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Question about XMP and 2 separate sets of RAM

Timmah!

Senior member
Jul 24, 2010
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65
91
Hi there,

i have a quick question. I have 32GB of Corsair LPX 3000MHz DDR4 Ram. 4 modules, 8GB each. Running with XMP on without issues at the advertised 3000 MHz.

I recently found out, to my surprise i could use up all that RAM while doing arch viz rendering - its surprising, cause i am actually using GPU based renderer, which is primarily VRAM dependent. However there is a moment, as you load up your data into VRAM and compile the scene, where system RAM comes into play and as i found out, quite heavily. I could not for example compile the scene from 3GB source file, cause the process would hit 25GB of RAM, at which point i got a low on memory warning and the app either crashed or the process simply would not continue. Funny thing, cause of this, i cant actually use up entire 8GB of my GPU´s VRAM (or around available 7GB to be more precise), cause i would never compile scene that big. The only exception would be, if the majority of those 7GB were textures and not the geometry needed to compile.

Anyway, as a result, i am looking at getting more RAM, pretty much the same kit as i already have. Thankfully I still have another 4 empty RAM slots on my board. My question is, from your experience, do you think i would still be able to run with XMP profile on, if its not a single matched 64GB kit, but rather 2 32GB ones? Although of the exactly same RAM? Or is this highly unlikely? I remember this used to be issue few years back, in the days of first i7s, but AFAIK, so was running XMP out of the box overall, which i dont think is a case anymore....

Thanks in advance for your advices...
 

Valantar

Golden Member
Aug 26, 2014
1,792
508
136
There's always a risk of addming more DIMMs forcing you to lower clocks/timings, but given that it sounds like you have four channels in use already, chances are it won't matter much. As for XMP, you can't be guaranteed that every single subtiming is matched on two kits (even if CAS and even the next 3-4 timings are the same - heck, even if the model numbers are identical), so you'll likely have to manually adjust timings to match the rating of the slowest rated kit. Still, this probably won't present issues. I'd expect to have to tweak things to get two 4-DIMM kits to work together perfectly, but it should be totally doable.
 

Timmah!

Senior member
Jul 24, 2010
759
65
91
Shamelessly necroing my own thread.

So i bought another set of RAM, same pack. Installed it into mobo, populating all the RAM slots now. Tried to switch the computer on and:

- it switched itself off after cca 5 seconds
- it switched itself on and off again after another cca 5 - 10 seconds
- it switched itself on and this time kept running, but the display never came on and i think the LEDs on the keyboard did not even flash (did not post?)

I tried to reset it with the reboot button on the case but did not seem to do anything. So i switched it on and off again by the main power button - no change in behavior however...

After i took all the new RAM sticks away, the computer worked as usual. Except all my overclocks are gone - i had CPU clocked to 4,2 and RAMs running on XMP before.

I assume the XMP being activated might have been a reason, but i am not sure. If that was the case i would expect the computer eventually boot with default 2133 MHz RAM speed and work just fine...

So i wonder whether I could have installed the RAM sticks wrongly- this was probably the first time i did it myself. I think all those locks clicked and i did it fine, but then again, when i moved my hand over all the sticks, it felt like the ones i installed were slightly "higher" - maybe i did not push them enough.

What do you think? How would the computer behave if it was badly installed RAM? I am bit scared to fiddle with it now, when it did not post yesterday, for a moment i feared i might have killed the board or something...

Thanks for any advices.
 

Timmah!

Senior member
Jul 24, 2010
759
65
91
Nevermind, gave it another try, and after bit of fiddling i got it running just fine. The shenanigans from my first failed attempt sort of screwed my BIOS settings or whatever, cause it the CPU was not actually clocking to 4,2GHz set in BIOS (just 3,8Ghz for whatever reason), so i had to go back to Optimized Defaults and reinstate my overclocks back again once more - including XMP, which i thought was the culprit in the first place, but to my surprise, it actually works! So i can run the RAM at 3000 MHz even though its not one matched pack - i might have got bit lucky here, so i am fairly satisfied. Hopefully i wont run into any instabilities now forcing me to downclock anything.
 

Timmah!

Senior member
Jul 24, 2010
759
65
91
Thank you, but it seems my joy was premature, i had:

- computer rebooting on its own while browsing web on me
- computer not being able to restart right away, when i prompted it to, ending with BSOD
- computer getting BSOD while browsing / shutting up webpage

Clearly something is wrong. Since the only change to my computer is the RAM, i assume the RAM is the culprit (or better said its XMP overclock) rather than CPU overclock, which was stable for the past half year. Gonna disable one thing at the time tomorrow and see whats gonna happen.
 

Valantar

Golden Member
Aug 26, 2014
1,792
508
136
Have you tried running only your new RAM, without the old? And just to state the obvious, if your two kits have different timings, you'll have to set both manually to the timings of the slowest kit, if not slightly slower than that again.
 
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Timmah!

Senior member
Jul 24, 2010
759
65
91
Thank you Valantar, for your previous response as well, to which i never replied and thanked, so my apology. Really appreciate your help.

Nope, i did not try to run the new kit on its own yet. Just added it to the old one. I installed it only on Saturday evening and yesterday dealt with computer only in the evening for about 3 hours, so there wasnt any time to do any trials yet. Anyway, i guess i would prefer to assume for the moment the new kit is not faulty and the instability is rather down to overclocks.

Regarding the timings, i kinda assumed they are the same across the board, as its 2 identical kits. I realize you said in your previous post i am not guaranteed that for all the timings and subtimings, anyway, how do i found out (what they precisely are)? Should i check values in CPU-Z or should i go to BIOS and check there? I dont have any experience in OCing RAM, thats why i am trying to rely on XMP, yet obviously, since the RAMs are capable at running 3000MHz, it feels as waste to run them just at stock speeds - did not my cold hard cash to run my computer slower than its capable of.

Thus, if you could keep guiding me as what exactly am i supposed to do, it would be very nice from you.
 

cool2hate

Junior Member
May 15, 2020
1
0
6
I think you need to enter timings manually for each pair of RAM on your MB. It means XMP profile can't be used anymore but still you can just enter values manually. You can use each pair alone with their own XMP profiles and record values. Also O.C. to see max. values and enter values manually for each pair with canceling XMP profiles.
 

john3850

Golden Member
Oct 19, 2002
1,418
17
81
On the last few mb I used the XMP profile cames only from the the first set of dimms try using ASRock Timing Configurator or Thaiphoon to see more info.
My Viper runs 4000MHz 19-19-19-39 while my kingston runs 4000MHz19-21-21-42.
Because I am running two different kits I run the slower set in the first two dimms which set all 4 slots to 19-21-21-42 and allows a 4000MHz.
 

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