I would like to equip my ProLiant DL380 Gen9 (2xE5-2680 v4) with more RAM. My plan was to have a total of 768GB (24x32GB R4). According to HP configurator, however, the clock rate drops to 1600 mhz instead of 2400. Is that because of the R4 ranks? Or is the CPU or server not able to cope with this? I hope someone can give me a good explanation, Google didn't help me.
You can find explaination here on page 3 http://www.supermicro.com/support/resources/memory/X10_memory_config_guide.pdf
"Because LRDIMM are buffered, a quad ranked LRDIMM appears to the Processor as a dual rank DIMM. Because of this it’s possible to populate all three DIMM slots without violating the logical limit. However by doing so, the memory speed drops to 1600MT/s as a result of the added memory."
A rank is a group of RAM chips that share the same data and address lines while sitting on the same DIMM. Each group puts load on the bus, potentially limiting its clock speed.
So, the RAM clock speed does not depend on the number of DIMMs you plug in but on the number of ranks that they represent in total. R-DIMMs and especially LR-DIMMs reduce the bus load to address this problem and to enable a larger number of ranks.