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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

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
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
hi
Loga
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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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