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Old   July 14, 2019, 00:36
Question Low quad channel memory bandwidth (pics)
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Greetings CFD-online

I built humble workstation based on your recommendations:

Asus x99 Strix
i7-6850k
4x8 Corsair Vengeance 2133 MHz @2800 mhz in quad channel

Theoretically I would have 89,6 GB/s of memory bandwidth but benchmarking my system on SiSoftware Sandra gives me 45.49 GB/s of memory bandwidth. With ram OCed at 2666 mhz Sandra gives 46,15 GB/s

CPU frequency is stock 3,6 GHz. I think I have everything correcly connected. Aida64 tells me my system is quad channel.

Thanks for your help

Rig and benchmark photos



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Old   July 14, 2019, 01:35
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I don't have any numbers at hand to compare for this benchmark.
With 4x8GB single-rank DDR4-2800 you should be in the ballpark of 60000MB/s using AIDA64 memory benchmark.

No synthetic memory benchmark will come close to theoretical results. The best you can get is usually 10-20% lower. And for sheer memory bandwidth, single-rank leaves another ~10% on the table compared to dual rank at the same frequency.

You checked the manual which DIMM slots to populate with 4 DIMMs? There is often a recommendation to use either slots 1 or 2 of each memory channel. I think I would use A1, B1, C1, D1.
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Old   July 14, 2019, 10:54
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Hello there flotus1, thanks for your answers

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Originally Posted by flotus1 View Post
You checked the manual which DIMM slots to populate with 4 DIMMs? There is often a recommendation to use either slots 1 or 2 of each memory channel. I think I would use A1, B1, C1, D1.
Yes I'm using slots A1, B1, C1, D1 according to the manual.

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I don't have any numbers at hand to compare for this benchmark.
Following this thread Duke711 gets 86.34 Gb/s with 6850k, I don't know what I'm doing wrong

Epyc 7551 vs 6850K; Ansys Mechanical Bench
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Old   July 14, 2019, 15:29
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Different memory benchmarks produce different results. You really can not compare them.
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Old   July 14, 2019, 16:15
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Comparing with same memory benchmarks, users get ~70 Gb/s , ¿could it be that my ram (Corsair 2133 MHz ) is not fast enough as Corsair Vengeance® LPX 16GB (4 x 4GB) DDR4 DRAM 3200MHz C15 ?

https://www.techpowerup.com/forums/t...186338/page-33
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Old   July 14, 2019, 16:21
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Didn't you get your results with a different benchmark? You only mentioned Sisoft. That's why I recommended Aida64, there are just more results to compare to.
Also: use CPU-Z tab "memory" to check how your memory is actually running. Maybe the overclock did not even apply.
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Old   July 16, 2019, 19:16
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I was gettings nearly the same results on Aida64. I managed to increase the memory bandwidth doing overclocking tweaks, now I reach 52 Gb/s which is comparable to other users results. Still lower than Corsair Vengeance® LPX 16GB (4 x 4GB) DDR4 DRAM 3200MHz C15.

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Old   July 17, 2019, 04:50
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The values seem to be a bit on the low side. But you have to be aware of one thing: when looking at results in the kind of forum you linked, you are comparing against very enthusiastic people with lots of experience. So don't expect to get close to their results while still learning how to overclock.

A few things you can do to (potentially) improve your benchmark results and real-world performance:
  • use the latest bios
  • make sure the system is idle while testing
  • set windows performance scheduler to maximum performance
  • try to get a command rate of 1 instead of 2. Should be possible with rather low memory frequencies
  • increase northbridge/uncore frequency. Might require higher voltages. Higher values than core frequency are generally not beneficial, so you will have to increase CPU frequency too in order to get more than 3.6GHz.
  • tighten the timings further, especially when using rather low memory frequencies like DDR4-2666. This might require increasing the memory voltage.
  • and of course: start reading in-depth guides on how to overclock memory and cache subsystem on X99.
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Last edited by flotus1; July 17, 2019 at 08:34.
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Old   July 17, 2019, 22:44
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Quote:
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[*]use the latest bios - updated to july 2019
[*]make sure the system is idle while testing - done
[*]set windows performance scheduler to maximum performance - done
[*]try to get a command rate of 1 instead of 2. Should be possible with rather low memory frequencies ¿what is command rate of 1?
[*]increase northbridge/uncore frequency. Might require higher voltages. Higher values than core frequency are generally not beneficial, so you will have to increase CPU frequency too in order to get more than 3.6GHz. - Done to 4.2 GHz
[*]tighten the timings further, especially when using rather low memory frequencies like DDR4-2666. This might require increasing the memory voltage.

[/LIST]
All this seemed to increase the memory bandwidth a little bit

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Old   July 18, 2019, 02:42
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From your picture I can see that command rate is still 2, uncore frequency is unchanged and timings are still the same. So I am not sure what you checked besides increasing core clock speed. Diagnosing issues like this from far away without full information is next to impossible. I honestly don't know what else I could do here.
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Old   July 30, 2019, 07:12
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Quote:
Originally Posted by pibil1 View Post
Greetings CFD-online

I built humble workstation based on your recommendations:

Asus x99 Strix
i7-6850k
4x8 Corsair Vengeance 2133 MHz @2800 mhz in quad channel

Theoretically I would have 89,6 GB/s of memory bandwidth but benchmarking my system on SiSoftware Sandra gives me 45.49 GB/s of memory bandwidth. With ram OCed at 2666 mhz Sandra gives 46,15 GB/s

CPU frequency is stock 3,6 GHz. I think I have everything correcly connected. Aida64 tells me my system is quad channel.

Thanks for your help

Rig and benchmark photos




hmm not sure if 4x8 is correct it should be 2x16, I read somewhere that a 4 sticks will run slower but the x99 should be able to handle it without a problem, I also notice it runs on 2133 MHz this must also the cause.
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Old   July 30, 2019, 09:25
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hmm not sure if 4x8 is correct it should be 2x16
No, it should not. This is a quad-channel platform, not dual-channel.
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Old   September 19, 2019, 08:21
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You get different results for different benchmark products. It is hard to compare all of them.
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