by Marty Deitch
Customers are always looking for MIPS/MSU charts, whether they come from Gartner, Cheryl Watson, or IBM. When used properly, these numbers provide value to the financial analyst and capacity planner. But they also have a dark side when misused. The problem goes like this. We propose a processor upgrade along with a capacity planning study, and the customer asks why our MIPS ratings are lower than what he sees in his MIPS table. That leads to a discussion of why you should never use a MIPS table for capacity planning. This isn’t a new thing. It’s been going on for as long as I’ve been in this business.
IBM does not measure MIPS. They measure the relative change in capacity between different processor models. Further, IBM takes these measurements using workload specific benchmarks and publishes these values in their LSPR (Large System Performance Ratios) tables. Since these measurements show relative capacity and not absolute MIPS values, they must be converted to MIPS by the following calculation. First, select a base processor and assign it a MIPS rating. Next, use the relative capacity values in the LSPR tables to extrapolate the MIPS values for all the other processor models. That is essentially how all MIPS tables are derived. These values have an inherent flaw in that they show a single MIPS value for every processor. But the actual capacity provided by a processor is variable and depends on how that processor is configured and how it is used. For example, the actual capacity delivered by a processor depends on the number of specialty CPs (ie ZIIPs, IFLs, ICFs), the number of active LPARs, the number of Logical CPs, and the kind of work that runs on the processor. The only tool that considers all of these effects when estimating the relative capacity between different processors is the IBM z/PCR tool. And that is why a single MIPS value from a MIPS table should never be used for a capacity planning study.
You can read more details about how capacity planning works in the article: "Mainframe Capacity Planning: Systems Management's Stepchild"