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I measured memory throughput of my computer with MemTest86+, and one result surprised me.

I use DDR3-8500 (533 Mhz) modules. I have one 4 GiB and two 2 GiB. The timings seem to be 7-7-7-20 fixed by the motherboard (a WMCP78M motherboard in an Acer X3300 with AMD Athlon II X2 215 Processor).
The motherboard has 4 memory slots, divided in two groups supporting dual channel.

A single module (2 GiB or 4 GiB) gives 3566 MB/S (I presume this means 3566 MiB/s).
Two identical modules (2 x 2 GiB) on separate slots give 3566 MB/S. I'm okay with that because dual channel is not active.
Two different modules (2 GiB + 4 GiB) on whatever slot give 3566 MB/S. Okay too.
Two identical modules (2 x 2 GiB) on adjacent slots gives 4122 MB/S. Better throughput because dual channel is in action.

Now if I put 2 x 2 GiB on adjacent slots, plus 1 x 4 GiB on another slot, I expect dual channel being active for half the memory. But I have 4122 MB/S. So dual channel is working even with the lonely 4 GiB module.
It could be a bug into MemTest86+: the throughput can be computed only on a part of the memory. So I inverted the modules, but same result (I don't think the motherboard relocates the slots depending which ones are in dual channel).

I updated the BIOS to latest officiel version.
I checked hidden BIOS settings with ctrl+F1 but nothing more appear.
Finally I used AMIBCP to open the BIOS's ROM and see if any option still remain invisible, but everything was already shown. Note that "everything" is not a lot because there is not a single setting about FSB, voltage or so.

What could be the explaination behind the fact I obtain a dual channel score when I use 3 memory modules ?

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I don't think you actually are. max transfer speed on DDR3 1066 PC3 8500 is approximately 8534MB/s, which is about twice what you're getting. – Frank Thomas May 17 '13 at 18:20
Huh... yes, I didn't noticed that. This is a very low transfert rate indeed. – Gregory MOUSSAT May 17 '13 at 19:18
up vote 2 down vote accepted

This smells like you have a problem. As Frank Thomas said, the transfert speed you have is way too low. Even a cheap motherboard perform better.

Have a look into your BIOS settings. You could find the reason. Or try to reset the BIOS to default values or optimal values. You can also reset the settings with the jumper on the motherboard.
Some BIOSes have hidden settings. Ask Google if you can find them for your.

You can also try to update the BIOS. But a so wide gap between your results and theory is probably not a BIOS version problem.

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Question updated – Gregory MOUSSAT May 18 '13 at 14:48

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