Read speed drops 50% with more than 32GB installed - F3-1600C7Q-32GTX

[goose114]

I'm measuring memory read speed as reported by AIDA64 and memtest86+. With 32GB installed, my read speed is ~19000MB/s. As soon as I install another DIMM, the read speed drops to ~9000MB/s. It doesn't matter if the total memory is 40GB or 64GB, the speed remains around 9000MB/s. It is as if the system is going from quad-channel back to single-channel.

The motherboard I am using is an ASRock X79 Extreme6 with BIOS 2.20 (latest). CPU is an i7-3820. I don't have other RAM to test, but moving DIMMs to other slots changes nothing, so I suspect either a configuration issue or a problem with the motherboard. Does anyone have any suggestions for what to check?

[Tradesman]

What are you adding over and above the 32GB set? The timings, voltages, etc in the SPD of the sticks is for 32GB, so if adding another stick, even of the same exact model it can be problematic...i.e. my 32GB (4x8GB) requires a tRFC of say 314, if I were to add another stick or two that number would go up, another scenario would be the 32GB calls for 1.5 volts, adding additional sticks might then make it need 1.53 or whatever. Same with the MC (memory controller), more memory may require more voltage than what is set by the SPD in the sticks or what the BIOS calls for under AUTO.

[goose114]

I have two 4x8GB F3-1600C7Q-32GTX kits. I was just using the XMP 1.2 option to set speeds. Is this something I'll need to test various settings with, or is there a straightforward way to estimate what would be required to get maximum bandwidth out of all 8 DIMMs? Am I better off from a performance standpoint just sticking to a single 32GB kit?

[Tradesman]

Can you post your BIOS settings for voltages and timings (both base and advanced), and I'll see if I can figure out what may work well

[goose114]

Thanks for looking. All of the settings in these pictures are auto. Voltage is 1.5.

http://imgur.com/a/OKVQH#0

[Tradesman]

Hmm, no system voltages so we'll sort of wing it for the moment, let's try the tRFC at 310, bump the DRAM voltage to 1.57, imagine VCCSA is around .940 let's initially go to 1.2, and take the VTT to 1.15

[goose114]

Here are the other voltages before any changes.

http://imgur.com/CKQO5YM

[goose114]

Changed tRFC to 310, DRAM voltage to 1.575, and VTT voltage to 1.15. VCCSA voltage stayed the same at 1.2.

AIDA64 memory read benchmark before the changes was 11,332 MB/s, and after the changes dropped very slightly to 11,145 MB/s. I'm currently testing with 40GB installed.

[Tradesman]

Might raise VTT to 1.2 and then test with tRFC up + 10 and then - 10 from the 310 and see if tighter or looser helps.....with 3 sets it's basically a guessing/testing game

[goose114]

Doesn't seem to be having any effect, the benchmark is staying in the 11,000 MB/s to 11,300 MB/s range regardless of the settings.

[Tradesman]

Might try checking with GSkill or GSkill tech via PM to see if they have an idea, may just be the sets not wanting to play together