<< Chapter < Page Chapter >> Page >

The barbershop problem

The original barbershop problem was proposed by Dijkstra. A variation of it appears in Silberschatz and Galvin’s OperatingSystems Concepts. A barbershop consists of a waiting room with n chairs, and the barber room containing the barber chair. If there are no customers to be served,the barber goes to sleep. If a customer enters the barbershop and all chairs are occupied, then the customer leaves the shop. If the barber is busy, but chairsare available, then the customer sits in one of the free chairs. If the barber is asleep, the customer wakes up the barber. Write a program to coordinate thebarber and the customers.

Scheduling

Until now we have talked about processes, from now on we will talk about resources, the things operated upon by processes. Resourcesrange from cpu time to disk space to channel I/O time.

Resources fall into two classes:

  • Preemptible: processor or I/O channel. Can take resource away, use it for something else, then give it back later.
  • Non-preemptible: once given, it cannot be reused until process gives it back. Examples are file space, terminal, and maybe memory.

OS makes two related kinds of decisions about resources:

  • Allocation: who gets what. Given a set of requests for resources, which processes should be given which resources in order to make most efficientuse of the resources? Implication is that resources are not easily preemptible.
  • Scheduling: how long can they keep it. When more resources are requested than can be granted immediately, in which order should they beserviced? Examples are processor scheduling (one processor, many processes), memory scheduling in virtual memory systems. Implication is that resource ispreemptible.

Cpu scheduling

Processes may be in any one of three general scheduling states:

  • Running.
  • Ready. That is, waiting for CPU time. Scheduler and dispatcher determine transitions between this and running state.
  • Blocked. Waiting for some other event: disk I/O, message, semaphore, etc. Transitions into and out of this state are caused by variousprocesses.

There are two parts to CPU scheduling:

  • The dispatcher provides the basic mechanism for running processes.
  • The scheduler is a piece of OS code that decides the priorities of processes and how long each will run.

This is an example of policy/mechanism separation.

Goals for Scheduling Disciplines

  • Efficiency of resource utilization (keep CPU and disks busy).
  • Minimize overhead (context swaps).
  • Minimize response time. (Define response time.)
  • Distribute cycles equitably. What does this mean?

Fcfs (also called fifo)

Run until finished.

  • In the simplest case this means uniprogramming.
  • Usually, "finished" means "blocked". One process can use CPU while another waits on a semaphore. Go to back of run queue when ready.
  • Problem: one process can monopolize CPU.

Solution: limit maximum amount of time that a process can run without a context switch. This time is called a time slice.

Round robin

Run process for one time slice, then move to back of queue. Each process gets equal share of the CPU. Most systems use somevariant of this. What happens if the time slice is not chosen carefully?

Questions & Answers

what is mutation
Janga Reply
what is a cell
Sifune Reply
how is urine form
Sifune
what is antagonism?
mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
how many types of solid did we have
Samuel Reply
what is an ionic bond
Samuel
What is Atoms
Daprince Reply
what is fallopian tube
Merolyn
what is bladder
Merolyn
what's bulbourethral gland
Eduek Reply
urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
State the evolution relation and relevance between endoplasmic reticulum and cytoskeleton as it relates to cell.
Jeremiah
what is heart
Konadu Reply
how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
Pelagie Reply
what is the causative agent of malaria
Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
where does digestion begins
Achiri Reply
in the mouth
EZEKIEL
what are the functions of follicle stimulating harmones?
Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
what are the functions of Endocrine and pituitary gland
Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
Achor
what's biology?
Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Operating systems. OpenStax CNX. Aug 13, 2009 Download for free at http://cnx.org/content/col10785/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Operating systems' conversation and receive update notifications?

Ask