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I'm looking at the RAM compatibility table for my motherboard:

RDIMM

and it shows that dual-rank (DRx4) memory modules can operate at twice the speed of quad-rank (QRx4) memory modules (1600 MT/s vs 800 MT/s with 2 DIMMs per Channel).

Does this mean DRx4 DIMMs' connection to the CPU has twice the bandwidth of QRx4 DIMMs, or are the higher MT/s numbers offset by each transfer being twice smaller?

If the bandwidth is actually lower for QRx4 DIMMs, then why?

As I understand it, at least with registered RAM, the CPU is not directly connected to the RAM chips, so any additional load inside the DIMM should be a separate issue.

  • The chart only indicates Dual/Triple channel where exactly do you see Quad channel? – Ramhound Sep 17 '14 at 11:51
  • quad-rank = QR (leftmost column), the CPUs/sockets are quad-channel, but I didn't mention quad-channelness – MaxB Sep 17 '14 at 18:41
  • A very interesting question, which I suspect has to do with the load on the memory controller which is higher is you use more ranks (either more DIMMs, e.g. 3x dual rank DIMMs per channel) or just DIMMs with more ranks. (Notice the per channel, not DIMMs total). – Hennes Sep 17 '14 at 18:48
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Quad-rank DIMMs are slower than dual-rank DIMMs because there are more loads attached to each signal. More load means more inductance and capacitance which means it takes longer for voltages to stabilize.

"MultiRank Modules do have higher loading on the DQ (and on unbuffered DIMMs on CA) bus. ... [I]f there are more Ranks connected in one channel the speed will be reduced." -- Memory Rank

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    DIMM ranks seem to be well discussed on Server Fault. But on Super User it is rare enough to see them mentioned. I wonder if this is because board with many memory sockets are more common in server then in consumer stuff. – Hennes Sep 17 '14 at 21:35

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