ideal memory configuration 4 bank, ddr3, AM3+ FX - 1 vs 2 vs 4 dimms?

Question

Ok, so ive been looking around, trying to learn and understand the way that ram works. Ive gotten one answer that said
"The addressing is best for 2 sticks, and when you use 4; it slows down"

Another answer said something like:
Theres bank/channel interleave that makes the memory read like one stick
Also I read something about the memory density also being a factor.

I dug further and found out that theres a higher speed limit on my board for 2 sticks vs 4, so now im trying to put an image in my head of how and why, and... pfft.

Can anyone explain, or recommend a resource that would answer these questions?

Answer 1

There is no 'ideal' answer since to much depends on what someone considers ideal.

Memory access speed depends on a lot of things, including:

1. Type of Memory (Fast page RAM, SDRAM, DDR, DDR2, DDR3, ...)
2. Speed of the RAMs IO (e.g. 667MHz for DDR3-1333)
3. Efficiency of the RAM (DDR 1,2 and 3 are twice as efficient as SDRAM, which is why you often see RAM sold at higher IO speeds than they actually run at. E.g. DDR1-400 runs at 200MHz, DDR3-1333 at 667Mhz).
4. Latencies on the RAM. Most often specified as DDRX-ABCD A-B-C-D.

Highest speed is a combination of these, but since you are usually stuck with a specific type of memory points 2 and 4 are the ones to check. Point 2 (memory IO speed) makes the biggest difference.

I dug further and found out that there is a higher speed limit on my board for 2 sticks vs 4.

This is because your memory controller needs to do 'more work' if it has to address more DIMMS. I realise this is an imprecise definition of work. For details check the posts on memory ranks.

Usually, it comes down to this.

• If your memory controller has one channel and you plug in an average rank 2 DIMM then it just works.
• If your memory controller has one channel which has to drive more than 2 ranks then it may slow down. On consumer board this is often 2 DIMMs for a total of 4 ranks.
  This can differ. E.g. on the Intel 5100 chipset it can drive up to 2 dual rank DIMMs at full speed on only has to slow down when a third DIMM is added to the memory controller.

• If your board has two memory controllers then using 1 rank 2 DIMM on one channel is just fine.
• If your board has two memory controllers then using 1 rank 2 DIMM on both channel is also fine. This often allows for the use of dual channel mode which is slightly faster.
• If your board has two memory controllers then using 2 stick on both channels is usually means more work and the IO frequency is slowed down. (Same situation as 2 DIMMs per single controller, but now twice the same situation).

In the case of your AM3 setup you can gain a slight speed gain by using only two DIMMs over 4 DIMMs with an equal memory size. But more RAM (even more RAM at a slower speed) usually results in a faster experience. So go for more RAM first, and if the price difference is small then consider using the 2 DIMM solution.