performance of the processor

source:  http://en.wikipedia.org/wiki/Instructions_per_cycle

instructions per cycle


The number of instructions executed per clock is not a constant for a given processor; it depends on how the particular software being run interacts with the processor, and indeed the entire machine, particularly the memory hierarchy. However, certain processor features tend to lead to designs that have higher-than-average IPC values; the presence of multiplearithmetic logic units (an ALU is a processor subsystem that can perform elementary arithmetic and logical operations), and short pipelines.


 When comparing different instruction sets, a simpler instruction set may lead to a higher IPC figure than an implementation of a more complex instruction set using the same chip technology; however, the more complex instruction set may be able to achieve more useful work with fewer instructions.


clock speed (measured in cycles per second or Hertz [Hz])


The number of instructions per second is an approximate indicator of the likely performance of the processor.


The number of instructions per second for a processor can be derived by multiplying the instructions per cycle and the clock speed (measured in cycles per second or Hertz [Hz]) of the processor in question.


A given level of instructions per second can be achieved with a high IPC and a low clock speed (like the AMD Athlon and Intel Core 2), or from a low IPC and high clock speed (like theIntel Pentium 4).

The useful work that can be done with any computer depends on many factors besides the processor speed. These factors include the processor architecture, the internal layout of the machine, the speed of the disk storage system, the speed of other attached devices, the efficiency of the operating system, and most importantly the high level design of the application software in use.

For users and purchasers of a computer system, instructions per clock is not a particularly useful indication of the performance of their system. For an accurate measure of performance relevant to them, application benchmarks are much more useful. Awareness of its existence is useful, in that it provides an easy-to-grasp example of why clock speed is not the only factor relevant to computer performance.