Worth mentioning a Haswell system with QDR i helped build. I discovered that it was needed to use an older version of libpsm to avoid a bandwidth boost tweak that increased latency which isn't the competitive advantage of IB QDR. Also highly disappointing they down clocked Haswell as soon as AVX2 was touched. Given no temperature increase, there isn't a thermal argument for it just for using more of the micro instructions on the die. Was an early system, and needed to use end-of-life CentOS for the correct library versions.