星期二, 11月 03, 2009

CS3161 (A) OPERATING SYSTEM PRINCIPLES (DR. LIU WENYIN) (09CS3161_LW) - Answers to Tutorial 8 Questions

Q1 Explain the difference between internal and external fragmentation in memory management. Suggest ways to reduce or solve of both types of fragmentation.


Internal fragmentation is the area in a region or a page which is not used by the job occupied that region or page. This space is unavailable for use by the system until that job is finished or the region is released.

External fragmentation is a region which is unused and available, but it is too small for any of the waiting jobs.

To reduce or solve problems of fragmentation :
Internal fragmentation - reduce size of individual region / allocation unit.
External fragmentation - break down request into non-contiguous portions, compaction, swapping.



Q2 What is compaction ? Why use it ?


Movement of processes to eliminate small free memory partitions. Compaction is used to eliminate memory fragmentation (external) and to increase memory utilization.

It allows smaller memory partitions to form fewer bigger ones, thus allowing larger processes to run.





Q3 Given memory partition of 100K, 500K, 200K, 300K, and 600K (in order), how each of the First-fit, Best-fit, and Worst-fit algorithms place processes of 212K, 417K, 112K, and 426K (in order) ? Which algorithm makes the most efficient use of memory ?


(1) First-fit :
212K is put in 500K partition
417K is put in 600K partition
112K is put in 288K partition (new partition 288K = 500K - 212K)
426K must wait
(2) Best-fit :
212K is put in 300K partition
417K is put in 500K partition
112K is put in 200K partition
426K is put in 600K partition
(3) Worst-fit :
212K is put in 600K partition
417K is put in 500K partition
112K is put in 388K partition
426K must wait

In this example, the best-fit turns out to be the best algorithm.



Q4 (i) What is paging ?(ii) What is a frame ?(iii) What is contained in the page table ?(iv) How many frames are needed for each page ?(V)Draw the diagram to show how paging works.


(i) Splitting program up into a group of fixed-equal-sized partitions, allowing the parts to be non-contiguous in memory, during the execution of the process.
(ii) Fixed-size block of physical memory, each block must be of the same size as one page.
(iii) Page number, frame number, base address of each frame, presence, protection (permission), dirty bit.
(iv) One.

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