Computing power has been measured in various ways over time. In the distributed computing field and for supercomputers, FLOPS (or teraFLOPS (TFLOPS) or petaFLOPS (PFLOPS)) are normally used. GIMPS has also used 3 other measurements over its lifespan (P90 years, PII-400 years, and GHz-days).
MIPS stands for "Millions of instructions per second"...
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FLOPS stands for FLoating point OPerations per Second. Supercomputer ratings usually derive theoretical peak FLOPS as a product of number of cores, cycles per second each core runs at, and number of double-precision (64 bit) FLOPS each core can ideally perform. Although not an SI unit, the convention is to use the SI prefixes, such as giga-, tera-, and peta-. GIMPS, via PrimeNet has been producing greater than 50 TFLOPS (as of February 2011).
The calculating effort one 90 MHz Pentium computer produces over the course of 1 calendar year (365 days). Because of the early adoption of this unit by GIMPS, other distributed computing projects also adopted this unit to measure the relative contribution of users.
A unit that was only briefly used. One PII-400 year equals 5.5 P90 years.
The work accomplished by one core of a hypothetical 1GHz Core 2 Duo CPU in one day. One P90 year equals 5.075 GHz-days. 1 TFLOPS equals 500 GHz-days, thus 1 P90 year is about 10 GFLOPS.