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Made the mistake of thinking 12GB was enough, but I keep crashing out of memory while running sparse matrix solver... a few more GB should do it.

Is it valid to replace 3 of the 2GB modules with three 4GB ram modules, for 18GB total ? I realize that its "probably" valid, and "might" break triple-channelness, but does anyone have a specific experience trying this?

Mods- I think this is the same question, but unanswered and questionable choice of title... could be merged with my question. http://superuser.com/questions/139090/semi-hybrid-triple-channel-ddr3

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Just out of curiosity, how big is your dataset? Instead of throwing memory at the problem, could you improve your algorithm? It might take a bit longer, but yield a better value in the long run. – MBraedley Oct 25 '10 at 22:59
well it would be stupid of me to accidentally be using full-matrix instead of sparse somewhere, but that would take up about 120GB... so 2.5-3.5 GB per matrix is not bad. Whether the matrix could be more smartly created is a difficult question. "maybe", but the bottom line is if I use 8 threads on core-i7, I can't even fit 2GB per thread... – peter karasev Oct 26 '10 at 1:02
well the other trick is to sequentially load into the matrix from disk, you just get a huge performance hit while loading, and another place to introduce bugs. – peter karasev Oct 26 '10 at 1:23
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In most cases, this sort of thing varies by motherboard. Most don't like too much mixing and matching, but can handle it in limited cases.

Mine, for example, has four slots. You're free to have two sorts of RAM, but slots 1/3 and 2/4 have to have identical sticks, otherwise Bad Things happen (slot 4 will be ignored, IIRC). This would obviously be a bit different with triple-channel, but the same principle applies.

My advice would be to check your mobo manual, try it and see what happens., and if it doesn't work, look into getting 2 or 4 of the 4GB sticks for balance.

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Triple Channel mobo's will have 6 Dimm slots, it sounds like he's already populated them all with 6 2gb Dimms (2 x 6gb triple channel set). Your information is still valid for dual channel motherboards but not applicable in this case. – chunkyb2002 Oct 25 '10 at 21:29
Ah, I missed the triple channel. Sounds like 3x2 and 3x4 should work, if the mobo likes that. I'll edit my answer a bit. – peachykeen Oct 26 '10 at 0:36
This is the motherboard: ASUS P6X58D Premium LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX Intel Motherboard. Actually upon RTFM, it says "You can install different RAM sizes in channels A,B,and C. The system maps the lowest size of the three to triple channel, and any excess is then used in single-channel mode". The above sounds much funkier than what I ask, so its likely that 2 + 4 in a channel would be ok... I guess that's one reason this board is kind of expensive, nice feature. – peter karasev Oct 26 '10 at 1:12
Yeah, it sounds like you should be good. With modern motherboards, it's not going to break anything if it's not supported, so there's no reason not to. Being able to use the excess is a pretty nice little features though. Mine definitely doesn't support that. – peachykeen Oct 26 '10 at 2:19
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up vote 3 down vote

There's no rule that says your RAM all has to be the same size. Additionally you won't break "triple-channelness" so long as you have 3 matching 4GB dimms to replace the 3 2gb dimms.

It is important that the speed and timings are the same though otherwise your performance will be as good as your weakest component.

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