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GDB of CS604 will be conducted on Thursday, 07 February, 2019. It will remain open for two days.

 

GDB Topic

Consider a situation in which system is using demand paging technique to load pages from Swap Space to Main Memory. Let’s suppose that a process needs to be loaded in Main Memory. Initially, there are no pages of that process in Main Memory and no free frames are available in main memory.

In that situation, which technique will be used so that process can be loaded in the main memory without affecting the degree of multi-programming and either the page fault rate will increase or decrease thereby increasing or decreasing the performance of system?  

You are required to give your own opinion. Keep your answer precise and to the point and should not exceed 6 to 8 lines.

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Yes but if you use like FIFO algorithm in page replacement then it cause more page fault then the Least Used algorithm.

hence if we use good page replacement algorithm then it reduce page fault rate.

Basic Page Replacement

  • The previously discussed page-fault processing assumed that there would be free frames available on the free-frame list. Now the page-fault handling must be modified to free up a frame if necessary, as follows:
    1. Find the location of the desired page on the disk, either in swap space or in the file system.
    2. Find a free frame:
      1. If there is a free frame, use it.
      2. If there is no free frame, use a page-replacement algorithm to select an existing frame to be replaced, known as the victim frame.
      3. Write the victim frame to disk. Change all related page tables to indicate that this page is no longer in memory.
    3. Read in the desired page and store it in the frame. Adjust all related page and frame tables to indicate the change.
    4. Restart the process that was waiting for this page.                                            This is helpful or not?

Thanks for posting but it not helping to find either page fault rage increase/decrease.

  • There are two major requirements to implement a successful demand paging system. We must develop a frame-allocation algorithm and a page-replacement algorithm. The former centers around how many frames are allocated to each process ( and to other needs ), and the latter deals with how to select a page for replacement when there are no free frames available.
  • The overall goal in selecting and tuning these algorithms is to generate the fewest number of overall page faults. Because disk access is so slow relative to memory access, even slight improvements to these algorithms can yield large improvements in overall system performance.
  • Algorithms are evaluated using a given string of memory accesses known as a reference string, which can be generated in one of ( at least ) three common ways:
    1. Randomly generated, either evenly distributed or with some distribution curve based on observed system behavior. This is the fastest and easiest approach, but may not reflect real performance well, as it ignores locality of reference.
    2. Specifically designed sequences. These are useful for illustrating the properties of comparative algorithms in published papers and textbooks, ( and also for homework and exam problems. :-) )
    3. Recorded memory references from a live system. This may be the best approach, but the amount of data collected can be enormous, on the order of a million addresses per second. The volume of collected data can be reduced by making two important observations:
      1. Only the page number that was accessed is relevant. The offset within that page does not affect paging operations.
      2. Successive accesses within the same page can be treated as a single page request, because all requests after the first are guaranteed to be page hits. ( Since there are no intervening requests for other pages that could remove this page from the page table. )
      • So for example, if pages were of size 100 bytes, then the sequence of address requests ( 0100, 0432, 0101, 0612, 0634, 0688, 0132, 0038, 0420 ) would reduce to page requests ( 1, 4, 1, 6, 1, 0, 4 )
  • As the number of available frames increases, the number of page faults should decrease                                                                                                                    Now?

these  reference string, will be used by algo 

and on the basis of algo further page fault rate will increase / decrease

Kalimullah: page fault is occurring before page replacement how can page replacement will help to reduces number of page fault. page fault had occured thats why process looks for pages in disk.

Bht difficult hai nahi samjh lag raha mjhe ma mada banda hun

Page fault is not occurred because of page replacement. page replacement occurred due to page fault and page fault occurred due to unavailability of frame.

Correct algo prevent more page fault whereas incorrect or inefficient algo cause more page fault.

if you read or watch lecture about FIFO Algo and Recent Past Algo you will see FIFO algo cause more page fault rate than Recent Past Algo

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Here is in written form.

Basic Page Replacement The previously discussed page-fault processing assumed that there would be free frames available on the free-frame list. Now the page-fault handling must be modified to free up a frame if necessary, as follows: 1. Find the location of the desired page on the disk, either in swap space or in the file system. 2. Find a free frame:  If there is a free frame, use it.  If there is no free frame, use a page-replacement algorithm to select an existing frame to be replaced, known as the victim frame.  Write the victim frame to disk. Change all related page tables to indicate that this page is no longer in memory. 3. Read in the desired page and store it in the frame. Adjust all related page and frame tables to indicate the change. 4. Restart the process that was waiting for this page.

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