Low Level Measurements
Before we start with the rather complex virtualization benchmarks, it is good to perform a few low level benchmarks. First, we measured the bandwidth in Linux with our Pathscale compiled Stream binary on the latest Ubuntu Linux. For more details about our Stream binary, check here.
To get a better understanding we tested the 8GB RDIMMs at both the rated speed (1600MHz) as well as configured (via the BIOS) as 1333MHz DIMMs. The comparison of 1333MHz RDIMM and LRDIMM allows us to measure the impact of the iMB buffer. That impact is small but measurable: RDIMMs offer about 5% more bandwidth than LRDIMMs at the same speed. 1600MHz RDIMMs offer 14% higher bandwidth than LRDIMMs.
Of course, bandwidth only matters when you run out of it. Latency always matters, although a 15MB (up to 20MB) L3 cache can hide a lot of memory latency. We tested memory latency with AIDA64.
The iMB adds about 11% if we disable turbo and compare the LRDIMM and RDIMM at the same clockspeed (1333MHz). Somewhat interesting is that the latency of the RDIMM at 1600MHz is higher. The memory chips are accessed with a significantly higher CAS and RAS to CAS latency at 1600MHz, which explains this counter intuitive result. Once we enable turbo, the latency differences get very small. The iMB causes only 2% extra latency, which is negligible.
The conclusion so far: the iMB decreases bandwidth by a measureable but small amount, but the latency impact is hardly measurable.
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