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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.


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 the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
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Source:  OpenStax, Operating systems. OpenStax CNX. Aug 13, 2009 Download for free at http://cnx.org/content/col10785/1.2
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