The uncore portion provides it's own performance gains from higher MHz (uncore = everything else except the core) but there is certainly a performance gain to be had going from DDR3-1600MHz to DDR3-1868MHz at the same uncore of 3604MHz. It won't 'bottleneck the ram to DDR3-1800MHz' as you put it, but there is an interaction between the IMC portion of the analogue controller and the ram speed.
Also, my R3E does not drop to low multi at no load. I can watch the C-state and its in 100% speed mode all the time with a lowest calculated multiplier of 29.997x when 30x is set in BIOS.
I'm keen for an efficiency test, to do it right and to standardize for any CPU speed we'd need to calculate the GFLOPS/MHz (we can of course also have brackets of results as you suggested). What could be fun is doing something like the low clock SPI32M thread, where there is categories of say 100GLFOPS, 90GFLOPS, 80FLOPS etc and the lowest speed required to hit as close to but not over that mark, ie: the 70GFLOPS bracket would have an ideal target of 79.99 GFLOPS etc.
In absolute terms, for my 4GHz run I just did, I get 78.3962/4005.1 = 0.01957 GFLOPS/MHz, or to make it more meaningful, 19.57 GFLOPS/GHz.
Looking at the data from the last 2 pages, here's the absolute efficiency rankings:
- mr too short - 92.1319/4610.5 = 0.01998 = 19.98 GFLOPS/GHz
- donmarkoni - 91.9528/4610.8 = 0.01994 = 19.94 GFLOPS/GHz
- zoson - 91.5017/4657.9 = 0.019644 = 19.64 GFLOPS/GHz
- CryptiK - 78.3962/4005.1 = 0.01957 = 19.57 GFLOPS/GHz
- Spoolindsm - 86.6040/4504.9 = 0.019224 = 19.22 GFLOPS/GHz
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