You are here: Home » Content » Review question: Protection and Security
Content Actions
Quality
Affiliated with  (?)
This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
Lenses
Tags  (?)
These tags come from the endorsement, affiliation, and other lenses that include this content.

Review question: Protection and Security

Module by: Duong Anh Duc

Summary: Review question of Protection and Security

Briefly explain how single, double, and circular buffering can be used to improve the performance of a process.

With single buffering, the operating system dedicates a buffer to a process. This allows the process to work on one block while the I/O system reads another block into the buffer.
With double buffering, the operating system dedicates two buffers to a process. The process can then empty one buffer while the I/O system fills the other. This allows the process to process data as fast as the I/O system can read it in.

Why is the average search time to find a record in a file less for an indexed sequential file than for a sequential file?

With a sequential file, the records are stored in order of a particular key, but searching requires going through all of the records sequentially until a match is found. In an indexed sequential file, the index contains pointers to every K records in the file. For a file with N records, searching for a record with a particular key requires searching only the N/K entries in the index, and then the K records in that section of the file. Depending on the size of K, this can be considerably less than sequentially searching through all N entries.

Briefly describe the four categories of information stored in a file directory. Give an example of an item from each category, along with a one sentence description.

  • Basic information is used to identify the file and includes the file name and type.
  • Address information is used to find the file on disk and includes the address and size of the file.
  • Access control information is used to provide sharing and protection and includes the owner and related access information.
  • Usage information is used for accounting and includes such items as the date of creation and last modification.

Problems

Perform the same type of analysis as that of Table 11.2 for the following sequence of disk track requests: 27, 129, 110, 186, 147, 41, 10, 64, 120. Assume that the disk head is initially positioned over track 100 and is moving in the direction of decreasing track number. Do the same analysis, but now assume that the disk head is moving in the direction of increasing track number.

Answer:
FIFO SSTF SCAN C-SCAN
Next track accessed Number of tracks traversed Next track accessed Number of tracks traversed Next track accessed Number of tracks traversed Next track accessed Number of tracks traversed
27 73 110 10 64 36 64 36
129 102 120 10 41 23 41 23
110 19 129 9 27 14 27 14
186 76 147 18 10 17 10 17
147 39 186 39 110 100 186 176
41 106 64 122 120 10 147 39
10 31 41 23 129 9 129 18
64 54 27 14 147 18 120 9
120 56 10 17 186 39 110 10
Average 61.8 Average 29.1 Average 29.6 Average 38

If the disk head is initially moving in the direction of increasing track number, the results change for only SCAN and C-SCAN:

SCAN C-SCAN
Next track accessed Number of tracks traversed Next track accessed Number of tracks traversed
110 10 110 10
120 10 120 10
129 9 129 9
147 18 147 18
186 39 186 39
64 122 10 176
41 23 27 17
27 14 41 14
10 17 64 23
Average 29.1 Average 35.1
Consider the disk system described in Problem 11.9 and assume the disk rotates at 360 rpm. A processor reads one sector from the disk using interrupt-driven I/O, with one interrupt per byte. If it takes 2.5 μs to process each interrupt, what percentage of time will the processor spend handling I/O (disregard seek time)? Repeat using DMA, and assume one interrupt per sector.
Answer:
If you have the disk rotating at 360 rpm, then one byte is read off the disk in:
T = b/rN = 1/(360 rpm*512 bytes/sector * 96 sectors/track)
T = 3.4 μs
If one interrupt occurs every byte, then the operating system will process an interrupt (2.5 μs), then do some work, then process another interrupt, etc. The time between interrupts is equal to the time it takes to read one byte. In other words, the disk will read one byte, then interrupt the operating system, then read another byte. The operating system will only be able to get work done if it has enough time to finish processing an interrupt before the next one occurs. In this case, the processor will spend 2.5 μs / 3.4 μs or 74% of its time handling I/O.
With DMA, there is only one interrupt for the entire sector. It takes 3.4 μs/byte * 512 bytes/sector = 1,741 μs to read the sector. Therefore the processor only spends 2.5 μs / 1741 μs = .14% of the time handling I/O.
Note: The above answer does not take into account rotational delay. If you include rotational delay, then the time to read a byte becomes very large:
T = b/rN + 1/2r = 83 ms
This means the time the processor spends processing interrupts is very small (approximately .003% in both cases).

It should be clear that disk striping can improve the data transfer rate when the strip size is small compared to the I/O request size. It should also be clear that RAID 0 provides improved performance relative to a single large disk, because multiple I/O requests can be handled in parallel. However, in this latter case, is disk striping necessary? That is, does disk striping improve I/O request rate performance compared to a comparable disk array without striping?

Answer:
Consider a disk array that does not use striping. If multiple I/O requests are issued for files on different disks, then performance is the same as a striped disk array. However, the advantage of striping is that it improves the chances that multiple I/O requests will in fact be for data on different disks. Without striping, all the files from a given directory will be on the same disk, so a single user operating in a single directory will be using one disk instead of all the disks in the array. Likewise, it is likely that all user files will be on one disk, and all binaries on another, so that multiple users accessing their files will use the same disk. Disk striping is a simple, automated way for the administrator of the array to ensure that files are spread out across all the disks.

You purchase a disk drive that spins at 9600 RPM, stores 300 KB per track, and has an average seek time of 8 ms.

  • What is the average transfer time to read a 1 MB file, assuming the file is stored on consecutive tracks?
T = Ts + 1/(2r) + b/(rN)
T = .008 + (60 s/m)(1 r)/ (2*9600 r/m) + (1 MB)(1024 KB/MB)(60 s/m)/(9600 r/m)(300 KB/r)
T = .008 s + .003125 s + .021333 s
T = 32.458 ms
  • What is the average transfer time to read a 1 MB file, assuming that the file is stored in 10 separate locations on the disk, and each location has 100 KB of contiguous data?
T = 10*(Ts + 1/(2r) + 1/(rN))
T = 10(.008 s + .003125 s + (100 KB)(60 s/m)/(9600 rpm)(300 KB/r)
T = 10(.008 s + .003125 s + .002083 s)
T = 132.083 ms

What file organization would you choose to maximize efficiency in terms of speed of access, use of storage space, and ease of updating (adding/deleting/modifying) when the data are:

  • updated infrequently and accessed frequently in random order?
Both the indexed file organization and the hashed file are efficient for frequent access to random parts of a file. Since the file is updated infrequently, the overhead of keeping indexes is reduced.
  • updated frequently and accessed in its entirety relatively frequently?
The indexed sequential file is efficient when access is usually to the entire file in sequential order. Keeping multiple indexes adds unnecessary overhead and the hash structure is not as useful for sequential access.
  • updated frequently and accessed frequently in random order?
The hashed file is efficient for frequent updates, and also is efficient for random access.

Consider a hierarchical file system in which free disk space is kept in a free space list.

  • Suppose the pointer to free space is lost. Can the system reconstruct the free space list?
Yes, it is easy to recover the lost space. Keep a bit map for every block on the disk, initially set to zero. Then traverse the file system starting at the root, and mark the bit mask with a 1 for every block that is used by every file. In the end, those blocks still marked by a zero are free.
  • Suggest a scheme to ensure that the pointer is never lost as a result of a single memory failure.
One solution is to keep a copy of the free space pointer in several different places on the disk.

Consider the organization of a UNIX file as represented by the inode (Figure 12.13). Assume that there are 12 direct block pointers in a singly, doubly, and triply indirect pointer in each inode. Further, assume that the system block size and the disk sector size are both 8K. If the disk block pointer is 32 bits, with 8 bits to identify the physical disk and 24 bits to identify the physical block, then

  • What is the maximum file size supported by this system?
The maximum file size can be found by calculating the space all the different types of block pointers can reference. First we need to calculate the number of pointers an indirect block can hold:
N = 8*1024*8/32 = 2048
Then:
Size = 12*8 + 2048*8 + 2048*2048*8 + 2048*2048*2048*8 KB = 64 TeraBytes
  • What is the maximum file system partition supported by this system?
The maximum file system partition is essentially equal to the size of a disk. Since we use 24 bits to address the blocks on each disk, the maximum size of a disk is 128 GB.
  • Assuming no information other than that the file inode is already in main memory, how many disk accesses are required to access the byte in position 13,423,956?
The address given is in the 13 MB range. The 12 direct pointers cover 96K, so the address is not located there. The singly indirect pointer covers the next 16 MB of the file, so the address is in a block referenced by the singly indirect pointer. This means we will need two disk accesses, one for the indirect block and another for the block containing the data.

What services are provided by TCP that are not provided by UDP? Briefly explain what each of these services does.

  • reliability: TCP ensures that whenever a source host sends a message, it will be received by the destination host in the order it was sent. If the network drops or corrupts the packet, TCP will retransmit it until it is received correctly.
  • flow control: TCP checks how much data the destination host can buffer, then makes sure the source host does not send more than this amount at a time. This keeps the source host from sending data faster than the destination host can receive it.
  • congestion control: TCP continually adapts to current network conditions, and slows down the source host if the network becomes congested. This keeps the source from sending data faster than the network can deliver it.

Explain how UDP traffic can "take over" the network when competing with TCP traffic.

UDP does not use congestion control, so all it has to do is send at whatever rate it desires. If this causes packet loss due to congestion in a router (a FIFO queue overflowing), then the TCP flows will slow down. If UDP sends at a high enough rate, it can force the TCP flows to slow down enough that they get nothing.

Why does a UDP server need only one socket but a TCP server needs one socket for each client?

A UDP server needs only one socket because anyone can send to it on that socket. It does not establish connections to each client. A TCP server, on the other hand, needs a socket for each client because it establishes a separate connection for each one.

Comments, questions, feedback, criticisms?

Send feedback