Question

Using the Critical Thinking Process, identify 5 disk scheduling options, and 5 disk scheduling criteria. Weigh the options against the criteria and select the best disk scheduling option for:

Identify and select a process-to-process communication technique that best both upward and downward scalability for a non-real time mission-critical system your company has been building.

In This Format:

An approach to making informed, unemotional decisions to is employ the following 4 step process: A) Identify your options B) Establish your criteria for this particular situation C) Weigh your options against your criteria D) Select and defend your option eping with Given the following problem statement, A system with two dual-core processors has four processors available for scheduling. A CP-intensive application is running on this system. All input is performed at program start-up, when a single file must be opened. Similarly, all output is performed just before the program terminates, when the program results must be written to a single file. Between startup and termination, the program is entirely CPU- bound. Your ask is to improve the performance of this application with concurrency and parallelism. applying the 4-Step Critical Thinking Process produces, for example, something like this: Identify Options Mutiple single-threaded processes using shared memory Establish Criberia Short Schedule time to Denign Code, Tet, Inbagrate, Deploy) Low Risk (diicuty, skall lewel, vender support Low Response time(tal time to complete tnk Low Resource comumption Imemory,bandwidth, devies C Weigh the Options against the Criteria Populating with narate amp Single multi-threaded process using shaned mamory processasScalscs using massaging cales across network wel espone Populatingwith relative numerical scoe Criteria Score Multiple singo-threaded Multiple single-threaded D) Select and Defend Option rmgerig with aptin 1, Sirglo mub-threaded process because this option can be rmplemented r, the ป。rted schedul. with the leant amount of risk while stll providing superioe eponse ime. Shorter schedule and lower risk outweigh the

Answer 1

Disk scheduling-It is done by a operating system to schedule I/O (input output) requests arriving for the Disk.It is also know as input output scheduling.

Disk scheduling is important because :

-Multiple input output requests may arrive by different process and only I/O request can be served at a time by disk controller .Thus other I/O requests need to wait in waiting queue and need to be scheduled.

-Two or more request may be far from each other so can result in greater disk arm movement.

-Hard drives are one of the slowest parts of computer systems and thus need to be accessed in an efficient manner.

There are five disk scheduling criteria:-

1)Seek time:seek time is the time taken to locate the disk arm to a specified track where the data is to be read or write .So the disk scheduling algorithm that gives minimum average seek time is better.

2)Rotational Latency -It is the time taken by the desired sector of disk to rotate into a position so that it can access the read/write heads.So the disk scheduling algorithm that gives minimum rotational latency is better.

3)Transfer time -Transfer time is the time  to transfer the data.It depends on the rotating speed of the disk and number of bytes to be transferred.

4)Disk Access Time-Disk access time is :

Disk access time = Seek Time +Rotational Latency+ Transfer Time

Followings are the five disk scheduling options:-

a)FCFS

b)SSTF

c)SCAN

d)CSCAN

e)LOOK

Now lets see the all five disk scheduling options in details

1)FCFS-(First come first serve) It is the most simplest disk scheduling algorithm .In this scheduling algorithm request are processed by the order of the request arrive in the disk queue ,means which request first that would be addressed/completed first then after next and so on.

Advantages of FCFS

1)No indefinite postponement

2)Every request gets a fair chance

Disadvantages of FCFS

1)It may not be providing the best possible service

2)Does not try to optimize the seek time

b)SSTF:It understands for shortest seek time first,In this scheduling algorithm requests having the shortest seek time are executed first.so the seek time of every request is calculated in advance in queue and then they are scheduled according to their calculated seek time.As a result,the request near the disk arm will get executed first.SSTF is certainly an improvement over FCFS as it decreases the average response time and increases the throughput of system.

Advantages of SSTF

1)Throughput increases

2)Average response time decreases

Disadvantages of SSTF

1) High variance of response time

2)overhead to calculate the seek time in advance

3)can cause starvation for a request if it has higher seek time as compared to incoming requests

c)SCAN:-In SCAN algorithm the disk arm moves into a particular direction and services the requests coming in its path and after reaching the end of disk ,it reverses its direction and again services the request arriving in its path.so the algorithm works like an elevator and hence also known as elevator algorithm.As a result ,the requests at the midrange are services more and those arriving behind the disk arm will have to wait.

Advantages of SCAN

1)High throughput

2)Low variance of response time

3)Average response time

Disadvantages of SCAN

1)Long waiting time for requests for locations just visited by disk arm

d)CSCAN:It stands for Circular SCAN.In Cscan Algorithm ,the disk arm again scans the path that has been scanned ,after reversing its direction.So,it may be possible that too many requests are waiting at the other end or there may be zero or few requests pending at the scanned area.

Advantages of CSCAN

1)Provide more uniform wait time compare to SCAN

e)LOOK:It is similar to the SCAN disk scheduling algorithm except the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be service in front of the head and then reverse its direction from there only.Thus it prevents the extra delay which occurred due to unnecessary traversal to the end of the disk.