Comparing DDR5 at single and dual ranks highlights differences in performance and stability.
Comparing DDR5 at single and dual ranks highlights differences in performance and stability.
Hello everyone, I'm trying to grasp what Dual Rank and Single Rank mean. Does Single Rank offer more bandwidth than Dual Rank? Is it just about where the NAND chips are placed or does it involve a difference in how the memory controller interacts with the memory? Also, is Dual Rank equivalent to having two channels with DDR5? Someone on YouTube said it works the same as two single-rank sticks, so I want to understand that.
Dual channel and dual rank differ. You can have both features. Dual channel allows your memory controller to use two channels simultaneously, effectively increasing bandwidth. Dual rank lets it access one rank while the other is ready, which helps lower latency. Yes. Putting two single-rank sticks on the same channel works just like having one dual-rank stick on that channel. You can choose either one dual-rank stick per channel (two sticks total) for dual channel, or use two single-rank sticks per channel (four sticks total) to achieve the same result.
Dual rank improves actual latency by affecting memory access, not just CL latencies. In terms of topology, DaisyChain can indeed perform worse than Dual Rank depending on the setup.
The difference between CL latency and real latency is often unclear. Latency can appear as clock cycles, yet it remains a tangible delay. Also, dual-rank configurations demand greater effort from the memory controller and can restrict the highest achievable bandwidth when using a memory overclock.
In summary, using identical clocks and timings, one dual-rank module per channel behaves similarly to two single-rank modules per channel. The maximum bandwidth stays consistent, but from my perspective, it can be enhanced by employing multiple ranks within a channel. Practical bandwidth is still constrained, yet I see potential for improvement with more than one rank per channel. Right now, I'm conducting tests with DDR4 to gather data. The main concern is that memory controllers might struggle to drive two modules simultaneously, which could reduce the reliable maximum clock speed compared to a single module. This issue becomes more noticeable at higher speeds, especially with DDR5, where sensitivity to loading may increase.
It's complicated. Assuming all the settings are the same, the more ranks you have the faster your RAM is because the memory controller has more places to get data from with waiting for certain bank groups to recover. That said, the performance gains scale logarithmically, where on DDR4 going from single rank to dual rank was roughly equivalent to adding 400MT/s worth of bandwidth for free but dual rank to quad rank was maybe 100MT/s at best. On DDR5 it's even worse, where dual rank is only barely faster than single rank. The problem that will result is that the more ranks present in a memory setup, the harder the memory controller has to work and your max memory speed can be greatly reduced. It depends on the platform for how much (Ryzen 5000 it goes from 4800+ being very possible with single rank memory, a good motherboard, and desynced infinity fabric down to 4000MT/s being about as much as you can hope for with dual rank, while on Ryzen 7000 it goes from 6200 to 6000MT/s with single rank to dual rank), and there are a lot of platforms where you can't outclock the benefits of dual rank, but on DDR5 with how little benefit there is from dual rank you want to aim for a max clock speed single rank setup more than a dual rank setup.
But what about choosing 1x Dual Rank versus 2x Single Rank lights? If I understand correctly, I've heard that Dual Ranks work similarly to Dual Channel RAM—so a 1x Dual Rank setup is like having two 1x Single Rank units. Even though the performance might be comparable, using Dual Rank would be preferable because it powers just one stick instead of two, which also reduces power consumption.
The improvement comes from reduced waiting time between read operations. Dual rank minimizes delays compared to reading the same rank, while dual channel lets each DIMM access its own rank simultaneously, boosting performance significantly. In contrast, power consumption is mainly driven by the number of memory chips; more RAM chips mean higher power draw. A mix of two single-rank sticks and one dual-rank stick should offer similar power usage.