Mainframe computer

Mainframes (often colloquially referred to as big iron) are large and "expensive" computers used mainly by government institutions and large companies for mission critical applications, typically bulk data processing such as censuses, industry/consumer statistics, ERP, and financial transaction processing.

Speed and performance

The CPU speed of mainframes has historically been termed as MIPS (million instructions per second). MIPS have been used as an easy comparative rating of the speed and capacity of mainframes. The smallest zSeries IBM mainframes today run at about 26 MIPS (z890 Model 110) and range up to well over 20,000 MIPS (in one System z9-109 Model S54 mainframe).

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However, MIPS is inherently misleading for at least two reasons:

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• Changes in processor architecture over the years mean that MIPS ratings have long ago lost all direct relevance to the actual number of instructions executed. The meaning of MIPS is now purely for historical rather than for technical reasons.
• A single value rating like MIPS is inaccurate when applied to different types of computer processing work. Mainframe design emphasizes "balanced performance," with tremendous I/O capacity in particular for superior online database and transaction performance.

Offically, IBM has long published a set of LSPR (Large System Performance Reference) ratio tables for mainframes that takes into account different types of workloads.

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An analogy can be easily seen in today's competitive UNIX server environment, where more demanding customers look at machine performance based on specific type of workload benchmarks, e.g. SpecInt or TPC-C, rather than looking at pure CPU clockspeed. However, increasingly even these benchmarks have their own problems.

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Unfortunately, it takes a fair amount of work (and maybe guesswork) for customers to determine what type of workload type they have and then apply only the LSPR values most relevant to them. Use of MIPS still persists today to the extent that IBM and other consultants continue to publish MIPS for general reference.

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