Why Parallel Processing? Why now?

Many software companies have applications which are in use by their customers that have significant runtime and for which fast runtime is a necessity or a competitive advantage. There has always been the pressure to make such applications go faster. Historically, as processors have increased their speed, the needed speedups could often be achieved by tuning the single cpu performance of the program and by utilizing the latest and fastest hardware. In the Electronic Design Automation industry, it has always been the case that the newest machines had to be used to run the design tools which were being used to design the next generation of processors. The speed and memory capability of the newest machines had always been just enough to design the next generation chips. Other types of cpu intensive software have also ridden the hardware performance curve in this way.

            We will no longer see significant increases in the clock speed of processors. The power consumed by the fastest possible processors generates too much heat to dissipate effectively in known technologies. Instead processor manufacturers are adding multiple processors cores to each chip. Why does this help? Power Consumed = Capacitance * Voltage^2 * Frequency. If a given calculation is perfectly moved from a processor running at N Gigahertz to 2 parallel processors running at N/2 Gigahertz where does the savings come from? It would seem that each processor runs in half the power but now there are 2 processors which would mean that the same power is used. The power savings comes from the fact that slower processors can run at a lower voltage. For example a processor running at half the frequency can run at around 8/10 the Voltage level. .8^2 is .64 which implies a 36% power savings. If you scale this up to 32 cpus then it will be possible to get a lot of compute power for much lower power consumption and therefore much lower required heat dissipation. Eventually it seems that even cell phones and other embedded devices will move to multi core processing for this reason. More compute capabilities or longer battery life for the same capabilities. Both are compelling values.

Part of the assumption that goes into the definition of how this power savings will be achieved is that the software implementation of the parallel program running on the 2 slower processors must be perfectly efficient. Well, nothing in the real world is perfectly efficient. Even if the coding is not perfectly efficient, as long as it is reasonably efficient, then there is a benefit. If the parallel coding is inefficient, then it might be that the parallel program will use more power on the slower processors than the serial program running on the fast single processor. However, since faster processors that won’t melt can no longer be made, we are kind of stuck with going parallel and need to do our best.

It stuck because from a software development perspective a large new burden is being placed on software developers. That burden is to write programs that are as efficient as possible and which make use of N processors, hopefully where N is configurable by the user and can be increased as new processors chips with more cores become available. For most developers this is something really new and really complex. It also presents a huge discontinuity for software companies with large investments in legacy code.