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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.

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

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
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
Daniel
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
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
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
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
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.
Tarell
what is the actual application of fullerenes nowadays?
Damian
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.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
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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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