Tuesday, May 30, 2006

It's Not Just About Moore's Law

Almost a decade ago I was working in a group that maintained a hierarchical mass storage system responsible for managing data to and from a facility full of CRAYs and other supercomputers, as well as to the desktops of several hundred researchers. Concerned about the scalability of the system over time, I was pondering Moore's Law. Gordon Moore, co-founder of Intel, observed that the complexity of integrated circuits doubles with respect to minimum component cost every twenty-four months. This is typically re-stated, somewhat inaccurately, that processor speed doubles every two years, and is sometimes misquoted as every eighteen months.

It occurred to me that processor speed could not be the only thing changing. After poking around a bit in a variety of sources ranging from NASA to the National Science Foundation to industry trade magazines, I built the following table of geometric progressions that I have tried to keep updated over the years.

Microprocessor speed doubles every 2 years.
Memory density doubles every 1.5 years.
Bus speed doubles every 10 years.
Bus width doubles every 5 years.
Network connectivity doubles every year.
Network bandwidth increases by a factor of 10 every 10 years.
Secondary storage density increases by a factor of 10 every 10 years.
Minimum feature size halves (density doubles) every 7 years.
Die size halves (density doubles) every 5 years.
Transistors per die doubles every 2 years.
CPU cores per microprocessor chip double every 1.5 years.

I plugged some of these into a spreadsheet and generated the following logarithmic graph. Note that some of the curves are hard to see because they fall on top of one another. (Click on the graph to see a larger version.)

Power Curves

What we have here is a set of mostly disparate power curves that illustrate how the performance of major components change with respect to one another over time. And not much time, either. For example, in a decade, processor speed will increase by a factor of thirty, while bus speed will merely double, and network bandwidth will increase by an order of magnitude.

Admittedly, some of the numbers from my original research may have gotten a little shaky as manfacturers have had to resort to more and more arcane measures to maintain their rate of improvement. But if I can at least convince you that different technologies are on very different curves of performance improvement, then you are pretty much forced to concede that the balanced system architecture you design today might not cut the mustard just a few years down the road as components and surrounding infrastructure are upgraded. The design decisions that made sense at the time, and the software implementations that were based on those assumptions, may not seem as wise later.

In fact, depending on the half-life of the system that is based on these assumptions, its basic architecture may have to be revisited more than once during its lifetime.



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