File management & how it relates to the internal operating system of a computer

1) What is the fundamental purpose of any operating system?

 

2) What is the role of the file manager?

 

3) What other basic functions must the operating system be able to perform?

  

For problems 4 and 5, a computer has the following four processes that have arrived in the ready queue in the sequence shown:

• Process 1 with a run time of 40 seconds, a priority of 1, and it will require 20 seconds of I/O after 20 seconds of execution.
• Process 2 with a run time of 15 seconds and a priority of 2.
• Process 3 with a run time of 25 seconds, a priority of 2, and it will require 15 seconds of I/O after 5 seconds of execution.
• Process 4 with a run time of 10 seconds and a priority of 1.

**The attached document could be used as supporting documentation.

4) If the Round Robin Scheduling algorithm is used, which process completes first and why?

 

5) If the Non-preemptive Priority Scheduling algorithm is used, which process completes first and why?

 

6) What are the advantages of partitioning a disk, rather than using the entire disk as one partition?

 

7) Using a variable-partitioned multiprogramming memory, which of the four holes shown below will be used to satisfy a 65 KB program requirement under the conditions of:

 

occupied

occupied

occupied

occupied

0-15 KB	15-75 KB	75-125 KB	125-210 KB	210-275 KB	275-345 KB	345-420 KB	420-495 KB	495-520 KB
				occupied	Hole A	Hole B	Hole C	Hole D

 

___ First-fit ___ Best-fit ___ Worst-fit

 

8) There are a number of different factors, both hardware and OS software that affect the operating speed of a virtual storage system. Explain carefully each of the factors and its resulting impact on system performance.

  

2.1.2Scheduling

Scheduling of processes is performed on two levels:

 high-level scheduling is used for long-term processing (anywhere from an hour to a day to a week)

 dispatching is short-term scheduling

High-level scheduling determines when a process will be admitted to the system.

 In an interactive (online) environment, processes will be automatically accep ted unless the job would
overload the system.

 In a batch environment, the high-level scheduler is used to control the long-term load on a system.

An example of high-level scheduling would be a system that runs interactive and short run-time batch processes during
the day, and schedules longer-running processes for the night and weekend periods.

Dispatching makes the instant-by-instant decisions on which of the processes that are ready should be given CPU
execution time. There are several algorithms that the dispatcher uses to make these decisions. These algorithms are
divided into two classes:

 Nonpreemptive scheduling allows a process to run to completion or until it voluntarily surrenders the CPU.
This method is unpredictable because there is no control over very long-running processes or processes that
are blocked while waiting for resources.

 Preemptive scheduling allows the CPU to be taken away from the process when it has become blocked or
after it has run a specified time. The method is both predictable and more efficient in its use of the CPU.

Six types of scheduling are discussed below and then their impact upon the sequence of executing a queue of processes
is shown in Example 6-2. You should click on the desired scheduling algorithm to see the order of processing.

Three non-preemptive algorithms are:

 First-in, first-out (FIFO) Each process is placed into a queue as it is admitted to the ready process. The
dispatcher selects the process at the head of the list to have access to the CPU with no consideration of
memory requirements, CPU time estimates, or priority requirements. Click on first-in, first-out in Example 6-
2 and see Figure 6-5a.

 Shortest job first The dispatcher selects the process with the shortest estimated run time to have access to
the CPU. This will maximize the number of processes that are handled in any given time. Click on shortest
job first in Example 6-2 and see Self-Assessment question 1.

 Nonpreemptive priority queue Processes are assigned a priority and then placed in a FIFO priority-based
queue. The dispatcher selects the highest priority queue first and then the job at the head of that queue for
access to the CPU. After the highest priority queue is emptied, the processes in the next highest priority
queue can be selected. Click on nonpreemptive priority queue in Example 6-2 and see Self-Assessment
question 2.

Three preemptive algorithms are:

 Round robin Incoming processes are placed in a FIFO queue. When a process is given access to the CPU, it
retains that access until it completes, is blocked, or exceeds a pre-established time limit. At that time it is
removed from the CPU. Timed-out processes are reentered into the queue. Blocked processes are reentered
into the queue when they wake up. Click on round robin in Example 6-2 and see Figure 6-2b.

 Shortest remaining time Processes are selected for the CPU based on the shortest estimated running time. If
the process becomes blocked while executing, it is removed, and then the process is allowed to reenter the
queue using the remaining estimated run time after it wakes up. Click on shortest remaining time in Example
6-2 and see Self-Assessment question 3.

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Batch

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Dispatch

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Nonpree-sch

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Preem-sch

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#FIFO

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Short_job

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#NPQ

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#R-robin

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Short_remain

 Round robin with priority queues Processes are placed in priority-based round robin queues. The highest
priority queues are emptied before lower priority queues. If a process times out, it is reentered into its
priority queue. If a process with access to the CPU becomes blocked, it is removed until it wakes up, and
then it reenters its priority queue.

Figures 6-5a and 6-5b demonstrate the increased efficiency of the preemptive scheduling algorithm over the non-
preemptive scheduling algorithm, and show the timing of switching processes in and out of the CPU. In order to
make the comparisons, we assume a computer has the following four processes that have arrived in the ready
queue in the sequence shown:

 Process 1 with a run time of 20 seconds, a priority of 1, will require 20 seconds of I/O after 10 seconds of
execution.

 Process 2 with a run time of 30 seconds and a priority of 2.

 Process 3 with a run time of 15 seconds, a priority of 2, will require 10 seconds of I/O after 5 seconds of
execution.

 Process 4 with a run time of 5 seconds and a priority of 1.

Figure 6-5a uses the first-in, first-out non-preemptive scheduling algorithm. Figure 6-5b uses the round robin
preemptive scheduling algorithm. Note that the preemptive scheduling algorithms complete in 70 seconds as compared
to 100 seconds for the non-preemptive algorithms. This is because the processes waiting for I/O are blocked and thus
removed from the run state in preemptive scheduling.

Figure 6-5a
First-in, First-out Scheduling Algorithm

Figure 6-5b
Round Robin Scheduling Algorithm

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#R-robin2

2.2.3 Variable-Partition Multiprogramming

A more flexible approach to memory management is variable-partition multiprogramming where the operating system
is allowed to add, subtract, or modify the size of partitions. As programs are moved into and out of memory, holes are
created. The holes are the wasted space not being used by programs.

Figure 6-10 shows holes could be created as programs are terminated and added

 shows memory with three programs and one hole

 shows memory after program 2 has been completed and removed from memory, leaving a second hole

 shows memory after program 4 has filled part of the hole, leaving a smaller hole

In Figure 6-10, there were two holes for the 60 Kb program, a 26 Kb hole and an 80 Kb hole, but the decision was an easy
one because only the 80 Kb hole was big enough. In most situations, there would be more than one hole big enough in
which to put a program.

Three basic algorithms can be used to determine which hole fits:

 First-fit selects the first available space in memory that can hold the program.

 Best-fit selects the memory location that is closest in size to the program.

 Worst-fit selects the biggest available hole so that the remaining hole will be the largest.

These three strategies are shown in Figure 6-11 with the holes shown in yellow. The decision of which strategy to use is
determined when the parameters for the operating system are established.

Figure 6-10
Variable-Partition Multiprogramming

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Holes

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#First-fit

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Best-fit

http://tychousa12.umuc.edu/IFSM310/0406/Modules/G1-Glossary.html?class=1309:IFSM310:6380#Worst-fit

Figure 6-11
Memory Placement Strategies

A problem with both partitioning schemes is that the final address in memory where a program will be loaded for
execution is not known. The resolution to that problem is to use base addressing where the operating system uses a
base register to hold the location that corresponds to the starting location of the program.