SCIzzL Information

The ANSI/IEEE Std Scalable Coherent Interface (#1596-1992) is often barely visible through the thick clouds of Fear, Uncertainty, and Doubt that seem to be stirred up by that central word, "Coherent". Responding to this situation, it is tempting to downplay the Coherence by pointing out that it doesn't cost you anything if you don't need or use it. (Analogy: the fact that your computer can do symbolic 4-D relativistic tensor algebra, by running Mathematica software, neither inhibits you from using it as a word processor nor motivates you to trade it in for a weaker one--you know your needs might change in the future.) But hiding the Coherence of SCI is a mistake. Most of those who fear it today are going to want and need it in the future, and then they'll be glad that the SCI they put in today because of its performance and cost-effectiveness already is compatible with coherence. Because of the speed of light. Because of advances in software. Because of advances in networking. Because of advances in communication. As machines get more tightly coupled by networks and people become more dependent on networking, bandwidth requirements go up. When bandwidth goes up but performance doesn't get better, people discover they are software limited. As software becomes more efficient, communication will become more intimate. The old concept of "I/O" becomes increasingly inapt--almost everything is just communication of information among processors, networks, memories, and (incidentally) disk drives and displays. As the amount of information becomes larger, it becomes increasingly shared--you don't want to copy all of it just to look at the bits and pieces that interest you. What does not change during this evolution is the speed of light (and signals): it takes more time to get data from places that are farther away. There are two strategies for handling this: Anticipate what data you will want, and ask for them early (prefetch hints) so they will be available from local (cache) memory when actually needed. Ask for more ...