3.2 Time

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3.2 Time

Time is the most fundamental of concepts needed for computer systems performance analysis. Without a clear concept of time our performance studies cannot take on quantitative qualities. Time as a quantity shows up in several ways when one investigates a computer system's performance. For example, we hear of concepts such as arrival time of an entity, the service time for an entity, time between failures, time to repair, entity lifetime, and numerous other quantities of time associated with computer systems performance. Each of these quantities requires us to have a reference point from which to determine their meanings.

In computer systems performance, we will be interested in the measurement of time related to various operational events in the computer system under study. These events will be marked by timestamp, and by using this timestamp we will have the capability to determine the relative ordering of these events in relation to each other. The timestamp of an event, E, would be represented as E(t). The measurement or marking of the time, t, will only be as good as the clock we use in representing the time of an event and our ability to match the time representation with the event.

Time in a real-world system is represented in two major ways: either as a continuum or as discrete intervals or steps. The best way to think of these two measures of time is that discrete time represents a single instance of a time clock's measures, whereas continuous time represents discrete time intervals, where the intervals approach zero or are infinitely small.

Computer systems work using the concept of fixed time intervals. These intervals represent the time frame or limit into which a computer system's clock breaks down a second. Typical computer systems clocks or cycle times are measured in nanoseconds (10-9 seconds) or in slices of about one-billionth of a second. Such fine gradations of a second help us to understand the speed of computer systems and related components.

I am sure you all can relate to hearing about a processor's speed. When we go to purchase a new personal computer, we are quoted a number of measures of computer relative time. For example, a 1.5-GHz processor implies that the processor will have a clock cycle of about 0.67 nanoseconds, or 6.67-10 seconds. This is not the only measure one needs to know when measuring or sizing up a personal computer for purchase. The CPU speed is important but is only one measure. We need to know how fast data and instructions can be transferred from the external devices into the internal primary memory, and then how fast the primary memory can transfer this information and instructions to the processor for actual execution. Even though one is quoted the CPU speed, this does not represent the actual measure of the machine's performance. We will see that the way devices are interconnected and how they interact will dictate overall speed. In this simple example, the slowest device in the system will ultimately dictate the real speed.



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Computer Systems Performance Evaluation and Prediction
Computer Systems Performance Evaluation and Prediction
ISBN: 1555582605
EAN: 2147483647
Year: 2002
Pages: 136

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