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Let:

  • h = fraction of time that a reference does not require a page fault.
  • tmem = time it takes to read a word from memory.
  • tdisk = time it takes to read a page from disk.

then

  • EAT = h * tmem + (1 - h) * tdisk.

If there a multiple classes of memory accesses, such as no disk access, one disk access, and two disk access, then you wouldhave a fraction (h) and access time (t) for each class of access.

Note that this calculation is the same type that computer architects use to calculate memory performance. In that case, theiraccess classes might be (1) cached in L1, (2) cached in L2, and (3) RAM.

Page selection and replacement

Once the hardware has provided basic capabilities for virtual memory, the OS must make two kinds of scheduling decisions:

  • Page selection: when to bring pages into memory.
  • Page replacement: which page(s) should be thrown out, and when.

Page selection Algorithms:

  • Demand paging: start up process with no pages loaded, load a page when a page fault for it occurs, i.e. until it absolutely MUST be inmemory. Almost all paging systems are like this.
  • Request paging: let user say which pages are needed. The trouble is, users do not always know best, and are not always impartial. They willoverestimate needs.
  • Prepaging: bring a page into memory before it is referenced (e.g. when one page is referenced, bring in the next one, just in case). Hard todo effectively without a prophet, may spend a lot of time doing wasted work.

Page Replacement Algorithms:

  • Random: pick any page at random (works surprisingly well!).
  • FIFO: throw out the page that has been in memory the longest. The idea is to be fair, give all pages equal residency.
  • MIN: naturally, the best algorithm arises if we can predict the future.
  • LFU: use the frequency of past references to predict the future.
  • LRU: use the order of past references to predict the future.

Example: Try the reference string A B C A B D A D B C B, assume there are three page frames of physical memory. Show the memoryallocation state after each memory reference.

Note that MIN is optimal (cannot be beaten), but that the principle of locality states that past behavior predicts futurebehavior, thus LRU should do just about as well.

Implementing LRU: need some form of hardware support, in order to keep track of which pages have been used recently.

  • Perfect LRU? Keep a register for each page, and store the system clock into that register on each memory reference. To replace a page, scanthrough all of them to find the one with the oldest clock. This is expensive if there are a lot of memory pages.
  • In practice, nobody implements perfect LRU. Instead, we settle for an approximation which is efficient. Just find an old page, not necessarilythe oldest. LRU is just an approximation anyway (why not approximate a little more?).

Clock algorithm, thrashing

This is an efficient way to approximate LRU.

Clock algorithm: keep "use" bit for each page frame, hardware sets the appropriate bit on every memory reference. The operatingsystem clears the bits from time to time in order to figure out how often pages are being referenced. Introduce clock algorithm where to find a page to throwout the OS circulates through the physical frames clearing use bits until one is found that is zero. Use that one. Show clock analogy.

Questions & Answers

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
Do somebody tell me a best nano engineering book for beginners?
s. Reply
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
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
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit 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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