Q1:What's CPU schedule criteria?

Q2:What are CPU burst time and  I/O burst time?

 

A1:

1) Knowledge about CPU schedule criteria from textbook

Many criteria have been suggested for comparing CPU-scheduling algorithms. Which characteristics are used for comparison can make a substantial difference in which algorithm is judged to be best. The criteria include the following:

• CPU utilization. We want to keep the CPU as busy as possible. Conceptually, CPU utilization can range from 0 to 100 percent. In a real system, it should range from 40 percent (for a lightly loaded system) to 90 percent (for a heavily loaded system).


• Throughput. If the CPU is busy executing processes, then work is being done. One measure of work is the number of processes that are completed per time unit, called throughput. For long processes, this rate may be one process per hour; for short transactions, it may be ten processes per second.


• Turnaround time. From the point of view of a particular process, the important criterion is how long it takes to execute that process. The interval from the time of submission of a process to the time of completion is the turnaround time. Turnaround time is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.


• Waiting time. The CPU-scheduling algorithm does not affect the amount of time during which a process executes or does I/O. It affects only the amount of time that a process spends waiting in the ready queue. Waiting time is the sum of the periods spent waiting in the ready queue.


• Response time. In an interactive system, turnaround time may not be the best criterion. Often, a process can produce some output fairly early and can continue computing new results while previous results are being output to the user. Thus, another measure is the time from the submission of a request until the first response is produced. This measure, called response time, is the time it takes to start responding, not the time it takes to output the response. The turnaround time is generally limited by the speed of the output device.

2) My opinion and summary to the question 1

1. CPU utilization – keep the CPU as busy as possible


2. Throughput – number of processes that complete their execution per time unit


3. Turnaround time – amount of time to execute a particular process


4. Waiting time – amount of time a process has been waiting in the ready queue


5. Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment

A2:

1) Knowledge about CPU burst time and I/O burst time from textbook

The success of CPU scheduling depends on an observed property of processes: process execution consists of a cycle of CPU execution and I/O wait. Processes alternate between these two states. Process execution begins with a CPU burst. That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on. Eventually, the final CPU burst ends with a system request to terminate execution (Figure 5.1).

2) My opinion to the question 2

1. The CPU burst time is the time taken by CPU to execute the process.


2. The I/O burst time is the time taken by the process to perform some I/O operation.