing stage = the job is "marked" as finished. Now, say the quantum ...
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193 views

How are system calls handled in a virtual machine?

Quoting wikipedia, a system call is: In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services (e.g. ...
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What guarantees do “soft” real-time operating systems actually provide

I think I know what a "hard" real-time operating system is. It is an operating system with a scheduler that provides a contract with the application programmer. An application provides a deadline ...
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Which process states exist a multi-processor machine?

In a single-processor architecture, A process might be New, Ready, Running, Blocked and Terminated. Are there any additional states in a multi-processor architecture?
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Does a multi cpu system not improve the first come first serve (fcfs) cpu scheduling algorithm?

I created a program that simulates the fcfs scheduling algorithm. First I implemented it for a single core, then modified it for four homogenoues cpus. I compared the results with several datasets, ...
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How does the OS know the physical address of a process' first memory page?

If I have a program, its instructions are stored on the hard drive. When I double-click the executable the pages of memory for this process must get loaded in to RAM. However, for the pages to get ...
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How can I talk about priority inversion if it's impossible to specify priority?

In the paper: Leslie Lamport: What It Means for a Concurrent Program to Satisfy a Specification: Why No One Has Specified Priority; ACM Symp Principles of Programming Languages, (POPL-12):78-83, ...
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162 views

How to build/recognise a deadlock-free order of resources

I have been faced with a problem where three concurrent processes X, Y ,Z execute three different code segments that access and update certain shared variables. Before entering the respective code ...
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Exponential averaging for SJF CPU scheduling

Calculate the exponential averaging with $T_1 = 10$, $\alpha=0.5$ and the algorithm is SJF with previous runs as $8,7,4,16$. 9 8 7.5 None I am getting 4. None as the answer. ...
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What happens when time slices are too short or too long?

What bad things can occur when the lengths of the time slices in a multitasking system are too small? What can happen when they are too big? How should the length of the time slice be chosen?
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processes response time confusion

An OS contains 10 identical processes that were initiated at the same time.Each process contains 15 identical requests. Each request consume 20 msec of CPU time.A request is followed by an I/O ...
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365 views

A variant of job assignment (scheduling) problem with variable time span

The problem is a scheduling problem with n jobs and k machines. Each job i can be started at any time, but its duration is not exactly known except a time span interval. For example, a job may take ...
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Disk scheduling and process scheduling

Whats the diffrence between Disk scheduling and process scheduling? I mean i know the roles of disks and processes, but it seems to me they both have the same aim. to reduce monopolies, they even both ...
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The convoy effect in process scheduling

As I understand the convoy effect, in the context of vehicular traffic in a road system. A slow moving group of vehicles passes through the system, slowing traffic even in areas which were not ...
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What is the difference between Multiprogramming and Multitasking

I am finding it difficult to clearly differentiate between Multiprogramming and Multitasking. My primary source has been Wikipedia, but the WP article seems to be a little at odds with some less ...
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What mechanisms prevent a process from taking over the processor forever?

Suppose a process keeps running code (e.g. an infinite loop). How can other programs take over? What prevents the process from remaining active forever?
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Process state in multi threaded process

What would be the process state in a multi threaded process, in which threads are in different states (running, waiting, blocked etc)
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Scheduling algorithms and quantum time

I would like to know what impacts does a scheduling algorithm say Round Robin of FIFO have on a Operating system. I understand the a scheduling algorithm has the processes run in burst then switch ...
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918 views

How are threads implemented in different OSs?

I was reading Linux Kernel Development by Robert Love, where I came across this Linux takes an interesting approach to thread support: It does not differentiate between threads and normal ...
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6k views

What is meant by interrupts in the context of operating systems?

I've decided to read Operating Systems Concepts by Silberschatz, Galvin Gagne (8th edition) over the summer. I've gotten to a topic that's confusing me - interrupts and their role as it relates to ...
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What is the average turnaround time?

For the following jobs: The average wait time would be using a FCFS algorithm: (6-6)+(7-2)+(11-5)+(17-5)+(14-1) -> 0+5+6+10+13 -> 34/5 = 7 (6.8) What would the average turnaround time be?
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Which queue does the long-term scheduler maintain?

There are different queues of processes (in an operating system): Job Queue: Each new process goes into the job queue. Processes in the job queue reside on mass storage and await the allocation of ...
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655 views

Round-robin scheduling: allow listing a process multiple times?

In a round-robin scheduler, adding a process multiple times to the process list is a cheap way to give it higher priority. I wonder how practical an approach this might be. What benefit does it have ...
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583 views

Are two-level schedulers only useful to manage swapping?

Two-level scheduling is useful when a system is running more processes than fit in RAM: a lower-level scheduler switches between resident processes, and a higher-level scheduler swaps groups of ...
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994 views

Does cooperative scheduling suspend processes when they perform an I/O operation?

Many operating systems references say that with cooperative (as opposed to preemptive) multitasking, a process keeps the CPU until it explicitly voluntarily suspends itself. If a running process ...