3.4 Review question: 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 theI/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/Osystem 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 therecords 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 Nrecords, 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 thefile. 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 oneSentence 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, butNow assume that the disk head is moving in the direction of increasing track number.

Answer:

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

Consider the disk system described in Problem 11.9 and assume the disk rotates at 360 rpm. A processor reads one sector from thedisk 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 spendhandling I/O (disregard seek time)? Repeat using DMA, and assume one interrupt per sector.