3.3 Events

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3.3 Events

Time is an important measure, but it can only become useful for us if we have a means to use it in measuring something within our computer system under evaluation. An event describes an entity of interest in our system. Events usually represent some actionfor example, the beginning of a clock cycle (Figure 3.1) or the end of a clock cycle. The beginning of a computer's instruction execution cycle is another event, as is the end of the cycle, the reading of a memory location, the initiation of a block data transfer from a secondary storage device, and the initiation of a process or task. All of these represent events of interest to the computer engineer or computer architect.

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Figure 3.1: Example of a computer clock.

Events, representing actions within our computer system, must all be controlled, so that the sequencing or ordering (partial or total ordering) of these actions contributes to the accomplishment of some larger event. For example, the simple computer clock cycle is used to mark the beginning of an instruction's execution in a computer. The rising edge of the clock is used by the processor to begin execution of the current instruction in the instruction register and to prepare the next instruction for execution. The multitude of parallel events being performed during each and every clock cycle of the computer system's clock must be synchronized so that the designed intentions are realized. For example, the instruction to perform was loaded into the instruction register during the last sequence, while at the same time the next instruction address was computed and possibly some parameters for the instruction moved into place. Each action must be designed and its sequencing in relation to other actions defined so that the computer will work as intended.

Each simple action, from the clock ticks to more complex actions such as an instruction's execution all become part of larger systems actionsfor example, the initiation of a direct memory transfer of data from a secondary storage disk drive; the DMA transfer being used as part of the systems memory management system's paging algorithms; and the paging algorithm's relationship to the movement of one process actively running on the CPU being replaced by another due to a process switch handled by the operating system. All of these represent actions of interest to the computer analyst. Each, however, has a different temporal relationship to the measure of time. The clock cycle is measured in fractions of nanoseconds, the single assembly-level instructions in tens of nanoseconds, main memory transfers in the range of 100 nanoseconds, disk transfers in the milliseconds range, operating systems file transfers in the tens of milliseconds range, and so on.

In terms of performance assessment the system analyst must have an understanding of the events within the system under study and the relationship of these events among each other. For example, we need to know that a file access event is composed of the disk access event, memory page replacement algorithm event, and the main memory load and store events. In addition to knowing the events involved with a higher-order event, event orderings must also be understood. For example, it is important to know that the page replacement algorithm must be accessed first, to determine which page to move, before the new page can be loaded into the primary memory. These event orderings can be represented by simple event lists or by more complex partial orderings (Figure 3.2). These orderings dictate what events need to be considered and how the events may need to be measured, so that an accurate picture of the system's performance can be determined.

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Figure 3.2: Event partial orderings.



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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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