2.5.3 Throughput and Latency

Throughput and Latency are two key terms used in discussing computer performance. Throughput refers to the rate at which data flows through the system. For example, a computer might execute 500 million instructions per second. Latency refers to the time required to complete an individual operation. For example, it might take five seconds to launch an application, or two minutes to reboot the entire system.

These measures are clearly related. For example, suppose you want your Web server to display a large graphic on a user's computer when they press a button. Latency is how much time passes from the button press until the image is displayed. If the image is a 1 MB file, then with a throughput of ten megabits per second, the latency to display the image will be at least 0.8 seconds. Whether latency or throughput is most appropriate to report depends on the setting.

If you are selecting a provider for your Web server's network connection, you will be more concerned with having enough throughput, or bandwidth, to serve your needs. If you are a user of the same web server, you will certainly be concerned with the latency for your request. It does not matter to you how many other requests the server processes per second (i.e., throughput). You are mainly concerned with your request.

As a server administrator, you might hear complaints from people whose latency was exceedingly high, even though your throughput was acceptable. Let us say your server handles two different kinds of requests: a request for a small file that can be sent quickly and a request for a large file that takes a long time to send. If the vast majority of requests are for small files, your throughput (the bits sent per second) and latency for small files may be fine, while the latency for large files may be unacceptable.

A simple way to think about latency is to model the time it takes to do a task as being equal to some constant startup time, plus a time that scales with the size of the task.

Assessments: Exercise 2

Unit 3. Operating System Software

An operating system performs a vital role in enabling users to interact with a computer system. In this unit, we will learn about the specific responsibilities of an operating system, such as device management, memory management, resource sharing, and process control. After learning about the operating system, you will understand some of the problems that can cause your computer to perform less than optimally and some of the solutions that can resolve those problems. You will also learn how to customize your computing environment to your personal taste.

Readings: 3.1 Structure 3.2 Device Management and Configuration 3.3 Resource Sharing 3.4 File Systems

              

Assessments: Exercise 3

3.1 Structure

Just as we previously discussed with respect to hardware, a computer system's software has its own structure. The operating system is not a single entity but is composed of multiple layers, each with its own area of responsibility. The operating system's layers and the applications the user runs are considered unique "tasks" or "processes," competing for access to the CPU and other hardware resources. The management of this competition through the proper scheduling of processes is a major responsibility of an operating system and can have a significant impact on system performance.

Reading Sequence: Parsons/Oja, Chapter 4-Section A. Remark: Learn about the importance of the operating system to a computer system. It might surprise you to see how many different operating systems exist in addition to Windows. Windows itself comes in several different versions that have evolved over the years. Details of operating system functions will be covered in a later unit. 3.1.1 Layers of Software. Learning Goal: Knowing why layers are an important design concept, and what the layers of software are that make up a computer system. 3.1.2 The BIOS: Life at the Bottom. Learning Goal: An understanding of what the BIOS is and how it provides a bridge between higher layers of the operating system and the PC's physical hardware. Parsons/Oja, Chapter 3-Section D: Learning Goal: Knowledge of installing software and software upgrades. 3.1.3 Process Control. Learning Goal: An understanding of the operating system kernel and how it performs one of its most vital functions, task scheduling. This function determines which application or operating system process should next have use of the CPU. 3.1.4 Lab: Task Manager. Remark: To view the list of currently executing processes on a computer running Windows, perform the Task Manager lab.

             

Assessments: Multiple-Choice Quiz 8