معمارية : المحاضرة السادسة - مقاييس الاداء

2. Performance Measures

In this section, we consider the important issue of assessing (تقييم) the performance of a computer by studying three measures:

a) CPI (clock cycles per instruction)
b) MIPS (million instructions-per-second)
c) MFLOPS (Million floating-point instructions per second)

2.1 CPI (clock cycles per instruction)

Performance analysis should help answering questions such as how fast can a program be executed using a given computer? In order to answer such a question ,we need to determine the time taken by a computer to execute a given job.

We define the clock cycle time as the time between two consecutive rising (صاعد) edges of a periodic clock signal :


The time required to execute a job by a computer is often expressed in terms of clock cycles.

We denote the number of CPU clock cycles for executing a job to be the cycle count (CC), the cycle time by CT, and the clock frequency by f = 1/CT. The time taken by the CPU to execute a job can be expressed as :





It may be easier to count the number of instructions executed in a given program as compared to counting the number of CPU clock cycles needed for executing that program. Therefore, the average number of clock cycles per instruction (CPI) has been used as an alternate performance measure. The following equation shows how to compute the CPI:


It is known that the instruction set of a given machine consists of a number of instruction categories: ALU (simple assignment and arithmetic and logic instructions), load, store, branch, and so on. In the case that the CPI for each instruction category is known, the overall CPI can be computed as :






where Ii is the number of times an instruction of type i is executed in the program and CPIi is the average number of clock cycles needed to execute such instruction.



Assuming the execution of 100 instructions, the overall CPI can be computed as :



2.2 MIPS (million instructions-per-second)

A different performance measure that has been given a lot of attention in recent years is MIPS (million instructions-per-second (the rate of instruction execution per unit time)), which is defined as :















2.3 MFLOPS (Million floating-point instructions per second)

Million floating-point instructions per second, MFLOP (rate of floating-point instruction execution per unit time) has also been used as a measure for machines’ performance. It is defined as :

While MIPS measures the rate of average instructions, MFLOPS is only defined for the subset of floating-point instructions. An argument against MFLOPS is the fact that the set of floating-point operations may not be consistent (ثابتة) across machines and therefore the actual floating-point operations will vary from machine to machine. Yet another argument is the fact that the performance of a machine for a given program as measured by MFLOPS cannot be generalized to provide a single performance metric for that machine.