Here and elsewhere, certain product names may be the property of their respective third parties.xxii ■ IntroduCtIon Use Interprocedural OptimizationAdd the compiler flag -ipo to switch on interprocedural optimization. This will give the compiler a holistic view of the program and open more optimization opportunities for the program as a whole. Note that this will also increase the overall compilation time.Runtime profiling can also increase the chances for the compiler to generate better code. Profile-guided optimization requires a three-stage process. First, compile the application with the compiler flag -prof-gen to instrument the application with profiling code. Second, run the instrumented application with a typical dataset to produce a meaningful profile. Third, feed the compiler with the profile (-prof-use) and let it optimize the code.

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Liquid cooling is now a mainstream approach to boost energy efficiency for high performance computing systems. Higher coolant temperature is usually considered to be an advantage, since it allows heat reuse/recuperation and simplifies datacenter infrastructure by eliminating the need of chiller machine. However, the use of hot coolant imposes high requirements for cooling equipment. A promising approach is to utilize coldplates with channel structure and liquid circulation for heat removal from semiconductor components. We have designed a coldplate with low heat-resistance that ensures effective cooling with only 20-30°t emperature difference between the coolant and electronic parts of a server. Under the stress-test conditions the coolant temperature rose up to 65°C while server operation remained. We also studied power efficiency (expressed in floating point operations per watt) dependence on the coolant temperature (19-65°C) on the individual server level (based on Intel Grantley platform with dual Intel Xeon E5-2697 v3 processors). The power performance ratio shows moderate (≈10%) efficiency drop from 19 to 65°C due to increase of leakage current in chipset components and reduction of processor frequency which resulted into proportional reduction of DGEMM benchmark performance. It must be taken into account by datacenter designers, that the amount of recuperated energy from 65°C should be at least ≈10% to justify the choice of high temperature coolant solution.

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