Build Advice Mixing RAM module densities

I have an Asus P7P55-M motherboard with core i5-750 and 2 x 2gb DDR3 Corsair xms3 ram modules rated at 1333Mhz. The motherboard has four slots and is dual channel. At the moment I have the two ram modules in the light blue slots. As I understand it, this means the two modules are on different channels, and this gives best performance, is that correct?

I ask because I have just bought a 4GB single module as they are so crazy cheap at the moment (£14.99 delivered from amazon, already gone up to £19!) If I leave the two old modules where they are and fit the 4GB module in one of the black slots, will that mean that 6GB of ram is on one channel and only 2GB on the other? So I should move one of the 2GB modules into a black slot, and the 4GB into the now empty blue slot. Im aware the difference is not great, but I may aswell get it right as I dont upgrade too often.

 

The system will revert to single channel (which will halve your memory bandwidth). Always, always, always by ram in pairs (or 3s,4s for skts1366/2011).

Either buy another 4GB stick, or RMA the one you have and get a dual channel kit.

 

It is absolutely irrelevant. Memory speed and/or bandwidth doesn't really translate in anything noticeable, so singlechannel, dualchannel, flex (mixed single and dual channel), different memory speeds translate to max single digit percentage differences. The upgrade from 4 to 8GB will give you much more than thinking about single/dualchannel and speeds .

http://www.intel.com/support/motherboards/desktop/sb/cs-011965.htm
http://www.atomicmpc.com.au/Feature...y-kits-vs-dual-channel-on-p55-platform.aspx/4

 

According to the link provided by faugusztin, it is the overall amount of memory per channel that needs to be equal for dual channel mode to work. See the fourth picture example. I would not have thought that would work but its on the intel site so seems legit! If I put 6gb on one channel and 2 on the other then it would only run in single channel mode I believe (again from reading the intel page)

 

That logic worked on the pentium 4s, where going from 512mb to 1.5gb easily outweighed the loss of memory bandwidth. With 4GB of existing memory, the bandwidth loss will be far more noticable than the extra 4GB.

EDIT: something on that atomicmpc link dosn't look right. CPU-z is still saying its in dual channel mode. Has intel modified the way dual channel works (no longer requireing matched pairs per channel?)

 

The intel link suggest that equal number of DIMMs is not important, but equal size of total RAM per channel is. I dont have the extra module yet so I wont know until I install it.

 

Do you realize that are two separate channels, right ? One (2+2GB) running in dual channel, second (2GB) running in single channel. Same applies to the OP, the 2+2GB will run dual channel, the 4GB will run in single channel. Or 2+4GB will run 2+2GB in flex mode (2+2GB in dual channel, 2GB in single channel) and the remaining 2GB module will run single channel. In any combination, the OP will have 4GB in dual channel and 4GB in single channel performance.

But anyway, the real world performance difference between single channel and dual channel is single digits at best, unless you use an application which uses memory heavily and can saturate the memory bandwidth. Which is pretty much none of typical desktop applications.

 

So there are 2 channels, that can run in either dual or single channel mode? and if all four slots were full you would effectively have four channels open? (2 x dual channel)

 

Yes, exactly. Same for X58, where you can run single,dual or tripple channel, and then same for second set. And same for X79, where you can do same again on boards with 8 slots, where you will have quad channel too .

For example here is my 2x4GB+4GB setup :

 

ok, try sticking the 2x 2GB modules in the A channel, and the 1x 4GB module in the B channel, and see if that works.

There should be 2 groups of DIMM slots on the motherboard each with a black and blue slot. The black and blue pair closest to the CPU should be the A channel, fill those with the 2x2GB slots. Add the new 4GB module to the black slot in the second grouping of DIMM slots (farthest from CPU).

Reboot, and check the memory tab in CPU-z to make sure dual channel is enabled.

 

Oh by the way, i think it is exactly this mode :

And this scenario (except he has 2+2GB and 4GB) :

 

Yes it looks like I will get dual channel through both DIMMs. I will run CPUz to confirm when the new module arrives.