IBM mainframe speed has long been measured in MIPS, although the term fell out of favour even at IBM for awhile. Many IBM customers came up with alternate meanings for the acronym, poking fun at the world's largest builder of mainframes. The relative inaccuracy of using the MIPS measurement for anything meaningful prompted the humorous definitions, but MIPS as a measurement, while not accurate for measuring true throughput, is back in vogue as IBM's pricing measurement.
MIPS, an acronym standing for "Millions of Instructions Per Second," is a measurement of the speed of a mainframe computer's central processors. Customers invented their own meanings, such as "Meaningless Indicator of Processor Speed," and "Marketing Indicator of Processor Speed," among others. The reason the alternate meanings were invented was that MIPS is not an accurate indicator of total throughput capability, which consists of more than raw processor speed.
Mainframe computer processors are capable of billions of instructions per second, as are modern personal computers. Personal computers are typically measured in gigahertz, which is actually a measurement of machine instruction cycles per second. As any PC user knows, a slow hard drive can drag a really fast processor to a crawl. An old adage states that all processors, no matter how fast, wait at the same speed.
Total throughput capability includes several factors, including processor speed, input/output device speed, memory speed, workload type and transaction volume. Workload type can be online, batch or a combination of the two. All of these other functions are a part of the total workload that the mainframe has to manage. Slow or insufficient memory can slow down the total throughput, which brings into question the practice of capacity planning by using only the MIPS rating.
Future of MIPS
Although the term fell out of favour for awhile, even at IBM, it has enjoyed a resurgence in popularity over the first decade of the 21st century. IBM is using MIPS in its pricing model, and the company has made it easy for its customers to "turn up" the speed on a mainframe by purchasing more MIPS on the fly. While not a great solo measurement for mainframe performance, MIPS has found its place in the IBM marketing model.
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