3. Storage Management
1 What
are the disadvantages of linked allocation technique of files? [4]
Ans:--
i.
Slow direct accessing of any disk block - To find out the Nth block of a file,
we must start at the beginning of that file and follow the Dointer until we get
to the Nth block.
ii. Space requirement for pointers - If a pointer requires two words out of a 512 words block, then around 39 percent of the disk is being pond only for pointers, not for information. Each file therefore requires more space.
iii. Reliability - Since disk blocks are linked by pointers, a single damaged pointer can make thousands of disk blocks inaccessible. Some operating systems address this problem by storing pointers in a dedicated file and making redundant copies of it. The idea is to copy the list of pointers into the main memory and thus facilitate faster access of disk blocks. Redundancy of pointers files is employed for safer recovery.
ii. Space requirement for pointers - If a pointer requires two words out of a 512 words block, then around 39 percent of the disk is being pond only for pointers, not for information. Each file therefore requires more space.
iii. Reliability - Since disk blocks are linked by pointers, a single damaged pointer can make thousands of disk blocks inaccessible. Some operating systems address this problem by storing pointers in a dedicated file and making redundant copies of it. The idea is to copy the list of pointers into the main memory and thus facilitate faster access of disk blocks. Redundancy of pointers files is employed for safer recovery.
2 A 2500 byte long segment begins at
physical address 9010. Assuming that a location can hold one byte. what will be
the physical address of byte 187. byte 3004? [4]
3 Describe briefly what happens after a
user types the command "delete myfile" In a system. [4]
4 When a process terminates
It releases its memory space where a "hole" is created. If there are
several such “holes" In memory and another process Is scheduled to run,
then what are the advantages and disadvantages of using the
"best-fit" and the "first-fit" strategies for. .allocating
memory space to the, new process? [5]
5 Consider the following page reference
during a given time interval for a memory consisting of 5 frames:
12.16,13.16.17.14.12.14.18.14.12.19.17.18. Using the I) FIFO replacement
strategy and the' II) the LRU replacement strategy compare the results. Which
page replacement strategy is considered to be the best one and what
disadvantage does it have? [11]
6 Assuming the size of logical address
to be 232 and the page size to be 212. find the bits required for page number
and page offset In the logical address. [2]
7 How does the combined paging-segmentation
scheme work? 4
Ans :
- Combine Paging and Segmentation
- Structure
- Segments correspond to logical units: code, data, stack. Segments vary in size and are often large.
- Each segment contains one or more (fixed-size) pages.
- Two levels of mapping to make tables manageable (2 look-ups!)
- Page table for each segment.
- Segments Pages Advantages
- Advantages of Segments
- Supports sparse address spaces. If segment is not used, no need for page table. Decreases memory required for page tables.
- Advantages of Paging
- Eliminate external fragmentation.
- Advantages of Both. Increases flexibility of sharing. Share at two levels: Page or segment (entire page table).
- Segments + Pages Disadvantages
- Internal fragmentation increases. Last page of every segment in every process .
- Increases overhead of accessing memory
- 1 or 2 overhead references for every real reference.
- Large page tables
- Two potential solutions: Page the user page tables (multilevel page table), Inverted page table.
8 Consider a system where free space is
kept in a free-space list. Suppose the pointer to the free-space list is lost.
Can the system reconstruct the free-space list? Explain, Why must the bit map
for file allocation be kept on mass storage rather than in main memory? [4]
9 What is TLB? Mention the disadvantage
of using it. Find out the effective memory access time with an 85% hit ratio
and the following access times: TLB access time: 25ns; MM access time: 100ns. [7]
10 What is the concept behind
the working set strategy? What are the probiems associated with the strategy? [6]
11 Consider the following page reference during a given time
interval for a memory consisting of 3 frames: b,c,a,a,c,d,b,e,f,a,b,d,a,f,a.
Using the i) First In First Out (FIFO) replacement strategy and the ii) the
Least Recently Used (LRU) replacement strategy compare the results. Does always
the page hit increase if the number of frames is increased? Give reasons for
your answer. [12]
12 When
files are deleted, the blocks freed are put on the free list without being
erased. Should these blocks be erased of all the data on them? If yes, when
should it be done? [4]
13 The overhead required to read an entry
in a page table (including the time to access associative memory) is 50
nanoseconds. To reduce this overhead the computer has an associative memory
that can hold 16 entries and can be accessed in 10 nanoseconds.
i) Find
a formula that expresses the effective access time as a function of the hit
ratio.
ii) What
hit ratio is needed to reduce the effective access time to 70 nanoseconds? [10]
14 What problems could occur if a system
allowed a file system to be mounted simultaneously at more than one location? [6]
15 What
are the various modes that can be used to perform Static Address binding?
What are the pros & cons of such Static Address binding? [4]
16 Illustrate
the notions of temporal locality of reference as well as spatial
locality of reference in a high level program by some example. [4]
17 Suppose the Operating System would like
to mark a frame (main memory copy of a page) as Execute only i.e. Reading
& Writing on that page/frame is not allowed. What feature of the
System Architecture is needed to implement this? Discuss with justification. [4]
18 Specify,
in proper sequence, the steps involved in translating a CPU
generated Logical Address to its equivalent physical address in a Demand
Paged Memory Management System. Assume that each of the Process involved does
possess a huge Virtual Address Space such that the entire Page Map Table
(P.M.T.) of any process cannot be accommodated in the existing
Physical Memory System. Assume that the Physical Memory system consists
on 2 level Cache L1 & L2 as well as the main memory
while the secondary memory media (Disk) acts as the Virtual Memory.
Consider ALL possible Hit as well as Miss situations. [14]
19 What is TLB? Find out the effective
memory-access time with an 80% hit ratio and the following access time:
TLB access time: 20ns; MM access time: 100ns [8]
20 Describe the public-key encryption
scheme and mention how is it advantageous to the data-encryption standard. [10]
21 Consider
the following page reference during a given time interval for a memory
consisting of 5 frames: y,c,z,c,d,a,y,a,e,a,y,f,d,e. Using the i) FIFO
replacement strategy and ii) the LRU replacement strategy compare the results.
Repeat both FIFO and LRU replacement strategies for memory with 3 frames and
same page reference pattern. Comment on the findings and draw a conclusion
justifying the adoption of a particular replacement strategy. [18]
22 What
is dynamic loading? Mention its advantage. How is dynamic linking performed?
Mention any disadvantage that you can think of for both the schemes. [8]
23 Where
and how “bit vector/table” is used? What are the advantages and disadvantages
of the technique? [5]
24 What is the difference between a
physical address and a virtual address? [4]
25 A minicomputer uses the
buddy system for memory management. Initially it has one block of 256K at
address 0. After successive requests of 7K, 26K, 34K and 19K come in, how many
blocks are left and what are their sizes ad addresses? [6]
26 Explain working of virtual memory. [6]
27 A computer whose processes have 512
pages in their address space keeps its page tables in memory. The overhead
required for reading a work from the page table in 50 nsec. To reduce this
overhead, the computer had a TLB that holds 32 entries and can do lookups in
5nsec. What hit rate is needed to reduce the mean overhead to 20nsec? [6]
28 Contiguous allocation of
files leads to disk fragmentation. Is this internal or external fragmentation? [3]
29 Can we implement symbolic links in DOS
(FAT files system)? If so, show how, and if not, explain why.
[3]
30 Describe the problems with contiguous
allocation method for allocating disk space. How does linked allocation method
overcome the disadvantages of contiguous allocation method? [4]
31 Explain External Fragmentation. How the
compaction can be used to solve the problem of external fragmentation? [4]
32 What is a thrashing? How can be problem
of thrashing be prevented? [4]
33 Consider
the following page reference string:
1, 2, 3, 4, 2, 1, 5, 6, 2, 1, 2, 3,
7, 6, 3, 2, 1, 2, 3, 6
How many page faults would occur for
the LRU page replacement algorithm, assuming three frames? [8]
34 When do page faults occur? Describe the
actions taken by the operating system when a page fault occurs? [6]
35 Differentiate between logical address
and physical address. [4]
36 Distinguish
between logical and physical address space.
[4]
37 What is TLB? Find out the effective memory-access time
with an 87% hit ratio and the following access times:
TLB
access time: 12ns; MM access
time: 90ns [8]
38 Consider the following page
reference during a given time interval for a memory consisting of 3 frames :
v.w.u.u.w.x.v.y.z.u.v.x.u.z.u. Using the i) First In First Out (FIFO)
replacement strategy and the ii) the Least Recently Used (LRU) replacement
strategy compare the results. Does page hit increase if the number of frames is
increased? Give reasons for your answer. [10]
39 What is dynamic loading? Mention its
advantage. How dynamic linking is performed? Mention any disadvantage that you
can think of for both the schemes. [8]
40 Distinguish between paging and
segmentation schemes of memory management. [4]
41 Where and how "bit
vector/table" is used in free space management of disks? What are the
advantages and disadvantages of the technique? [5]
42 State the advantages of segmentation
over paging. [4]
43 What
do you understand by reentrant code? How does it support sharing? [6]
44 Given
memory partitions of 100K, 500K, 200K, 300K and 600K (in order). How would 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? [6]
45 Distinguish between internal
fragmentation and external fragmentation. Which of the two is prevalent in
paging system? [4]
Ans:-- Fragmentation occurs in a dynamic memory allocation
system when many of the free blocks are too small to satisfy any request.
External Fragmentation: External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.
Internal Fragmentation: Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used
External Fragmentation: External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.
Internal Fragmentation: Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used
46 What is the cause of thrashing? How can
the OS reduce the effects of thrashing and improve overall performance? [4]
47 Consider
the following page reference string.
1, 2, 3, 4, 5, 3, 4, 1, 6, 7, 8, 9,
7, 8, 9, 5, 4, 5, 4, 2
How many page faults would occur for
the following replacement algorithms for five frames?
1) LRU
Replacement
2) FIFO
Replacement [12]
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