$1.2 A REVIEW OF EXTERNAL FACTORS Efficiency Definition:efficiency Efficiency is the ability of a software system to place as few demands as possible on hardware resources,such as processor time,space occupied in internal and external memories,bandwidth used in communication devices. Almost synonymous with efficiency is the word"performance".The software community shows two typical attitudes towards efficiency: Some developers have an obsession with performance issues,leading them to devote a lot of efforts to presumed optimizations But a general tendency also exists to downplay efficiency concerns,as evidenced by such industry lore as"make it right before you make it fast"and "next year's computer model is going to be 50%faster anyway". It is not uncommon to see the same person displaying these two attitudes at different times,as in a software case of split personality (Dr.Abstract and Mr.Microsecond). Where is the truth?Clearly,developers have often shown an exaggerated concern for micro-optimization.As already noted,efficiency does not matter much if the software is not correct (suggesting a new dictum,"do not worry how fast it is unless it is also right", close to the previous one but not quite the same).More generally,the concern for efficiency must be balanced with other goals such as extendibility and reusability;extreme optimizations may make the software so specialized as to be unfit for change and reuse. Furthermore,the ever growing power of computer hardware does allow us to have a more relaxed attitude about gaining the last byte or microsecond. All this,however,does not diminish the importance of efficiency.No one likes to wait for the responses of an interactive system,or to have to purchase more memory to run a program.So offhand attitudes to performance include much posturing;if the final system is so slow or bulky as to impede usage,those who used to declare that "speed is not that important"will not be the last to complain. This issue reflects what I believe to be a major characteristic ofsoftware engineering, not likely to move away soon:software construction is difficult precisely because it requires taking into account many different requirements,some of which,such as correctness,are abstract and conceptual,whereas others,such as efficiency,are concrete and bound to the properties of computer hardware. For some scientists,software development is a branch of mathematics;for some engineers,it is a branch ofapplied technology.In reality,it is both.The software developer must reconcile the abstract concepts with their concrete implementations,the mathematics of correct computation with the time and space constraints deriving from physical laws and from limitations of current hardware technology.This need to please the angels as well as the beasts may be the central challenge of software engineering§1.2 A REVIEW OF EXTERNAL FACTORS 9 Efficiency Almost synonymous with efficiency is the word “performance”. The software community shows two typical attitudes towards efficiency: • Some developers have an obsession with performance issues, leading them to devote a lot of efforts to presumed optimizations. • But a general tendency also exists to downplay efficiency concerns, as evidenced by such industry lore as “make it right before you make it fast” and “next year’s computer model is going to be 50% faster anyway”. It is not uncommon to see the same person displaying these two attitudes at different times, as in a software case of split personality (Dr. Abstract and Mr. Microsecond). Where is the truth? Clearly, developers have often shown an exaggerated concern for micro-optimization. As already noted, efficiency does not matter much if the software is not correct (suggesting a new dictum, “do not worry how fast it is unless it is also right ”, close to the previous one but not quite the same). More generally, the concern for efficiency must be balanced with other goals such as extendibility and reusability; extreme optimizations may make the software so specialized as to be unfit for change and reuse. Furthermore, the ever growing power of computer hardware does allow us to have a more relaxed attitude about gaining the last byte or microsecond. All this, however, does not diminish the importance of efficiency. No one likes to wait for the responses of an interactive system, or to have to purchase more memory to run a program. So offhand attitudes to performance include much posturing; if the final system is so slow or bulky as to impede usage, those who used to declare that “speed is not that important” will not be the last to complain. This issue reflects what I believe to be a major characteristic of software engineering, not likely to move away soon: software construction is difficult precisely because it requires taking into account many different requirements, some of which, such as correctness, are abstract and conceptual, whereas others, such as efficiency, are concrete and bound to the properties of computer hardware. For some scientists, software development is a branch of mathematics; for some engineers, it is a branch of applied technology. In reality, it is both. The software developer must reconcile the abstract concepts with their concrete implementations, the mathematics of correct computation with the time and space constraints deriving from physical laws and from limitations of current hardware technology. This need to please the angels as well as the beasts may be the central challenge of software engineering. Definition: efficiency Efficiency is the ability of a software system to place as few demands as possible on hardware resources, such as processor time, space occupied in internal and external memories, bandwidth used in communication devices