Lower RAM Performance with Less DIMMs?

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Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 2:51 am

Hi gerbils. I was playing around with my PC for the past few days trying to troubleshoot choppy gameplay. While I was at it I've realized that my PC was running slower when only 2 of the 4 DIMM slots were populated. This was also chronicled in my recent post about this problem (General Hardware > PC Becoming Slow). Since I believe this deserves a separate thread under RAM, here I am with this new thread.

My specs are as follows: AMD Phenom II X3 720 (unlocked to X4, no OC), MSI 785GM-E65, 4GB or 8GB of Apacer DDR3-1333, AMD HD4670 512MB, 1TB Hitachi HDD, 2 x LG ODDs, ACBel 600+W PSU.

I'm using Passmark (Memory Mark) to test the RAM's performance. When only 2 DIMMs are filled, I get a score of around 800 (Dual Channel mode, as reported by CPU-Z and also following the manual's recommendation for Dual Channel operation). With all 4 DIMM slots filled, the score jumps to about 1,200. I'd hate to think that memory performance is actually lower with fewer DIMMs installed.

I also tested 2 x 4GB DDR3-1333 (instead of 2 x 2GB DDR3-1333) from another PC and I still got low scores of ~800 in Passmark.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 3:15 am

I'm guessing you have both sticks in one channel rather than one stick in one channel and the other in the other.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 5:52 am

If you're certain you're running dual channel correctly, maybe there's some kind of additional interleave between 1 stick per channel and 2 sticks per channel? Or is it possible there is a memory timing changing on you without you changing it?
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 7:34 am

Have you looked to see what mode the IMC is operating at in the BIOS? It should be 2-3 choices:, ganged, unganged, and maybe auto. If yours is set to auto (if that's an option) it's possible when 4 DIMMs are installed the BIOS chooses the "better" one for you. Try toying around with this setting.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 9:45 am

I followed the manual's instructions on how to populate the slots for dual channel operation, and CPU-Z does report that the DIMMs (in this case, two of them) are running in dual channel mode. Also, the IMC is set to Unganged, which is 2 x 64-bit operation. I'm also using the BIOS Optimal Defaults, and I never touch the memory timing settings there.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 10:28 am

ronch wrote:I never touch the memory timing settings there.


You should. Is your motherboard setting the memory to the correct timings? Check the specs on your RAM and then compare to what's in there.

Also, what is the model of the motherboard? There were boards that would clock down memory if all 4 dimms were populated.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 11:24 am

@TheEmrys: Yes, some boards would clock down RAM when there are more DIMMs installed, but in this case, installing more DIMMs actually makes performance better according to Passmark, not worse. I checked the timings and it's still 9-9-9-24, which I believe is standad for DDR3-1333.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 12:06 pm

ronch wrote: I checked the timings and it's still 9-9-9-24, which I believe is standad for DDR3-1333.


Not true at all. Timings are more linked to technology than to speed of DIMM. Back in the day, DDR would often run at CAS 2. As we've had huge increases in bandwidth and higher technology, timings get more and more loose. My DDR 1333 ran at 7-7-7-24. I'd look into it, if nothing else to just know what speed you should be able to get out of it. I know that for my old 870 board, AUTO would force my RAM down to 9-9-9-24.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Wed Dec 12, 2012 1:11 pm

How about run a real benchmark that outputs throughput and latency numbers, instead of just some fairly arbitrary number with no meaning??

For all you know, Passmark is giving a higher number when you have 8GB installed precisely because you have 8GB installed instead of 4GB, with no difference in speed whatsoever.

I'll give it a shot tonight. I have identical setups for my Z68 rig, 4x4GB 1600MHz DIMMs and 4x8GB 1600MHz DIMMs, I'll lock the timings before I switch, too. I bet the passmark score goes way up.
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Re: Lower RAM Performance with Less DIMMs?

Postposted on Mon Dec 17, 2012 6:14 am

I asked AMD about it and they said it's highly unlikely there's something wrong with the memory controller. Memtest86 also showed no problems with the RAM, and if there's a problem with the IMC chances are the PC will fail Memtest86. I'm banking on the idea that Passmark's RAM benchmark isn't very reliable.
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