DDR3 Memory Population Guide
Proper memory population is important to maximize the performance of your memory kit. There are three factors that determine memory performance:
- Memory Capacity (GB)
- Memory Channel (Dual, Triple or Quad)
- Memory Frequency (MHz)
The best way to boost memory performance is to add more of it (i.e. increase the Memory Capacity by adding more DIMMs or replacing current DIMMs with ones that have more GBs). But before you do that, make sure to consider the other two factors in memory population.
Each motherboard should specify whether it has a Dual, Triple or Quad Channel memory bus. The more Memory Channels a motherboard has, the greater the data throughput from the memory to the memory controller (i.e. higher performance). But the memory must be populated properly to take advantage of this. Whatever configuration your motherboard has, you can only take advantage of its (Dual, Triple, or Quad) Memory Channels by populating the memory DIMMs in multiples of that configuration (for each processor on the motherboard).
So, for example, if you have a motherboard with Triple Channels of memory bus, you have to populate the memory with either 3, 6 or 9 DIMMs (for each processor) to use Triple Channel. Otherwise, if you populate each processor with, say, 4 DIMMs (or any other non-multiple of 3), then each of those DIMMs will run on Single Channel and will lose the performance boost. For Dual Channel, make sure to populate in multiples of 2, and for Quad Channel, in multiples of 4, if possible.
The third factor to consider is Memory Frequency. Memory DIMMs will usually specify three different frequencies on which they can operate: for example, "1333 MHz / 1066 MHz / 800 MHz". The higher the frequency, the higher the bandwidth and performance of the memory. Memory DIMMs will only run at their highest frequency if the motherboard supports that frequency and if there are no more memory DIMMs populated per processor than the number of Memory Channels (Dual, Triple or Quad) of that motherboard.
Let's look at an example. On a motherboard with Triple Channel memory bus, if there are 3 or less DIMMs populated for each processor, they will run at the highest supported frequency, say 1333 MHz. If you add 1 to 3 more DIMM to this configuration, all of these added DIMMs and the original 3 DIMMs will run at a lowered frequency, 1066 MHz. If you add 4 to 6 DIMMs to the original 3, then all of the DIMMs together will run at the lowest frequency possible for this memory, 800 MHz. This is the general rule: if you populate beyond a multiple of the Memory Channel, all of the memory DIMMs will run at a lowered frequency.
Those are the three factors of memory population. There are cases when you may want to sacrifice Memory Frequency for more Memory Capacity. The two tables below show how different usage needs may call for different memory configurations.
Finally, here are some last points to remember when populating memory:
- To use Memory Channels (Dual, Triple, or Quad), all memory DIMMs must be of the same Memory Capacity (GB) and of the same type for any given processor.
- For motherboards running multiple processors, if one processor's memory configuration is taking advantage of (Dual, Triple or Quad) Memory Channel, it is best if the memory DIMMs on the other processors are populated to use the (Dual, Triple or Quad) Memory Channel as well.
- Make sure to populate the DIMMs in the proper order noted on the motherboard: 1A, 1B, 1C, 1D, 2A, 2B, 2C, 2D, etc.
|Usage||Workloads||Memory Frequency||Max B/W||DIMMs per Channel||Max DIMMs||Max Capacity||CPU|
|Maximum Bandwidth||HPC||1333 MHz||32 GB/s||1||6||48 GB||
|General Purpose||Various apps||1066 MHz||25.5 GB/s||2||12||96 GB||
|Maximum Capacity||Virtualized platforms||800 MHz||19.2 GB/s||2 QR RDIMM 16GB||12||192 GB||All|
|3 SR/DR RDIMM 8GB||18||144 GB|